Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.stsci.edu/~webdocs/STScINewsletter/2008/spring_08.pdf Äàòà èçìåíåíèÿ: Mon Jan 19 23:40:14 2009 Äàòà èíäåêñèðîâàíèÿ: Mon Apr 6 00:32:28 2009 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: orange soil

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Spa ce T elescope Science Institute

The Last Confessions of a Dying Star Image Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA) http://hubblesite.org/newscenter/archive/releases/2008/13/

Synergy Between Webb and Hubble

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M. S tiavelli, mstiavel@stsci.edu, and H.S. S tock man resolution spectra dow n to the V band, but the per for mance below 1.6 m i c r o n i s n o t g u a r a n t e e d b y s p e c i f i c m i s s i o n r e q u i r e m e n t s . N e v e r t h e l e s s, we expect Webb to be signi f icantly more sensitive than Hubble in the overlapping wavelengths bet ween 0.6 and 1.7 microns. Webb is capable o f m u l t i - o b j e c t s p e c t r o s c o p y a t s p e c t r a l r e s o l v i n g p o w e r s u p t o 3 0 0 0. Webb also has a complement of coronagraphs for high- contrast imaging. Meanwhile, Hubble will remain unique amongst space obser vatories for visible and ultraviolet imaging ( W FC 3/ACS) and spectroscopy (COS/ S T IS). Hence, most Webb science will not repeat Hubble science, but ex tend and complement it in the in f rared. W hat Hubble data might be needed to enable Webb science for broad science areas? High-redshift galaxies Webb will identi f y candidates for high-redshi f t galaxies us band f ilters to identi f y spectral features due to the Ly man Ly man- alpha forest. Unfor tunately, this technique is subjec detections of low-redshi f t interlopers (e.g., a B almer break at low redshi f t). Deep visible imaging dow n to t he B b and and p er h aps in to t he ul t r av io le t c an ing bro ad bre ak and t to f als e
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ubble will have restored and enhanced science capabilities af ter Ser vicing Mission 4 (SM4), planned for late summer 2 0 0 8. C u r r e n t S M 4 p l ans c all f or t h e r e p air o f t h e Sp a c e Telescope Imaging Spectrograph (S T IS) and the Advanced C amera for Sur veys (ACS), and the installation of t wo new instruments: W ide F ield C amera 3 ( W FC 3) and the Cosmic Origins Spectrograph (COS). W FC 3 will of fer a signi f icantly wider f ield in t he ne ar in f r ared t h an t he Ne ar In f r ared C amer a and M ul t i - Obje c t Spectrometer (NICMOS), and COS will be more sensitive in the ultraviolet than S T IS. T hese instrument changes and the other Hubble repairs will s u s t a i n k e y o b s e r v a t o r y c a p a b i l i t i e s f o r t h e n e x t f i v e y e a r s, u p t o t h e planned launch of the James Webb Space Telescope. Never theless, even though the Hubble and Webb science missions may overlap, astronomers would be wise to assume that the overlap will be brief, and they should consider the implications for Webb science programs. T he Webb telescope is optimized for operations in the near- and midin f rared, and for imaging and low- to medium-resolution spectroscopy. Its n o m i n a l w a v e l e n g t h r a n g e is b e t w e e n 1 a n d 2 8 m i cr o ns. T h e t e l e s c o p e is di f f r ac tion limi ted at w aveleng ths longer than 2 microns. Webb instruments are capable of tak ing high- angular resolution images and low-

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Synergy from page 1

r ule out such interlopers, but these wavelengths are not accessible to Webb. T herefore, Webb searches for high-redshi f t galaxies must take place in areas of the sk y where Hubble data are a l r e a d y a v a i l a b l e . (G r o u n d - b a s e d s u r v e y s c a n n o t r e a c h t h e n e c e s s a r y d e e p m a g n i t u d e s , A B ~ 30 ­ 31.) Several deep Hubble f ields already exist, and these will be prime candidate f ields for deep Webb obser vations. T hey may be su f f icient, but they entail t wo potential issues. F irst, we expect g a l a x i e s a t h i g h r e d s h i f t ( z > 7 ) t o b e r a r e, b u t w e h a v e y e t t o l e a r n h o w r a r e . T h e p r o b l e m o f eliminating low-redshi f t interlopers becomes more impor tant i f the likely number of bona fide sources is small. Similarly, it may be necessar y to have larger f ields than the Hubble visible and B -band images cur rently available. Second, Webb searches for high-redshi f t super novas could be most ef f icient in the continuous viewing zones (C V Z s), which can be obser ved at all times. T he Webb C V Z s lie within 5 degrees of the ecliptic poles. T he most impor tant area for ex tragalactic applications is the nor ther n C V Z, which does not contain any of Hubble deep ex tragalactic f ields. T hus, unless additional Hubble data are taken in the nor ther n C V Z, the deep f ields f rom Webb that come as byproducts of super nova searches will not be f ully usef ul for high-redshi f t galax y studies. To ensure a foundation for Webb high-redshi f t galax y and cosmology programs by resolving these issues would require additional Hubble obser vations. Nearby galaxies A complete census of nearby galaxies of all major mor phological t ypes requires obser ving gala xies up to distances of 10 ­20 Mpc. Un for tunately, Hubble data are complete only for objects within approximately 4 Mpc. At larger distances, Hubble has obser ved many galaxies, but the data s e t i s m i x e d w i t h r e s p e c t t o f i l t e r s, e x p o s u r e t i m e s, a n d s e l e c t i o n c r i t e r i a . M o s t o f t h e e x i s t i n g

Figure 1. Comparison of the angular resolution of Hubble and Webb in the near infrared. The left image has been obtained with Hubble's Near Infrared Camera and Multi-Object Spectrometer as part of the Hubble Treasury program on the Ultra Deep Field (PI: R. Thompson). The right image is a simulation of the same field as seen by Webb. The image was produced by deconvolving the NICMOS image using 40 iterations of the Richardson-Lucy algorithm and then convolving it with the expected Webb point- spread function. The spectral bands are F110W and F160W for NICMOS and F110W and F150W for Webb's Near Infrared Camera.

d a t a o n n e a r b y g a l a x i e s w e r e o b t a i n e d i n s n a p s h o t s u r v e y s, w i t h l i m i t e d a v a i l a b l e u l t r a v i o l e t o r B -band data. Un for tunately, some of the standard f ilters combinations used to disentangle age, metallicit y, and dust content also require imaging at wavelengths below the V band. T hus, unless alter native diagnostics are developed in the inf rared, astronomers will need Hubble data in these bands to suppor t detailed study by Webb. A n alter native diagnostic approach could be to rely more ex tensively on spectroscopy by exploiting the multi-object capabilities of Webb. T he study of objects i n t h e L o c a l G r o u p t h a t c a n b e r e s o l v e d i n t o s t a r s f a c e s s i m i l a r d i f f i c u l t i e s, a s m a n y o f t h e e x i s t i n g d e e p f i e l ds d o n o t i n c l u d e u l t r a v i o l e t o r B - b a n d i m a g e s. T h e L a r g e M a g e l l a n i c C l o u d l i e s l a r g e l y w i t h i n the Webb southern CV Z, and it is likely to be the object of detailed population studies. Once again, any required ultraviolet or B-band images should be obtained beforehand with Hubble. Galactic objects M a n y d a t as e t s ex is t i n t h e planetar y nebulae, super nova r e quir ing n ar row - b and im ages near-in f rared lines for the usua Hubble arc remnants, to ex am in e l v isi b l e l i n e h i v e f o r a w i d e a s s o r t m e n t o f g a l a c t i c o b j e c t s, globular clusters, and star-for ming regions. For s p e c i f i c e m i s s i o n l i n e s, i t m a y b e n e c e s s a r y t o s u s , s i n c e t h e s e t o f v i s i b l e, n a r r o w - b a n d f i l t e r s a v a su ch as studies bs t i t u te il ab le to

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Webb is quite limited. As for all other areas of science, availabilit y of blue or shor ter wavelengths might become a limiting f actor for some Webb science projects, unless those data are obtained by Hubble. Webb also lacks the astrometric and polarimetric capabilities of Hubble, and Webb obser vations requiring such data must rely on prior Hubble obser vations. QSO absorption systems Due to its limited spectral resolution, Webb is incapable of studying in detail the Ly man- alpha forest and the metal absorbers seen in the spectra of Q SOs. However, the systematic study of f e e d b a c k a n d t h e i n t e r p l a y b e t w e e n g as a n d s t a r s i n g a l a x i e s r e q u i r e s a s t a t is t i c a l s t u d y o f g a l a x i e s in f ields w here Q SO spectra have been measured. Hubble provides unique data on Q SO absor ption systems at z < 3. Webb will be capable of studying the proper ties of the galaxies associated with those absorbers. T he combination of these complementar y measurements by Hubble and Webb will p r o v i d e e s s e n t i a l c o n s t r a i n t s t o s t u d y h o w t h e I G M is e n r i c h e d b y g a l a x i e s . In summar y, Webb science will rely on Hubble data primarily for improving the photometric selection of f aint gala xies for the study of galactic evolution and cosmology. In other f ields, Webb science goals will be complementar y to those of Hubble. Regardless, astronomers should consider the needs to obtain adequate, homogenous Hubble data sets in the years just ahead. W

AC S Report

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R . Gilliland, gillil@stsci.edu

h e A d v a n c e d C a m e r a f o r S u r v e y s ( A C S) c o n t i n u e s t o o p e r a t e w i t h t h e s o l a r b l i n d c h a n n e l (SBC) receiving some 12 % of the Cycle 16 obser ving time on Hubble. ACS repair (ACS-R) of the wide -f ield camera ( W FC) and the high-resolution camera (HRC) are planned for Ser vicing Mission 4 later this year. T he ACS-R development is proceeding well and is on t r a ck . T h e A C S t e a m h as su c c e s s f u l l y d e m o ns t r a t e d t h a t t h e A C S - R d e sig n c a n d e l i v e r the expected per for mance f rom the charge coupled devices (CCDs), and is integrating and testing a f ully f light-representative engineering model. Fabrication of the f light hardware has star ted. In parallel, team members are work ing on the hardware car rier, sof t ware, operational aspects, and planning tests and ex tra-vehicular activities, among other areas critical to ACS-R success. So f ar, so good! P reparations for the Ser vicing Mission Orbital Veri f ication (SMOV ) period for the repaired ACS are well under way. C alibration programs executed as par t of SMOV for the ACS will establish the basic characteristics of the instr ument on orbit. In conjunction with early calibrations in Cycle 17, w e e x p e c t t o r e s t o r e k n o w l e d g e o f i n s t r u m e n t c h a r a c t e r i s t i c s -- l i k e r e a d o u t n o i s e, f l a t f i e l d s , sensitivit y, and charge trans fer ef f iciency (C T E) -- and calibrations to a level comparable to that before loss of the CCD -based channels of the ACS on Januar y 27, 20 07. We expect to f ully suppor t Cycle 17 science obser vations with the ACS. A l ar g e n u m b e r o f ch ar a c te r i z a t io n an d c alib r a t io n a c t i v i t ie s h a v e b e e n c o m p l e te d si n c e t h e previous Newslet ter ar ticle and detailed in cor responding instr ument science repor ts (ISRs). M . C h i a b e r g e a n d M . Si r i a n n i p r o v i d e d u p d a t e d m e asu r e m e n t s o f r e d l e a k s f o r u l t r a v i o l e t a n d nar row-band f ilters for the ACS CCDs (ACS ISR 07- 03). V. Kozhurina-P latais, P. Goud f rooij, and T. P uzia discussed di f ferential C T E cor rections for both photometr y and astrometr y for non- drizzled W FC images in ACS ISR 07- 0 4. A contribution f rom the communit y by K. Collins et al. (ACS ISR 07- 05) repor ted the detection and characteristics of a new ghost in F122M used with the SBC. R . B o h l i n p r o v i d e d a n e w a b s o l u t e p h o to m e t r i c c a l i b r a t i o n o f t h e A C S C C D c a m e r as i n A C S IS R 07- 06 based on obser vations of spectrophotometric standard stars and comparison to sy nthetic photometr y. In a program anticipating Webb astrometric calibration needs, R. van der Marel et al. (ACS ISR 07- 07) ref ined the absolute W FC scale and rotation. In ACS ISR 07- 08, J. A nderson e s t a b l is h e d s m a l l t e m p o r a l c h a n g e s i n t h e l i n e a r s ke w t e r m s o f t h e W F C si n c e i t s i n s t a l l a t i o n i n 2 0 0 2 , a n d i n c o r p o r a t i o n o f t h is e f f e c t i n p i p e l i n e Continued processing is under way. J. MaÌz A ppellÀniz described preparator y work (ACS page 4 ISR 07- 09) establishing astrometric f ields in NGC 60 4 and NGC 6681, which

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ACS from page 3

w i l l b e us e d t o r e v is e t h e g e o m e t r i c d is t o r t i o n c a l i b r a t i o n o f t h e S B C. M . C r a cr a f t a n d B . S p a r k s deter mined the polarization calibration of the ACS to 1% levels and discussed weighting schemes for use with MULT IDRIZ ZL E in ACS ISR 07-10. In ACS ISR 07-11, A . Fr uchter established wavelength calibration checks for the ramp f ilters. In ACS ISR 07-12, K. Sahu derived cor relations of the widths of the W FC point-spread f unction and optical telescope assembly temperatures. In ACS ISR 08 - 01, H. Kuntschner, M. Kummel and J. Walsh provided an updated f lu x calibration for the W FC G800L g r ism, a n d e s t a b l is h e d l i m i t s t o t h e i m p o r t a n c e o f f r i n g i n g. T he ACS-W FP C2 team (AW T ) continues to work on a large number of calibration updates for both instruments, while working to prepare for utilization of a repaired ACS. W For f ur ther information v isi t: ACS Web page: h t tp: // w w w.st sci.edu / hst /acs ACS Bulletin Board: h t tp: // forums.st sci.edu /phpbb / v iew forum.php? f=14 W F P C2 Web page: h t tp: // w w w.st sci.edu / hst / w f pc2 W F P C2 Bulletin Board: h t tp: // forums.st sci.edu /phpbb / v iew forum.php? f=20 F o r a n y q u e s t i o n s a b o u t t h e A C S o r t h e W i d e F i e l d P l a n e t a r y C a m e r a 2 ( W F P C 2) p l e a s e c o n s u l t t h e bulletin boards or send e -mail to help@stsci.edu.

N IC MOS: Improving Science by Removing Bright- Earth Persistence

N IC MOS: Improving Science by Removing Bright- Earth Persistence
A . Koekemoer, koekemoer@stsci.edu and A . Riess, ariess@stsci.edu

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he detectors in the Near-Inf rared C amera and Multi- Object Spectrometer (NICMOS) are susceptible to an ef fect k now n as "persistence," which can occur when the f lu x incident o n t h e d e t e c t o r is b r i g h t e n o u g h t o p r o d u c e s a t u r a t i o n . T h e c h a r g e t h a t a c c u m u l a t e s i n t h e saturated pixels is mani fested as a decaying, "persistent " signal for timescales up to a few hours. T h i s p e r s i s t e n t s i g n a l c a n s i g n i f i c a n t l y d e g r a d e t h e s c i e n t i f i c q u a l i t y o f s u b s e q u e n t e x p o s u r e s, s i n c e i t p r o d u c e s a n e l e v a t e d b a c kg r o u n d f l u x , w h i c h c h a n g e s w i t h t i m e a n d c a n b e s u b s t a n t i a l l y b r i g h t e r than f aint sources of interest. A s p e ci f i c m a n i f e s t a t i o n o f p e r sis t e n c e o c c u r s w h e n N I C M O S is e x p o s e d t o e m is si o n f r o m t h e bright Ear th, which can happen w hen NICMOS obser ves in parallel with the Advanced C amera for Sur veys (ACS). In this case, the exposure to the bright Ear th occurs at the end of the orbit, while the ACS detectors are being read out. Since NICMOS has no shut ter, its detectors are illuminated with bright-Ear th emission for up to several minutes, and the decaying signal is visible as large scale background str ucture in NICMOS exposures obtained during subsequent orbits. T he problem is illustrated in F igure 1a, and documented more f ully in NICMOS ISR 20 08 - 01 (A . Riess and L . E. Bergeron, h t tp: // w w w.st sci.edu / hst /nicmos /document s / isr s / isr_ 20 08_01.pdf/ ). T his ef fect was noticed in a number of exposures in the Cycle 15 "SHOES" program (P rogram ID: 10802; P I: A . Riess). O ther Hubble programs with parallel NICMOS exposures can be similarly af fected. A s o f t w a r e a l g o r i t h m t o ch a r a c t e r i ze a n d r e m o v e t h is p e r sis t e n c e f r o m a f f e c t e d N I C M O S exposures has now been developed and tested by the NICMOS Team at the Institute. We are now mak ing it available to the general communit y. T he sof t ware uses existing infor mation about the c h a r g e - t r a p p r o p e r t i e s f o r a l l t h e p i x e l s i n t h e N I C M O S d e t e c t o r s , m o d e l s t h e d e c a y i n g b a c kg r o u n d , a n d r e m o v e s i t f r o m e a c h e x p o s u r e. I n i t i a l t e s t s s h o w t h a t i t c a n r e m o v e u p t o 9 9. 5 % o f t h e variance due to this background, as illustrated in F igure 1b. Accurate removal of persistence substantially improves the scienti f ic qualit y of NICMOS exposures, since f ainter sources can be reached and photometric accuracy is increased. Initially, we are mak ing the sof t ware available within P Y R AF to enable obser vers to run it of f-line on their


Figure 1. (a) An example of the impact of decaying, persistent bright-Earth emission in a single NICMOS exposure, in this case from Hubble Program 10258, visit 9, obtained using the NIC2 camera. (b) The same exposure after applying the correction software to model and remove the low-level residual emission. It is clear that the quality of the exposure is substantially improved, with the background now being essentially flat.

ow n data. We are also investigating w hether it may be automatically incor porated into the Hubble N I C M O S a r c h i v a l p i p e l i n e, w h i c h w i l l r e s u l t i n a n o v e r a l l i m p r o v e m e n t o f t h e l o n g - t e r m s c i e n t i f i c l e g a c y v alu e o f ar ch i v al N I C M O S d a t a. W For fur ther information, please v isi t: NICMOS Web P age: h t tp: // w w w.s t sci.edu / hst /nicmos / NICMOS Bulletin Board: h t tp: // forums.s t sci.edu /phpbb / v iew forum.php? f=13 F o r a n y q u e s t i o n s a b o u t N I C M O S p l e a s e f e e l f r e e t o p o s t a m e s s a g e o n t h e N I C M O S B u l l e t i n B o a r d, or send an email message to help@stsci.edu.

STIS Update
Charles Prof fit t , prof fit t@stsci.edu

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n August 3, 20 0 4, the Space Telescope Imaging Spectrograph (S T IS) was rendered inoperable w hen a component f ailed on a circuit board in a low-voltage power supply. T his f a i l u r e m a d e i t i m p o s s i b l e f o r S T I S t o m o v e i t s m e c h a n i s m s, a n d n o l i g h t c o u l d r e a c h t h e detectors. At the time, S T IS was already operating with the redundant, side -2 electronics because the primar y, side -1 electronics had f ailed previously, in 2001. F o l l o w i n g t h e 2 0 0 4 i n ci d e n t , N A S A d e v is e d a p l a n t o r e s t o r e S T IS t o o p e r a t i o n a l s t a t us b y replacing the f ailed circuit board during the upcoming Ser vicing Mission 4 (SM4). T he repair w i l l r e q u i r e r e m o v i n g t h e c o v e r o f a n e l e c t r o n i c s b ox , e x t r a c t i n g t h e f a i l e d b o a r d, i n s e r t i n g t h e replacement board, and installing a replacement cover. Initially, plans also included a new radiator to c o o l t h e S T IS m u l t i - an o d e m u l t i ch an n e l ar r ay (M A M A ) d e te c to r s by s e v e r al d e g r e e s to r e d u c e dark cur rent. N ASA dropped this "S T IS Cooling System," however, to devote more time and resources to higher priorit y activities, including an at tempt to restore the charge coupled device (CCD) detectors of the Advanced C amera for Sur veys. T he replacement circuit board for S T IS -- as well as the replacement cover for the electronics box and the tools the astronauts will use to per for m the repair-- have been Continued m a n u f a c t u r e d a n d t e s t e d . We c u r r e n t l y a n t i c i p a t e t h a t t h e r e p a i r o f S T I S page 6 w i l l o c c u r d u r i n g t h e 4t h e x t r a - v e h i c u l a r a c t i v i t y o f S M 4 . A f t e r t h a t , b r i e f

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f u n c t io n al an d ali v e n e s s te s t s w ill v e r i f y t h e su c c e s s o f t h e repair. Testing the actual scienti f ic capabilities of S T IS will begin af ter Hubble is released f rom the shut tle Discover y. We e x p e c t t h a t t h e r e p a i r e d S T I S w i l l h a v e c a p a b i l i t i e s s i m i l a r t o t h o s e p r i o r t o t h e f a i l u r e. N e v e r t h e l e s s, a c c u m u l a t e d r adi a t io n d am ag e to t h e d e te c tor s an d hig h e r te m p e r a t u r e s in the af t-shroud of Hubble will probably elevate dark- cur rent l e v e l s, a s w e l l a s i n c r e a s e t h e n u m b e r o f h o t p i x e l s a n d r e d u c e t h e c h a r g e -t r a n s f e r e f f i c i e n c y f o r t h e S T I S C C D detector. E x trapolating the previous trend, we expect the m e d i a n d a r k cu r r e n t i n t h e C C D w i l l h a v e i n cr e as e d f r o m a v a l u e o f 0 . 0 0 4 4 e ­ /s i n 2 0 0 4 t o a b o u t 0 . 0 0 9 e ­ /s f o r C y c l e 17, w hich may signi f icantly af fect users obser ving targets ne ar t he f ain t limi t. For the near-ultraviolet M A M A , the dark cur rent is mostly due to a phosphorescent window glow, and changes to this glow should b e m o des t. Figure 1. The average of 125 1380- sec FUV MAMA T he f ar-ultraviolet (F U V ) M A M A dark cur rent has a complex dark exposures taken between May 2003 and August and poorly understood behavior, (see S T IS ISR 2007- 002 for 2004. The upper green line marks the standard aperture more details). It star ts out near 6 x 10 ­6 counts /s /pixel when location, while the lower green line shows the "D1" f i r s t t u r n e d o n, b u t t h e g l o w -- w h i c h c o v e r s m u c h o f t h e aperture location, which is positioned below the region detector-- increases with the amount of time the detector is of highest dark current. t u r n e d o n . We p r e d i c t t h a t a f t e r S M 4 t h is g l o w w i l l i n c r e as e t w i c e as q u i c k l y as i t h a d p r i o r t o t h e 2 0 0 4 f a i l u r e. P r o g r a m s obser ving f aint, compact targets with the FU V M A M A may wish to use the D1 aper ture positions d e f in e d to p l a c e t h e t ar g e t in a r e gio n o f su b s t an t i all y l o w e r d ar k cu r r e n t . O u r b e s t e s t im a te s f or t h e ex p e c te d d ar k cu r r e n t ar e i n clu d e d i n t h e cu r r e n t v e r sio ns o f t h e S T IS ex p o su r e t i m e c al cu l a to r s and are documented in the STIS Instrument Handbook. T he image show n in F igure 1 shows the average of 125 1380 -sec FU V M A M A dark exposures t a ke n b e t w e e n M a y 2 0 0 3 a n d A u g us t 2 0 0 4. T h e u p p e r g r e e n l i n e m a r k s t h e s t a n d a r d a p e r t u r e location, w hile the lower green line shows the "D1" aper ture location, which is positioned below t h e r e g i o n o f h i g h e s t d a r k c u r r e n t . F i g u r e 2 s h o w s t h e d i f f e r e n c e t h i s w o u l d m a k e t o t h e b a c kg r o u n d seen in a t y pical point source spectral ex traction; the dot ted line shows the background count rate d u e t o t h e d e t e c t o r d a r k c u r r e n t f o r a s p e c t r u m t a ke n a t t h e s t a n d a r d p o s i t i o n, w h i l e t h e s o l i d l i n e shows the reduced count rate that would be seen at the "D1" position. We have designed post-repair activities for S T IS to deter mine i f the capabilities of the instrument are close enough to those expected that general- obser ver science programs can proceed. To retur n S T I S t o s c i e n c e o p e r a t i o n s a s q u i c k l y a s p o s s i b l e, many of the more time - consuming calibrations -- 1x10 ­ 4 s u c h a s d e t a i l e d, h i g h s i g n a l -t o - n o is e f l a t f i e l d s a n d d e t a i l e d s e nsi t i v i t y m e asu r e m e n t s f o r e v e r y c e n t r a l 8 x10 ­ 5 w av e l e n g t h se t t in g -- w ill b e d e f e r r e d. T h e S T IS p ip e li n e an d c alib r a t io n r e f e r e n c e f il e s 6 x10 ­ 5 ar e r e a d y to su p p o r t d a t a p r o c e s si n g w i t h o u r b e s t estimates for the Cycle 17 calibration. Improved 4 x10 ­ 5 r e f e r e n c e f i l e s, b a s e d u p o n o n - o r b i t m e a s u r e m e n t s af ter the repair, will be delivered as quick ly as 2 x10 ­ 5 p o s s i b l e. N e v e r t h e l e s s, u s e r s s h o u l d b e a w a r e t h a t 0 t h e i n i t i al p r o c e s si n g o f d a t a p r o d u c t s i m m e d i a te l y 0 200 400 600 800 1000 a f ter t he S T IS r ep air w ill use t he pr elimin ar y Column c a l i b r a t i o n f i l e s, a n d t h a t a f e w m o n t h s m a y e l a p s e before f inal calibrations are available. Figure 2. The difference made to the background seen in a typical point Because we expect the calibration of the older, source spectral extraction. The dotted line shows the background count rate p r e - r e p a i r S T I S d a t a t o b e r e l a t i v e l y s t a b l e, t h e due to the detector dark current for a spectrum taken at the standard position, "on-the -f ly" re - calibration of S T IS data has been while the solid line shows the reduced count rate that would be seen at the tur ned of f, in f avor of a static archive. T his change "D1" position. all o w s us e r s to r e t r ie v e S T IS d a t a m u ch m o r e q u i ck l y t h a n w as p r e v i o u s l y p o s s i b l e. T h is s t a t i c a r c h i v e o f S T IS data is also being incor porated into the Hubble Legacy A rchive (HL A). T he HL A provides a n u m b e r o f n e w c a p a b i l i t i e s f o r a r c h i v a l r e s e a r c h, i n c l u d i n g a d v a n c e d b r o w s i n g a n d t h e a b i l i t y t o v ie w t h e f o o t p r i n t s o f S T IS ap e r t u r e s ov e r l aid o n i m ag e s f r o m sk y su r v e y s. Su ch n e w w ay s o f a c c e s si n g t h e l ar g e ar ch i v e o f c alib r a te d S T IS d a t a sh o u l d p r ov id e ab u n d an t o p p o r t u n i t ie s f o r n e w archival research. W
STIS from page 5

Average dark rate in 7-pixel-high box (counts/pixel/s)

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Improving Hubble's Pointing and Astrometry
M. L allo, lallo@stsci.edu

Improving Hubble's Pointing and Astrometry

A

b s o l u t e as t r o m e t r i c a c c u r a c y is i m p o r t a n t i n t h e e r a o f m u l t i m is si o n a r ch i v e s a n d campaigns to cross-match objects obser ved at di f ferent wavelengths by various missions. For example, an astronomer might wish to study Hubble images of the optical counter par t of an X-ray source found by Chandra, or of an in f rared source found by Spit zer. A lso, the accuracy of Hubble's pointing will be operationally cr ucial af ter the installation, later this year, of the Cosmic Origins Spectrograph (COS), w hich has a tiny aper ture --1.25 arcseconds in radius. T h e a c cu r a c y w i t h w h i ch a m ar k sm an c an h i t a t ar g e t d e p e n ds o n t h e c alib r a t io n o f t h e gunsight. L ikewise, Hubble's astronomical "shar p shooting" relies on an accurate calibration of its F ine Guidance Sensors (FGSs). To point the telescope, the FGSs lock onto selected guide stars f alling into their large, banana-shaped f ields of view. T he goal of such calibrations is to accurately compute the astrometric position of any point within the f ield- of-view of a science instr ument, g i v e n t h e m e a s u r e d l o c a t i o n s o f t h e s e l e c t e d g u i d e s t a r s, w h o s e p o s i t i o n s a r e t a b u l a t e d i n G u i d e Star C atalog 2 (GSC2). I n J a n u a r y 2 0 0 8, w e i n s t a l l e d a n i m p r o v e d F G S c a l i b r a t i o n . A s a r e s u l t , t h e p o i n t i n g s y s t e m n o w places a target closer to its intended position in a science instr ument-- nearer a speci f ied pixel in o n e o f i t s c a m e r a s , f o r e x a m p l e , o r n e a r e r t h e c e n t e r o f a s m a l l a p e r t u r e i n a s p e c t r o g r a p h . A l s o, w e n o w ar ch i v e t h e s cie n c e d a t a w i t h m o r e a c cu r a te i n f o r m a t io n o n t h e l o c a t i o n a n d o r i e n t a t i o n o n t h e s k y a t w h i c h i t w as t a ke n . Obser vations made earlier in Hubble's mission can have their Median Pointing Offset for FGS2R in V2 & V3 astrometr y improved retroactively, and the staf f of the Legacy A rchive has recently done this for some existing Hubble images 1.5 by registering f ield objects to sources in GSC2 (see Institute Newslet ter Vol. 24, Summer 20 07). 1.0 For much of Hubble's mission, its 1-sigma pointing er ror was 1­2 arcseconds. Our aim now is for cur rent and f uture Hubble 0.5 us e r s to e n j o y su b s t a n t i a l l y i m p r o v e d p o i n t i n g a n d as t r o m e t r y g o o d t o ~0. 2 a r c s e c o n d -- a p p r ox i m a t e l y t h e a c c u r a c y o f t h e 0.0 G S C 2. T h is is a ch i e v e d b y c a l i b r a t i n g t h e a l ig n m e n t o f b o t h t h e FGSs and the science instruments (SIs) f requently enough to m a i n t a i n t h e as s o c i a t e d e r r o r s b e l o w t h e 0. 2 a r c s e c o n d l e v e l. ­0.5 T h is p o i n t i n g p e r f o r m a n c e w i l l a ls o a l l o w us t o r o u t i n e l y d e l i v e r a t ar g e t dir e c t l y in to t h e C O S ap e r t u r e in t h e l ar g e m ajor i t y o f ­1.0 cases, reducing the need for the SI's more time - consuming search mode.
Offset from Reference Point (arcseconds)

Median Pointing Offset for FGS2R in V2 and V3

Calibrating Hubble's Field of View

­1.5

2000.0

2000.5

2001.0

2001.5

2002.0

2002.5 2003.0

T he domin an t sources o f Hubble's poin t ing er ror s ar e Figure 1. Motion of FGS2R over two years following its installation in 2000. (1) er rors in the operational guide star catalog, (2) incomplete We expect comparable movements of the FGS and the two new SIs once k nowledge of optical distor tions in the FGSs, and (3) uncer taint y they are installed during Hubble's upcoming servicing mission. Corrections of the exact alignments of the FGSs and the SIs. to maintain catalog-limited pointing will require continuing calibrations of the To reduce the catalog's role in pointing er ror, GSC2 was made kind described in this article. the def ault database for guide star operations during Cycle 15, i n s u m m e r 2 0 0 6. G S C 2 is t i e d t o t h e I n t e r n a t i o n a l C e l e s t i a l R e f e r e n c e S y s t e m, w h i ch h as b e e n a d o p t e d b y o t h e r m o d e r n c a t a l o gs. T h e a b s o l u t e e r r o r i n a star 's position in GSC2 is ~0.25 arcsecond, and the relative er ror over an area of sk y the size of Hubble's focal plane is ~0.18 arcsecond. T hese er rors are about a f actor of three smaller than the cor responding er rors previously encountered with Guide Star C atalog 1. To ensure the f ull scienti f ic benef it of the increased accuracy of GSC2, we needed to recalibrate t h e a l ig n m e n t s o f t h e F G S s a n d SIs w i t h a c cu r a c y g r e a t e r t h a n t h a t o f G S C 2. T h e a c t u a l l o c a t i o n of a science instr ument can var y up to ~0.5 arcsecond, and its orientation by up to ~0.1° -- and the FGSs can exhibit changes f ive or more times greater. Fur ther more, the FGSs show signi f icant c h a n g e s o f s c a l e. W h i l e a l l t h e s e v a r i a t i o n s t e n d t o g e t s m a l l e r as t i m e Continued p a s s e s, w e m u s t c o n t i n u e t o c h a r a c t e r i z e t h e m , e v e n a f t e r y e a r s o n o r b i t , i n page 8 order to maintain alignment er rors at the desired level (see F igure 1).

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Pointing from page 7

T he most recent alignment calibration was based on data f rom calibration proposal 11021. C o m b i n a t io ns o f t ar g e t s t ar s an d g u id e s t ar s w e r e o b s e r v e d a t v ar io us l o c a t io ns a cr o s s t h e f ie l ds of-view of the FGSs and SIs (see F igure 2). We used the open cluster M35, one of our regular calibration targets. M35 is well matched to Hubble's f ield of view of ~0.5° in diameter, and contains a number of bright stars suitable for precise measurement and good sampling of the instr uments' f ields of view. Our analysis used star positions f rom the U.S. Naval Obser vator y's CCD Astrograph C atalog 2 (UCAC2), with astrometric er rors of ~0.02 arcsecond, well below those found in GSC2. T h e p r e cis e l y k n o w n p o si t i o n s o n t h e sk y o f t h e s e s t a r s a r e us e d t o g e t h e r w i t h t h e i r m e asu r e d locations in the FGSs and SIs to deter mine the instr uments' alignments in Hubble's f ield of view. Together with our colleagues at the Goddard Space Flight Center, we found and quanti f ied the r e c e n t c h a n g e s t o t h e a s s u m e d l o c a t i o n s, o r i e n t a t i o n s, a n d m a g n i f i c a t i o n s o f t h e F G S s . T h e s e s c a l e an d alig n m e n t ch an g e s c o m b in e d to in t r o du c e p o in t in g e r r or s o f u p to h al f an ar c se c o n d a t t r ib u t ab l e t o e a c h F G S. T h e S I s w e r e a l s o m e a s u r e d, b u t n o u p d a t e s w e r e m a d e a s t w o o f t h e t h r e e c h a n n e l s on the Advanced C amera for Sur veys became unavailable, and the W ide F ield P lanetar y C amera-2 and Near-In f rared C amera and Multi- Object Spectrometer showed minimal evolution. We will be amassing statistics to f ur ther assess the benef it of this work, but the data examined t h us f a r i n d i c a t e s w e h a v e i m p r o v e d t h e p o i n t i n g e r r o r i n d u c e d b y t h e F G S s b y a t l e as t a f a c to r o f 3, meeting our goal of ~0.2 arcsecond. I f the positions of the SIs and FGSs af ter Hubble Ser vicing M is s i o n 4 a r e c a l i b r a t e d a n d m a i n t a i n e d a c c o r d i n g t o o u r p l a n, t h e n w e c a n r o u t i n e l y e x p e c t t o associate this value with Hubble's total obser ved pointing and astrometr y er ror for the remainder of its mission. T his represents a f actor of 5 to 10 improvement compared with the ~1­2 arcseconds, 1-sigma er ror t y pically seen over the mission's histor y. W

Figure 2. The alignment strategy uses combinations of guide stars and target stars with well-known positions to map out the fields of the instruments. In this example, FGS2 observes target stars (green) while the multiple guide star pairs (red) span FGS1 and FGS3. This process is repeated for each of the three FGSs. Once the FGSs are mutually aligned, the positions of the SIs are determined by comparing target- star locations in the camera to the associated guide stars. Alignment of the ACS Wide Field camera is shown here.

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Figure 1. Fully assembled WFC3 being transported for fit check.

WFC3 Status
John W. MacKen t y, mackent y@stsci.edu

T

he W ide F ield C amera 3 ( W FC 3) is f ully assembled with its f light detectors and is undergoing f inal checkouts and calibrations prior to transpor t to Kennedy Space Center. Januar y 20 08 saw the electromagnetic compatibilit y and inter ference tests, followed by the Ser vicing Mission Ground Test in Febr uar y. T hese tests validate W FC 3's compatibilit y with Hubble, and check our abilit y to operate it following installation. A lso, f it- checks were per for med bet ween W FC 3 and t h e H ig h F id e li t y M e ch an i c al Si m u l a to r an d t h e W id e F ie l d S cie n c e I ns t r u m e n t P r o te c t i v e E n cl o su r e (the " box" that protects W FC 3 in the shut tle cargo bay). Meanwhile, the Ser vicing Mission 4 astronauts have been f amiliarizing themselves with W FC 3. A n exhausting, 127- day r un of ther mal vacuum ( T V ) testing in summer 20 07 tested W FC 3 ex tensively, and validated most subsystems. Issues were discovered with the visible -light calibration lamps, and with the ther mal control circuitr y for the cooler of the in f rared detector. New lamps were procured that cor rected a design f law, and they have completed li fe testing and are now installed in the instr ument. The ther mal control circuitr y was modi f ied and recent testing has validated its per for mance. Both the ultraviolet-visible and in f rared detector assemblies have been replaced with much superior f light detectors. T he ultraviolet-visible focal plane contains the most sensitive ultraviolet CCD ever manu f actured. Fur ther, the packaging of the focal plane has been Continued e x t e n si v e l y r e w o r ke d t o m i t i g a t e s e v e r a l d e f i ci e n ci e s d is c o v e r e d d u r i n g page 10 t e s t i n g. T h e i n f r a r e d d e t e c to r as s e m b l y n o w c o n t a i ns t h e f i n e s t i n f r a r e d

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WFC3 from page 9

detector produced by the Hubble program. It removes susceptibilit y to proton-induced glow an d o f f e r s ab o u t t w i c e t h e s cie n c e p e r f o r m an c e o f t h e d e te c to r s a v ail ab l e i n 2 0 0 4. Q u an t u m ef f iciency at the shor ter inf rared wavelengths (850 ­14 00 nm) is greatly improved, with essentially f lat response over the entire band. Dark cur rent is also well below the Hubble and zodiacal light b a c kg r o u n d i n t h e b r o a d b a n d f i l t e r s . A f i n a l T V t e s t i n F e b r u a r y ­ A p r i l 2 0 0 8 h as o b t a i n e d t h e f l ig h t s ci e n c e c a l i b r a t i o ns a n d c o m p l e t e d the optical, ther mal, and operations validation of W FC 3 for f light. Obser ver suppor t activities for W FC 3 are ramping up. T he Instrument Handbook was released as par t of the Cycle 17 Call for Proposals. W FC 3 now has A P T suppor t, an exposure time calculator, and, thanks to our colleagues at the S T-ECF, a grism simulator for spectroscopic obser vations. T he W FC 3 data pipeline is included in the upcoming S T SDAS release. W

Figure 2. Final inspection of the interior of the WFC3 by the project management team (l-r: John MacKenty, Randy Kimble, Jackie Townsend).

Figure 3. STS -125 (SM4) flight crew and GSFC engineers practicing removal of WFC3 from the Wide Field Science Instrument Protective Enclosure.

Figure 4. WFC3 being lifted from the Wide Field Science Instrument Protective Enclosure.

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COS Status
Tony Keyes, keyes@stsci.edu

P

reparations are proceeding rapidly to install the Cosmic Origins Spectrograph (COS) on Hubble during Ser vicing Mission 4 (SM4), and to conduct science obser vations with COS during Cycle 17. To suppor t the Cycle 17 Call for Proposals (CP) in December 20 07, the Institute issued the f irst COS Instrument Handbook ( IHB) and released the COS E xposure T ime C alculator (E TC) for spectroscopy, imaging, and all t y pes of target acquisition. The Institute is ramping up suppor t for COS users on several fronts. Following the distribution of the CP, the Institute helpdesk fielded many COS questions. At the Januar y A AS meeting in Austin, the Institute's COS instrument scientists presented posters about optimizing TIME-TAG obser vations, bright-object protection, target acquisition, pipeline calibration, and output data formats. COS team members helped distribute the latest version of the instrument information brochure, and they were available continuously at the Institute booth to answer questions from potential COS obser vers. Regarding COS hardware and operations, the Institute developed a suite of programs to test the end-to-end per formance of the ground systems. These tests were run on the instrument at Goddard Space Flight Center (GSFC). Engineers at Ball Aerospace and GSFC have now completed hardware testing and made changes to the flight sof t ware to make instrument turn-on more robust. The COS principal investigator, Dr. James Green of the Universit y of Colorado, leads the Instrument Development Team (IDT). The Institute collaborates with the IDT to craf t early programs for instrument star tup, checkout, and optical alignment, followed by calibration and characterization programs to verif y the scientific per formance of COS. The IDT has delivered the first suite of calibration reference files, which will be used in the calibration pipeline to process data from early obser vations. Updated reference files, based upon on-orbit per formance, will be delivered as COS is bet ter characterized in the ser vicing mission obser vator y verification period immediately af ter SM4 and in the early stages of the Cycle 17 calibration program. Users should be aware that early COS science obser vations will be processed initially using the best-available -- but preliminar y-- calibration information, and that it will take a few months before all on-orbit calibrations are available through the "on-the-fly" recalibration of the Hubble archive. Also in collaboration with the IDT, the Institute has developed a comprehensive plan for pipeline verification. We are developing the COS Data Handbook, which we plan to publish in December 2008, t o s u p p o r t t h e b e g i n n i n g o f C O S s ci e n c e o p e r a t i o n s. The COS IHB, E TC, instrument brochure, and a variet y of additional user-suppor t information are available via the Institute COS instrument website (ht tp: // w w w.st sci.edu / hst /cos) W

COS Optical Design

Figure 1. COS optical design. All external light enters at lower left and follows a common path to first optical element. The subsequent far-ultraviolet path is green, and the near-ultraviolet path is blue. The beam from the internal platinum-neon and deuterium calibration sources is tan. (Drawing courtesy of GSFC.) Continued page 12

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COS from page 11

COS vs. STIS: Far UV Sensitivity
­12.5

STIS E140M +0.2 x 0.2 aper ture R = 45,000

­13.0 ­13.5 ­14.0 ­14.5 ­15.0 ­15.5

COS G130M R~20,000­24,000 COS G160M R~20,000­24,000 1200 1300 1400 1500 1600 1700 1800 1900

1100

Wavelength (å)

Figure 2. Comparison of COS and STIS sensitivities in the far ultraviolet, showing the flux for which S/N = 10 can be achieved in a 3600- sec exposure with uniform binning at resolving power R ~ 20,000 (0.08 å per bin). This assumes the COS Primary Science Aperture and STIS medium resolution (M mode). The COS performance is based upon thermal-vacuum testing, and the STIS on-orbit performance includes aperture losses. Corresponding limits for the COS Bright-Object Aperture are more than 150 times brighter, due to attenuation in that aperture. For the most up-to-date instrument performance characteristics, please refer to the ETCs (http://etc.stsci.edu/webetc/index.jsp).

Testing the M I R I Verification Model

log Flux (erg cm­2 sec­1 å­1)

Testing the M I R I Verification Model
Gillian Wright , gsw@roe.ac.uk, Scot t Friedman, A listair Glasse, Tanya L im, George Rieke, Mike Ressler, and the MIRI team

J

12

ust before Christmas 2007, the veri f ication m o de l ( V M) o f t h e Webb M id In f r ar e d I n s t r u m e n t ( M I R I) s t a r t e d t h e r m a l - v a c u u m testing at the Ruther ford A ppleton L aborator y ( R A L ) i n t h e U n i t e d K i n g d o m . T h e V M is a f unctioning, f light-like version of the instrument, built for a variet y of pur poses, including: (1) demonstrating processes and procedures for manu f acturing, assembly, a n d a l i g n m e n t ; ( 2) d e v e l o p i n g m e t h o d s f o r t e s t i n g a n d calibrating the f light model; and (3) testing end-to - end f unctionalit y, f rom commanding the f light sof t ware t h r o u g h t h e r e c e i p t o f d a t a . We a r e t e s t i n g t h e M I R I V M to b u il d c o n f id e n c e i n t h e d e sig n -- b o t h i n te r ms o f s cie n c e an d o f o p e r a t io ns. W i t h i n t h e li m i t a t io ns o f t h e f id e li t y s t an d ar ds f o r t h e V M, w e w an t c o n f id e n c e t h a t t h e f l i g h t m o d e l ( F M ) o f M I R I w i l l m e e t i t s f u n c t i o n a l, per for mance, and inter f ace requirements. A l t h o u g h t h e V M is a f u l l y f u n c t i o n i n g i n s t r u m e n t , i t dif fers f rom the FM in a number of areas. For instance, t h e m e c h a n is m s a n d f o c a l p l a n e s a r e n o t d e s i g n e d to withstand launch vibrations. Also, only the shor t-wavele most dif ficult one) is populated with optics, and not all of

Figure 1. The MIRI VM, just prior to the installation of the second thermal shroud, is ready for testing in the vacuum chamber at Rutherford Appleton Laboratory.

n g t h ch an n e l o f t h e sp e c t r o m e te r (t h e t h e f il te r s i n t h e i m ag e r ar e i ns t all e d.


N e v e r t h e l e s s, t h e d a t a f r o m V M t e s t i n g a r e o f s u f f i c i e n t q u a l i t y f o r t h e M I R I s c i e n c e t e a m t o b e g i n defining calibration files and algorithms for pipeline data reduction. We began testing the V M with a complete functional check-out at MIRI's nominal 6.8 K operating temperature. Nex t, we tested optical per formance, using a simple ex ternal point source for the imager and the on-board calibration source for the shor t-wavelength channel of the spectrometer. Because MIRI's blanket of multilayer insulation is a critical inter face bet ween the optical system and the cr yo-cooler, we ex tensively investigated its thermal per formance. Finally, we investigated electromagnetic compatibilit y by measuring conducted susceptibilit y and emissions. A l t h o u g h t h e M I R I t e a m h as o n l y j us t b e g u n t o a s s e s s t h e 0. 5 T b y t e o f d a t a o b t a i n e d i n V M t e s t i n g , w e h a v e a l r e a d y l e a r n e d a g r e a t d e a l. A l l i n d i c a t i o n s ar e t h a t w e h a v e an i ns t r u m e n t t h a t w ill o p e r a te w e ll a n d b e c a p a b l e o f g r e a t s c i e n c e. We h a v e s u c c e s s f u l l y d e m o n s t r a t e d t h e e n d -t o - e n d f u n c t i o n a l i t y o f t h e d a t a c h a i n a t c r y o g e n i c t e m p e r a t u r e s, e x e r c i s e d m o s t o f t h e d e t e c t o r f e a t u r e s a n d m o d e s, a n d v e r i f i e d t h a t t h e o p t i c a l p e r f o r m a n c e is as e x p e c t e d f r o m modeling. L a t e r i n 2 0 0 8, w e w i l l c o n d u c t a s e c o n d t h e r m a l v a cu u m t e s t o f t h e M I R I V M w i t h m o r e ex t e nsi v e per formance testing enabled by the MIRI telescope simulator. T his simulator will permit point-source ill u m i n a t io n o f t h e i m ag e r an d sp e c t r o m e te r an y w h e r e in their respective fields of view, and will also allow for adjustment of the source temperature and for flatfield illumination of the imager. W h i l e t h e V M is b e i n g t e s t e d, t h e c o ns t r u c t i o n o f t h e F M is w e ll u n der w ay a t t h e Je t P r op u lsio n L abor ator y and ins ti t u tes in t he MIRI Europe an Figure 2. The MIRI VM medium-resolution spectrometer, illuminated c o n s o r t i u m . I n 2 0 0 9, w e w i l l u s e t h e s a m e f a c i l i t y by the on-board flat-field calibration source. The overall uniformity of a t R A L to tes t and c alibr a te t he F M, and ver i f y t h a t the image indicates that the calibration system performs well. The t h e M IRI m e e t s all i t s r e quir em e n t s an d is r e ad y dispersion direction is vertical, and the spatial direction is horizontal. to f l y. W
Each vertical stripe is the spectrum from one slice of the imageslicer. The stripes are in focus, and their position and curvature are as predicted by modeling. Three slices in the right-hand spectrum are faint, due to the intentional under- sizing of their pupil stops to test the end-to-end optical model. The fringing is typical of mid-infrared detectors operating at a spectral resolution of a few thousand. (Image courtesy MIRI European Consortium and JPL)

Figure 3. A point- source image from the MIRI VM imager. Initial analysis confirms that the optical performance meets the design goals. (Image courtesy MIRI European Consortium and JPL)

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Institute Honors John Bahcall
R . W illiams, wms@stsci.edu

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Above: Neta Bahcall and Bob Williams unveil the auditorium's new name, honoring John Bahcall.

h e n J o h n B a h c a l l p a s s e d a w a y u n e x p e c t e d l y i n A u g u s t 2 0 0 5, a s t r o n o m y l o s t a t r u e leader, and the Hubble and the Institute lost a great f riend. John was one of the most in f luential scientists of his time. He is remembered for many scienti f ic discoveries and professional activities, and especially for his tireless advocacy and stead f ast suppor t-- over long years -- of the Hubble Space Telescope. In the 1970s, John --together with Ly man Spit zer-- was instr umental in convincing N ASA , Congress, and the astronomical communit y that a large optical telescope in space could be both a unique window on the universe and a natural application for N ASA's space shut tle program. He played a key role in developing the strategies and alliances that produced Hubble. Af ter the launch of Hubble in 1990, John remained a staunch suppor ter during the di f f icult time of spherical aber ration -- both before the f irst servicing mission and during subsequent budgetary problems within N ASA that threatened to cur tail Hubble's scienti f ic program. He was a f requent visitor to Washing ton and the halls of Congress to testi f y on the ways in which Hubble was c h a n g i n g o u r u n d e r s t a n d i n g o f t h e u n i v e r s e, a n d o n t h e a m a z i n g i m p a c t i t w a s h a v i n g o n t h e p u b l i c w o r l d w i d e, r e a c h i n g e v e r y l e v e l of societ y. In commemoration of John B ahcall's seminal role over three decades in championing the Hubble Space Telescope, the Hubble pr oje c t a t G o d d ar d Sp a c e F lig h t C e n te r jo in e d w i t h t h e Ins t i t u te to create the John N. B ahcall Lectureship. T he Lectureship will b e a w a r d e d a n n u a l l y t o a n o u t s t a n d i n g s ci e n t is t , w h o w i l l d e l i v e r p r o f e s s i o n a l t a l k s a t t h e I n s t i t u t e a n d a t G o d d a r d, a s w e l l a s a p u b l i c l e c t u r e a t t h e N a t i o n a l A i r a n d S p a c e M u s e u m . I n 2 0 0 6, t h e f irst B ahcall Lecturer was P rof. Richard Ellis of C altech, and the most recent, during 10 ­14 December 2007, was P rof. Geof f rey Marcy of UC -Berkeley. To f ur ther recognize John B ahcall's f riendship and ser vice to the I n s t i t u t e, t h e s c i e n t i f i c s t a f f r e c o m m e n d e d t o t h e d i r e c t o r t h a t t h e I n s t i t u t e a u d i t o r i u m b e n a m e d t h e J o h n N . B a h c a l l A u d i t o r i u m . O v e r t h e y e a r s, J o h n h i m s e l f d e l i v e r e d l e c t u r e s i n t h a t r o o m o n many occasions. T he dedication occur red on the occasion of Geof f Marcy's lecture in December 2007, in the presence of John's wi fe, Neta, and daughter, Orli. W

Above: The crowd looks on, after the formal dedication of the STScI auditorium.

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Introducing John Mather to the Science and Operations Center
P. S tockman, stockman@stsci.edu and K athy Flanagan

Introducing John Mather to the Science Operations Center

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s the Webb project reaches its peak- development years, the Webb Science and O p e r a t i o n s C e n t e r (S & O C) a t t h e I n s t i t u t e i s r a m p i n g u p w o r k o n t h e g r o u n d s y s t e m a n d preparing for science operations. John Mather, the N ASA senior project scientist for the James Webb Space Telescope, visited the Institute on December 5, 2007, to meet the S& OC staf f. We provided an over view of the ground system and several protot ype demonstrations, including selection of NIRSpec aper tures for multi- object spectroscopy, with on-board scripts actually controlling obser vations. F o l l o w i n g t h e d e m o n s t r a t i o n s , J o h n m e t w i t h a h a l f- d o z e n a s t r o n o m e r s a n d s h a r e d h i s s e c r e t s for winning the Nobel P rize (teamwork and a great project). Af ter his colloquium presentation, "From the Big B ang to the Nobel P rize and on to James Webb Space Telescope," John enjoyed dinner with the director, senior staf f f rom the Institute and the JHU physics and astronomy depar tment, and the 2007 Brick wedde speaker, David Gross (U.C. Santa B arbara). John and David shared stories about the good old days at Berkeley and Nobel trivia (P rof. Gross won the 200 4 prize in physics). John's visit was a great success, with both visitor and staf f f inding deeper appreciation o f t h e ir r e sp e c t i v e s t r e n g t hs. W

Above: Webb senior project scientist, Dr. John Mather (left), with Prof. David Gross (U.C. Santa Barbara, center), and the Institute director, Dr. Matt Mountain (right). (Courtesy of Coyle Commercial Photographics, John J. Coyle, Jr.)

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Plumbing the Depth of the Hubble Ultra Deep Field

This edition of the Institute Newslet ter continues to reprint science ar ticles from NASA's annual Hubble200X Science Year in Review. We are pleased to continue this series with "Plumbing the Depth of the Hubble Deep Field," by Sangeeta Malhotra, "The Hubble Ultra Deep Field," by Steve Beck with, and "Stellar Chronology," by Ruth Peterson, all of which appeared in "Hubble2005."

S a n ge e t a M a l h o t r a i s a n associate pro f essor at the Arizona State Universit y, where s h e l e a d s a t e a m t h a t is d o i n g spectroscopy of galaxies in the H u b b l e U l t r a D e e p F i e l d an d sur roundings to prob e g ala x y evolu tion and re -ionization. Dr. Malhotr a wor ks on gala x y formation in the early universe, st ar for mation, dust, and interstellar gas in nearby galaxies, including our own.

Plumbing the Depth of the Hubble Ultra Deep Field

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Sangee t a Malhotra, Sangeeta.Malhotra@asu.edu

hat is the depth of the Hubble Ultra Deep F ield (HUDF )? To an astronomer, " depth " usually refers to f aintness of the sources that can be detected. A deeper obser vation detects f ainter sources. T he f aintest sources in the HUDF are about 4 b i l l i o n t i m e s f a i n t e r t h a n a n a v e r a g e h u m a n e y e c a n s e e. B u t i n c o m m o n p a r l a n c e, " d e p t h " o f t e n r e f e r s t o a d i s t a n c e, a s i n " h o w d e e p i s t h e w a t e r ? " D e p t h i n t e r m s o f d is t a n c e is i n f o r m a t i o n n o t e asi l y o b t a i n a b l e f r o m i m a g e s. T h e f i r s t s t e p i n measuring distances is to obtain the spectra, i.e., split the light f rom these sources into f iner colors, like sunlight through a prism or in a rainbow. W hen you do that, the light of ten shows features like breaks or shar p lines (see f igure on nex t page). T he distance in for mation for galaxies can then be o b t a i n e d b y c o m p a r i n g t h e s p e c t r a o f n e a r b y a n d d is t a n t g a l a x i e s a n d m e as u r i n g s h i f t s o f t h e s e s p e c t r a l f e a t u r e s . D u e t o t h e e x p a n s i o n o f t h e u n i v e r s e, a l l f e a t u r e s s h i f t t o w a r d s t h e r e d f o r d i s t a n t g a l a x i e s -- a p h e n o m e n o n o r i g i n a l l y d is c o v e r e d b y E d w i n H u b b l e. I t is f i t t i n g t h e n t h a t t o d a y w e a r e able to measure the distances ("redshi f ts") to some of the f aintest and most distant galaxies using the telescope named af ter him. Being in space, the Hubble Space Telescope is able to see f aint galaxies that are di f f icult to detect f rom the ground even with the largest telescopes. T his is especially tr ue for distant galaxies, w h i c h a r e v e r y r e d, t h e i r b l u e l i g h t h a v i n g b e e n s c a t t e r e d b y t h e i n t e r g a l a c t i c m a t t e r b e t w e e n t h e r e and here. T he reasons for Hubble's sensitivit y are mani fold: ground-based obser vatories su f fer f rom blur ring of the image by small motions in the Ear th 's atmosphere. Hubble for ms shar per images a n d s o c a n s e e s m a l l, d i s t a n t g a l a x i e s . L e s s w e l l k n o w n i s t h e f a c t t h a t o u r a t m o s p h e r e a l s o g l o w s in the red and in f rared. Hubble is above the atmosphere, so it looks through much darker skies in t h e in f r ar e d. Two years ago, all these strengths of Hubble were used to car r y out deep spectroscopy of t h e H U D F f o r t h e f i r s t t i m e. T h e i m a g i n g i n t h e H U D F t o o k a b o u t a m o n t h o f t e l e s c o p e t i m e. T h e spectroscopy was done under the GRism ACS P rogram for E x tragalactic Science project (GR A P ES), u s i n g a b o u t o n e -t e n t h o f t h e t i m e t h a t w e n t i n t o i m a g i n g . T he Hubble Advanced C amera for Surveys is equipped with a device called a grism, which is a p r is m o f g l as s e t c h e d w i t h t i n y g r o o v e s t h a t a c t t o d isp e r s e t h e l ig h t i n t o i t s c o m p o n e n t c o l o r s. ( T hese tiny grooves work the same way as the holograms on the f ace of most credit cards, w hich change color because the grooves disperse the di f ferent colors of light at di f ferent angles.) T he c o m p o n e n t c o l or s o f lig h t ar e c all e d a sp e c t r u m. T h e g r ism g i v e s l o w - r e s o l u t i o n s p e c t r a -- i t d o e s n o t s p l i t t h e l i g h t i n t o as m a n y c o m p o n e n t s as s o m e o f t h e g r o u n d - b as e d sp e c t r o g r a p h s. I t is o n l y a b l e t o i d e n t i f y a n d m e asu r e d is t a n c e s t o about 10 ­20 % of gala xies: ones with a lot of star for mation (where star-for ming gas produces a prominent emission line) or galaxies with light dominated by old stars (which produces steps in brightness at diagnostic wavelengths). Due to the redshi f ting of these prominent features, some d i s t a n c e i n t e r v a l s a r e e a s i e r t o e x p l o r e t h a n o t h e r s . I n s p i t e o f t h e s e l i m i t a t i o n s, t h e g r i s m p r o v i d e s a p o w e r f u l n e w p r o b e o f t h e d i s t a n t u n i v e r s e, a n i m p o r t a n t c o m p l e m e n t t o o b s e r v a t i o n s w i t h g i a n t ground-based telescopes. We d is c o v e r e d e a r l y o n t h a t H U D F a n d G R A P E S a r e n o t j u s t e x t r a g a l a c t i c s u r v e y s . T h e d is t a n c e scale star ts within our ow n galax y. T he closest object we have identi f ied is only 2000 light-years away, in the disk of the Milk y Way. Such nearby sources are low-mass stars, M dwar f s, w hich have d is t i n c t sp e c t r a l sig n a t u r e s a n d a r e v e r y f a i n t a n d c o m m o n .

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S AR YE <1 OF NS L IO BI L N EI 3 RS IVE UN 6 HE T OF E AG 8

PRESENT: 13.7

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A three-dimensional view of the Hubble Ultra Deep Field. Each plane is labeled with the age of the universe at the time when the light we now see lef t the galaxies.
Illustration by Ann Feild

F i n d i n g t h e d is t a n c e s t o s t a r s i n o u r g a l a x y is q u a l i t a t i v e l y d i f f e r e n t f r o m f i n d i n g d is t a n c e s t o remote gala xies, w here we use the Hubble expansion and redshi f ts. For stars, we use the spectral f e a t u r e s t o i d e n t i f y t h e i r t y p e, w h i c h t e l l s u s h o w i n t r i n s i c a l l y b r i g h t t h e y a r e, a n d t h e n u s e t h e r a t i o o f i n t r i n si c t o o b s e r v e d b r i g h t n e s s t o g e t t h e d is t a n c e s. M u ch f u r t h e r o u t o n t h e d is t a n c e s c a l e c o m e s m a l l g a l a x i e s t h a t a r e v e r y a c t i v e l y f o r m i n g s t a r s. T he t y pical distance is 6 billion light-years, or about hal f the age of the universe. Obser vations s u g g e s t t h a t t h is p e r i o d w as t h e p e a k o f s t a r f o r m a t i o n, a n d s i n c e t h e n g a l a x i e s h a v e b e e n s l a c k i n g of f. T he gala xies that can be identi f ied with the grism have prominent emission lines f rom gas that h a s b e e n i o n i z e d a n d h e a t e d b y a c t i v e s t a r f o r m a t i o n . T h e s e a r e s m a l l b l u e g a l a x i e s, m u c h l i k e some of the dwar f gala xies seen locally. Q uite interestingly, they do not show the increase in size with time seen in gala xies without prominent emission lines. A t a b o u t t h e s a m e d i s t a n c e s, w e a l s o s e e m a s s i v e e l l i p t i c a l g a l a x i e s, w h i c h a p p e a r t o h a v e s t o p p e d f o r m i n g s t a r s b y t h e t i m e t h e u n i v e r s e w as 2 b i l l i o n y e a r s o l d. W h i l e w e w e r e n o t su r p r is e d t o f i n d o l d, d i s t a n t g a l a x i e s, w e w e r e s u r p r i s e d t o f i n d a s m a n y a s w e d i d . Studies with Hubble and obser vatories on the ground independently con f ir m that old elliptical g a l a x i e s a r e f a i r l y c o m m o n w h e n t h e u n i v e r s e w as 3 b i l l i o n y e a r s o l d. T h e s e g a l a x i e s f o r m e d stars early and rapidly, w hile others for med their stars over much more prolonged periods. Fur ther s t u d y o f su ch g al a x ie s sh o u l d l e a d to i nsig h t i n to w h a t p r o m p t s o ns e t o f s t ar f o r m a t io n an d w h a t ultimately stops it. Continued We c a n i d e n t i f y t h e m o s t d is t a n t g a l a x i e s b e c a u s e o f a d r a m a t i c s p e c t r a l page 18 f e a t u r e t h a t r e d s h i f t s i n t o t h e o b s e r v e d c o l o r r a n g e. B e y o n d r e d s h i f t 4 -- w h i c h

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Spectra of Different Classes of High-redshift Objects
T h e s e f o u r p an e ls sh o w t h e sp e c t r a o f d i f f e r e n t cl as s e s o f o b j e c t s t h a t h a v e t h e r e d c o l o r s c h a r a c t e r is t i c o f high - r edshi f t g ala x ies. In all c as e s t h e r e is m o r e l ig h t o n t h e r i g h t (r e d d e r ) s i d e o f t h e s p e c t r u m than the lef t. However, only the g al a x ies t h a t show a sh ar p s tep f u n c t io n ar e t h e r e al h ig h - r e dsh i f t g al a x ie s. Sp e c t r o s c o py h e lp s us i d e n t i f y t h e n a t u r e a n d d is t a n c e s o f t h e s e o bje c t s.

Depth from page 17

cor responds to about 1.5 billion years af ter the Big B ang -- we can measure the distances of all star-for ming gala xies. In the HUDF, we see about 50 galaxies at these great distances -- seeing them as they were 12 to 12.8 billion years in the past, when the light reaching us today began its jour ney. T hese are some of the most distant galaxies ever seen. Because the HUDF goes ver y f aint, w e a r e a b l e t o s t u d y t h e t y p i c a l g a l a x i e s a t t h o s e d is t a n c e s. L o o k i n g a t t h e c o l o r s o f t h e s e r e m o t e g a l a x i e s r e v e a ls t h e i r y o u t h; t h e y h a v e y o u n g e r s t a r s t h a n the gala xies that we see in the nearby, older universe. T he appearance of these young galaxies is ragged and ir regular, par tly because we are originally seeing them in ultraviolet light, and par tly because they are still in the process of for mation. Untangling the t wo ef fects will be valuable. B e c a u s e s p e c t r a g i v e a c c u r a t e d i s t a n c e s t o g a l a x i e s, w e c a n d i s c e r n h o w t h e y a r e d i s t r i b u t e d i n t h r e e d i m e n s i o n s . We f i n d t h a t g a l a x i e s a r e c l u s t e r e d w h e r e v e r w e h a v e b e e n a b l e t o l o o k . In the HUDF, we see a cluster or "wall" of galaxies at a distance of 8 billion light-years (redshi f t 0.67). Massive, old, elliptical galaxies dominate dense regions at this distance, just as in the local universe. We also see the primitive for m of such a cluster at a look back time of 12.6 billion years away (at redshi f t ~6), w here we f ind four times as many galaxies as expected. I f we looked at s o m e o t h e r p a r t o f t h e s k y w e l i ke l y w o u l d n o t s e e s u c h a n a g g r e g a t i o n . I n f a c t , t h is c l u m p c o v e r s only hal f of the HUDF. To see how f ar this over- densit y ex tends, we obtained wide -f ield imaging at the ground-based telescopes of the National Optical Astronomy Obser vator y in Chile. At redshi f t 5. 8, w h i c h is a c c e s s i b l e f r o m t h e g r o u n d, w e s e e a w a l l o f g a l a x i e s w i t h a t r a n s v e r s e s i z e o f a t l e a s t 20 million light-years. T he HUDF is situated at the edge of this distribution. O n e o f t h e m a i n m o t i v e s f o r t h e d e e p i m a g i n g o f t h e H U D F w as t o t a ke a c e n sus o f g a l a x i e s a t redshi f t 6 (a look back time of 12.6 billion years). A massive ionization of the di f f use intergalactic gas may have occur red at this epoch (as discussed in the ar ticle by Steven Beck with). A census would deter mine w hether the galaxies could provide enough ultraviolet light to ionize the gas. T here is some controversy over the counting of redshi f t- 6 galaxies based on the images alone. One set o f p u b l is h e d e s t i m a t e s s a y s t h a t t h e r e a r e n o t e n o u g h g a l a x i e s t o i o n i ze t h e g as. A s e c o n d g r o u p

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t a ke s i n t o a c c o u n t t h e g a l a x i e s w e m ig h t h a v e m is s e d a n d s o m e t h a t a r e e v e n f a i n t e r t h a n t h e d e te c t io n li m i t , an d f i n ds t h a t t h e s e c an p r ov id e the remaining energy. Sp e c t r o s c o p y h as h e l p e d to c o n f i r m t h a t m o s t o f t h e c an d id a te g al a x ie s ar e i n d e e d a t r e dsh i f t 6 (a n d w e e d e d o u t t h e f e w i n t e r l o p e r s, w h i c h are mos t l y s t ar s in our ow n g ala x y and old red g a l a x i e s a t a b o u t h a l f t h e d is t a n c e t o r e ds h i f t 6), and has allowed us to understand the three d i m e n si o n a l s p a t i a l d is t r i b u t i o n a n d c l us t e r i n g of the f aint galaxies in the HUDF. W here they cluster together, the numbers of galaxies and photons are su f f icient to ionize the gas locally, even without accounting for the galaxies that w e m a y h a v e m is s e d . T h us t h e a n s w e r t o t h e ques t ion w he t her t here are or are no t enough g a l a x i e s to i o n i ze t h e d i f f us e i n t e r g a l a c t i c g as m ay depend on w here you look . S o, h o w d e e p i s t h e H U D F ? W e h a v e p l u m b e d to w i t hin 1 b illio n y e ar s o f t h e B ig B an g. T h e r e we f ind the seeds o f today 's gala x ies and c l u s t e r s o f g a l a x i e s, f o r m i n g s t a r s a t r a p i d r a t e s an d u n d o u b te d l y in f lu e n cin g t h e su r r o u n din g intergalactic gas. F or t y hours of Hubble observations have t aken us back 12.7 billion years. May all our endeavors be as f r uit f ul! W

Above: Expanded view of some of the HUDF galaxies. The light reaching us today from these galaxies left about 3­4 billion years ago.

Above: Mosaic of objects with redshift estimates from GRAPES, ranging from stars (at the top) to the most distant galaxies at the bottom.

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S teven Beck w i t h ser ved as director of the Space Telescope S c i e n c e I n s t i t u t e f r o m 19 9 8 u n t i l 2 0 0 5, a n d c u r r e n t l y h a s a joint appointment as an a s t ro nom e r a t t he I n s t i t u t e and professor of physics an d as t r o n o m y a t t h e Jo hns H o p k i n s U n i v e r s i t y. H e h a s b e e n a s t e ad f a s t p u b l ic advocate for astronomy f r o m sp ac e, e sp e ci all y f or t h e H ubb l e m is si o n, t h r o u g h app e ar an ces in t he m e dia and b e f o r e ad v is o r y c omm i t tees. H is p r i n c i p a l r e s e a r c h i n t e r e s t s ar e t h e f o r m a t io n a n d e ar l y evolution of planets, including t h o s e o u t s i d e t h e S o l a r S y s t e m, an d t h e b ir t h o f g al a x ie s in t h e early universe. He has published over 100 research ar ticles, and l e c t u r e s e x te n s i v e l y t o t h e general public and professional a u d i e n c e s. H e w as e l e c t e d as a member of the A mer ic an A c a d e m y o f A r t s a n d S ci e n c e s in 2004.

The Hubble Ultra Deep Field
S teven Beck wi th, sv wb@stsci.edu

T

he goal of exploration is to ex tend humankind 's reach f rom the k now n into the unk now n. For the past 15 years, Hubble has ex tended our vision to the f ar thest reaches of the visible universe, helping reveal how galaxies like our ow n have evolved over the past 13 billion y e a r s. T h e m o s t a m b i t i o us a t t e m p t t o v i e w g a l a x i e s i n t h e i r y o u t h is t h e H u b b l e U l t r a Deep F ield, or HUDF, completed in March 200 4 (show n below). In our work ing hy pothesis of the universe, the Big B ang theor y, stars and galaxies came into existence sometime bet ween about 10 0 million and 1 billion years af ter the universe for med. Gala xies for med f rom gas (mostly hydrogen) that was produced in the Big B ang, and was initially distributed throughout the universe with nearly uni for m densit y. As the universe expanded, the densest patches collapsed to for m galaxies. T his process took billions of years. Even today, less than 10 % of the original gas has been locked up in stars. O u r k n o w l e d g e o f t h e f i r s t b i l l i o n y e a r s o f g a l a x y e v o l u t i o n is q u i t e l i m i t e d. O v e r t h e p as t f e w y e a r s, a s t r o n o m e r s h a v e i d e n t i f i e d a f e w g a l a x i e s a n d q u a s a r s t h a t e x i s t e d w h e n t h e u n i v e r s e w a s a b o u t a b i l l i o n y e a r s o l d. Re s e a r ch i n d i c a t e s t h a t a t r e m e n d o us a m o u n t o f e n e r g y w as i n j e c t e d i n to the universe prior to this -- energy su f f icient to ionize nearly all of the gas bet ween galaxies (see sidebar on page 21). Astronomers think that the energy that powered this "epoch of re -ionization" came f rom the f irst generations of stars. T he galaxies hosting these stars were probably f ainter, b u t f a r m o r e n u m e r o u s, t h a n t h e f e w o b j e c t s s o f a r d e t e c t e d . I f t h e y w e r e n u m e r o u s e n o u g h a n d e m i t t e d e n o u g h u l t r a v i o l e t l i g h t , t h e y c o u l d h a v e i o n i z e d t h e i n t e r g a l a c t i c g as . I f n o t , m o r e e xo t i c processes -- such as energy released f rom mat ter f alling into black holes, or energy released by some unk now n decaying subatomic par ticle -- might have been responsible.

Right: The Hubble Ultra Deep Field, produced by stacking together hundreds of images taken in late 2003, is our most detailed view of the distant universe. More than 7000 galaxies have been detected in the final image, which encloses an area of the sky about the size of the period at the end of this sentence, viewed from normal reading distance.

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Re-ionization Epoch of

Radiation Era

First Galaxies

"Dark Ages"

First Stars

bble Deep Field

Normal Galaxies

Modern Universe 13.7

HDF 1.5

HUDF 0.9

Big Bang

Age of the Universe (billions of years)
Illustration by Ann Feild

The Epoch of Re-ionozation
Satellites such as the Cosmic Background E xplorer and the Wilkinson Microwave Anisotropy Probe (WMAP) have show n us that the universe at an age of 300,000 years consisted of hot gas of nearly uni for m densit y. T he ver y e a r l y u n i v e r s e w as s o h o t t h a t e l e c t r o ns a n d p r o to ns c o u l d n o t s t i ck to g e t h e r t o f o r m a t o m s . B y a n a g e o f 3 0 0, 0 0 0 y e a r s , t h e u n i v e r s e h a d c o o l e d e n o u g h t o a l l o w p r o t o n s t o c a p t u r e t h e e l e c t r o n s . W h e n t h is h a p p e n e d, t h e u n i v e r s e b e c am e t r ansp ar e n t to lig h t ; t h e p h o to ns r e a ch i n g us to d ay f r o m t h e C o sm i c Microwave B ackground emerged at this epoch. At some time bet ween a few hundred million and roughly a billion years later, m o s t o f t h e e l e c t r o ns w e r e ag a i n s t r i p p e d f r o m t h e i n t e r g a l a c t i c h y d r o g e n g as. T he era when this stripping occur red is called the "epoch of re -ionization." T he process required a tremendous amount of energ y. H o w d o w e k n o w t h e u n i v e r s e w as r e - i o n i ze d ? D is t a n t q u as a r s p r o v i d e strong evidence. Q uasars are bright, point-like objects that can be seen at g r e a t d i s t a n c e s, a n d t h u s p r o v i d e a s e n s i t i v e p r o b e o f t h e i n t e r v e n i n g g a s . T h e strongest atomic transition of the hydrogen atom is k now n as Ly man alpha, and occurs at a wavelength of 1216 å ngstroms. Intergalactic hydrogen produces thousands of absor ption lines -- k now n as the Lyman alpha forest-- in the s p e c t r a o f d is t a n t q u as a r s. B y m e as u r i n g h o w m u c h o f t h e q u as a r sp e c t r u m is e a t e n a w a y b y t h e s e a b s o r p t i o n l i n e s, a s t r o n o m e r s c a n e s t i m a t e h o w m u c h o f t h e i n t e r g a l a c t i c g as w as i o n i z e d . R e c e n t m e as u r e m e n t s o f t h e m o s t d is t a n t q u as a r s su g g e s t t h a t t h e i n t e r g a l a c t i c g as w as a l r e a d y h ig h l y i o n i ze d w h e n the universe was 1.3 billion years old, but that the gas was much more neutral w h e n t h e u n i v e r s e w as 1 b i l l i o n y e a r s o l d. T h e o r e t i c a l s t u d i e s p r i o r to 2 0 0 2 p r e d i c t e d t h a t t h e i n t e r g a l a c t i c g as sh o u l d transition f rom neutral to ionized in a relatively shor t period of time. However, in 20 03, astronomers using the WMAP satellite to measure scat tering f rom e l e c t r o ns i n t h e e a r l y u n i v e r s e e s t i m a t e d t h a t t h e g as w as a l r e a d y i o n i ze d b y a b o u t 2 0 0 m i l l i o n y e a r s a f t e r t h e B i g B a n g. T h is s u g g e s t s t h a t e i t h e r t h e q u as a r obser vations or the WMAP obser vations are being misinter preted, or that re -ionization is a longer and more complex process than previously thought. S o r t i n g t h is o u t is t h e su b j e c t o f i n t e n s e o b s e r v a t i o n a l a n d t h e o r e t i c a l e f f o r t .

W ith Hubble, we can, in principle, detect galaxies that are so distant that the light reaching us to d ay b e g an i t s jo u r n e y du r in g t h e e p o ch o f r e - io niz a t io n. A c e nsus o f t h e se o bje c t s c o u l d h e lp d e t e r m i n e w h e t h e r t h e e n e r g y e m i t t e d b y y o u n g g a l a x i e s w as su f f i ci e n t to i o n i ze t h e i n t e r g a l a c t i c gas. But f inding them is a challenge even for Hubble. P rior to the HUDF, our most sensitive obser vations were the Deep F ields, taken in 1995 and 1998. T h e s e o b s e r v a t i o n s p r o v i d e d a w e a l t h o f i n f o r m a t i o n o n d i s t a n t g a l a x i e s, b u t c o u l d d e t e c t v e r y f e w galaxies at look-back times (the time f rom then until the present) greater than 12 billion years. Such g al a x ie s w e r e g e n e r all y to o f ai n t to d e te c t , an d m o s t o f t h e lig h t r e a ch i n g us f r o m t h e s e g al a x ie s was at wavelengths redder than Hubble's camera could detect. T he Advanced C amera for Sur veys (ACS), installed in 20 02, provided the oppor tunit y for Hubble to detect galaxies at least six times f ainter. It is especially sensitive to f aint red galaxies. Creating the nex t- generation deep f ield, the HUDF, was a great adventure drawing on the exper tise of astronomers f rom around the world. Hubble pointed at a spot in the constellation of For nax and took obser vations over a span of 4 0 days star ting in September 200 3, paused for a month, and obser ved for another span of 4 0 days until Januar y 16, 200 4. T he f inal exposure lasted one million seconds, the longest exposure ever taken with an optical telescope, and using all of the Institute Director 's discretionar y time for one year. T he release of the images on March 9, 200 4, g e n e r a t e d t r e m e n d o us i n t e r e s t a m o n g b o t h as t r o n o m e r s a n d t h e g e n e r a l Continued public (nearly saturating the Space Telescope Science Institute's inter net page 22 connection for a few days). A group led by Rodger T hompson of the Universit y

21


Close-up images of some of the most distant galaxies in the Hubble Ultra Deep Field
G al a x ie s a t ver y e ar l y t im e s te n d to b e ver y sm all an d o f te n sh ow sig ns o f in ter ac t io ns. T h e H U D F c o n t ains n e ar l y 5 0 g al a x ie s at redshi f ts 5 ­ 6, compared to a few tent ative identi f ications in ear lier, shallower obser vations.

galaxies z ~ 3­4

Lookback time 11.4­12 billion years

(312 objects)

galaxies z ~ 4­5

Lookback time 12­12.3 billion years

(79 objects)

galaxies z > 5

Lookback time 12.3­12.6 billion years

(45 objects)

HUDF from page 21

22

of A rizona car ried out an additional set of obser vations with Hubble's Near In f rared C amera and Multi- Object Spectrometer (NICMOS), providing the oppor tunit y to search for even redder galaxies. A nother group, led by Sangeeta Malhotra of the Space Telescope Science Institute, used ACS to measure gala x y spectra. In the HUDF, we can see the entire span of the universe's histor y, f rom less than a billion years af ter the Big B ang until the present day. T he f aintest objects are roughly 4 billion times f a i n t e r t h a n t h o s e w e c a n s e e w i t h t h e u n a i d e d e y e. T h e s m a l l e s t a r e o n l y 5 0 m i l l is e c o n d s o f a r c a c r o s s, e q u i v a l e n t t o t h e s i z e o f a d i m e s e e n f r o m a d i s t a n c e o f 5 0 m i l e s . T h e H U D F i s a n i m a g e o f superlatives: the deepest, f ar thest, and earliest look into the histor y of stars and galaxies. T he most distant-- and thus the youngest-- galaxies in the HUDF are small, ir regular, relatively b r i g h t f o r t h e i r d i s t a n c e, a n d o f t e n c o m e i n p a i r s o r s m a l l c l u s t e r s . T h e i m a g e s a b o v e s h o w s o m e examples of these objects. The red colors are characteristic of distant galaxies whose spectra are s h i f t e d t o l o n g w a v e l e n g t h s b y t h e e x p a n si o n o f t h e u n i v e r s e -- t h e c h a r a c t e r is t i c r e ds h i f t us e d t o d e t e r m i n e h o w f a r a w a y t h e g a l a x i e s r e a l l y a r e. T he f igure on page 23 shows the spectr um of a distant galax y in the HUDF taken with Hubble (see the ar ticle by Sangeeta Malhotra). T he spectr um shows the intensit y of light versus its color o r w a v e l e n g t h . T h e r e i s a c l e a r b r e a k i n t h e s p e c t r u m: i t i s b r i g h t a t r e d w a v e l e n g t h s a n d f a i n t a t blue waveleng ths. T he lack of light at shor t wavelengths comes about because hydrogen atoms b e t w e e n t h e g a l a x y a n d us a b s o r b a l l t h e r a d i a t i o n . T h is s h a r p b r e a k is c h a r a c t e r is t i c o f v e r y d is t a n t gala xies, and the exact wavelength of the break provides a measurement of the galax y's redshi f t (see sidebar), and hence its distance.


M o s t o f t h e g a l a x i e s i n t h e H U D F a r e s o f a i n t t h a t w e c a n n o t m e a s u r e t h e i r s p e c t r a . N e v e r t h e l e s s, w e d o h a v e g o o d m e as u r e m e n t s o f t h e i r c o l o r s f r o m t h e f o u r f i l t e r s u s e d t o t a ke t h e i m a g e. T h o s e c o l o r s p r o v i d e r o u g h e s t i m a t e s o f g a l a x y r e d s h i f t s, a n d t h u s, d i s t a n c e s . T h e y a l l o w u s t o c o u n t t h e number of gala xies as a f unction of look back time. Moreover, we can use the obser ved brightness o f t h e g al a x ie s to e s t im a te h o w m u ch r adi a t io n t h e y e m i t te d a t di f f e r e n t t im e s to c o m p ar e w i t h t h e a m o u n t n e e d e d t o r e - i o n i z e t h e u n i v e r s e. T h e c e nsus o f d is t a n t g a l a x i e s i n t h e H U D F i n d i c a t e s t h a t y o u n g g a l a x i e s m a y p r o v i d e a l m o s t e n o u g h e n e r g y t o a c c o u n t f o r t h e i o n i z a t i o n o f t h e u n i v e r s e. B u t d i f f e r e n t g r o u p s o f as t r o n o m e r s an al y z i n g b o t h t h e A C S an d t h e N I C M O S H U D F o b s e r v a t io ns h a v e c o m e to d i f f e r e n t c o n cl usio ns a b o u t w h e t h e r t h e e n e r g y is a c t u a l l y su f f i ci e n t . T h r e e c o m p l i c a t i o n s h a v e l e d t o c o n t r o v e r s y w h e n a n a l y z i n g t h e i m a g e s. T h e f i r s t is t h a t w e c a n n o t a c t u a l l y m e as u r e t h e a m o u n t o f r a d i a t i o n t h a t p r o v i d e s t h e i o n i z a t i o n, b e c a u s e i t is a l l a t waveleng ths much shor ter than we can obser ve. To ionize hydrogen, light must be at a wavelength shor ter than 912 å . We must ex trapolate f rom the light we receive at longer wavelengths to e s t i m a t e t h e a m o u n t e m i t t e d a t sh o r t e r w a v e l e n g t hs. T h e as su m p t i o ns us e d to ex t r a p o l a t e t h e galax y spectra depend on the ages and chemical abundances assumed for the stars in these g a l a x i e s, a n d a r e a m a t t e r o f d e b a t e. T h e s e c o n d c o m p l i c a t i o n is t h a t t h e l i g h t f r o m m a n y f a i n t g a l a x i e s -- t h a t w e c a n n o t s e e -- m ig h t b e t h e m o s t i m p o r t a n t s o u r c e o f i o n i z i n g r a d i a t i o n . I n t h e n e a r b y u n i v e r s e, w e k n o w t h a t s m a l l (dwar f ) gala xies outnumber large ones. T here are hints in the HUDF that small galaxies might have o u t n u m b e r e d l a r g e o n e s b y a n e v e n l a r g e r f a c to r i n t h e p as t . T h e g al a x ie s t h a t w e s e e m ay jus t b e t h e t ip o f t h e i c e b e r g, s o to sp e ak , an i n d i c a t io n o f m an y m o r e u n s e e n g a l a x i e s t h a t a r e, n e v e r t h e l e s s , i m p o r t a n t t o t h e i o n i z a t i o n o f t h e u n i v e r s e . E s t i m a t i n g t h e n u m b e r o f t h e s e f ai n t g al a x ie s als o r e q u ir e s an ex t r ap o l a t io n f r o m w h a t Continued we see, and the assumptions used here are uncer tain and controversial.
page 24

Redshifts
T h e ex p ansio n o f t h e u n i v e r s e s t r e t ch e s t h e lig h t w a v e s e m i t te d f r o m distant galaxies during their passage to Ear th. T he light "redshi f ts"-- changes to l o n g e r w a v e l e n g t hs -- b e c aus e t h e lig h t w a v e s ar e s t r e t ch e d by t h e e x p a n s i o n o f u n i v e r s e. F o r v e r y d i s t a n t g a l a x i e s, b l u e l i g h t b e c o m e s r e d, a n d red light becomes inf rared. Astronomers can use this "cosmological " redshi f t to estimate the distances to gala xies: the larger the shi f t, the greater the distance. To do this precisely, as t r o n o m e r s d isp e r s e t h e l ig h t o f a d is t a n t g a l a x y i n t o a sp e c t r u m a n d l o o k f o r the telltale signatures of atoms, such as hydrogen and ox ygen. T he f igure below shows a spectrum of a ver y distant gala x y in the HUDF. T h e sp e c t r u m sh o w s li t t l e d e te c te d lig h t a t w av e l e n g t hs sh or te r t h an 8 3 0 0 å ngstroms (1 å = 10 ­8 cm), a shar p rise or edge, and a relatively constant f lu x o f lig h t a t l o n g e r w a v e l e n g t hs. T h e l a ck o f lig h t a t sh o r t w a v e l e n g t hs c o m e s ab o u t b e c aus e h yd r o g e n a to ms b e t w e e n t h e g al a x y an d us ab s o r b all t h e radiation. But this absor ption actually occurs at about 1216 å in the rest f rame o f t h e a t o m s. T h e r e as o n t h a t t h e a b s o r p t i o n e d g e is s e e n a t a w a v e l e n g t h o f 8 30 0 å is that both the galax y and the absorbing hydrogen are so distant that the ultraviolet wavelengths have been stretched to the red (long wavelength) par t of the spectr um. T he redshi f t, z, of this galax y is (8 30 0/1216) ­1 = 5.8. T he cor responding light-travel distance is 12.7 billion years, and the universe was only 1 billion years old when the light was emit ted. ( You can work out these numbers for arbitrar y redshi f ts on Edward Wright 's Cosmology C alculator web page: ht tp: // w w w.astro.ucla.edu /~wright / CosmoCalc.h tml)

10

Intensity

5 0 ­5

7000

8000 Wa v e l e n g t h ( å n g s t r o m s )

9000

Spectrum of one of the most distant galaxies in the Hubble Ultra Deep Field

The Hubble Ultra Deep Field, produced by stacking together hundreds of images taken in late 2003, is our most detailed view of the distant universe. More than 7000 galaxies have been detected in the final image, which encloses an area of the sky about the size of the period at the end of this sentence, viewed from normal reading distance.

23


HUDF from page 23

A f inal di f f icult y is that we do not k now for sure that the universe was ionized all at once during just a brief period. It could have happened more gradually. In the gradual picture, galaxies would create zones of ionization around them, essentially bubbles of ionized gas in a sea of neutral atoms. A s m o r e a n d m o r e g a l a x i e s w e r e b o r n , t h e s e z o n e s w o u l d g r o w a n d o v e r l a p l i k e S w i s s c h e e s e, e v e n t u a l l y m e r g i n g t o g e t h e r c o m p l e t e l y t o e l i m i n a t e a n y p o cke t s o f n e u t r a l a t o m s. I t w o u l d b e d i f f i c u l t t o s e e t h i s p a t t e r n i n a s i n g l e, n a r r o w f i e l d s u c h a s t h e H u b b l e U l t r a D e e p F i e l d . D e t e c t i n g su ch f l u c t u a t io ns w ill b e a ch all e n g e f o r f u t u r e o b s e r v a t io ns. A p a r t i c u l a r l y e xci t i n g p r o s p e c t is t h e p o s si b i l i t y o f d e e p o b s e r v a t i o n s w i t h t h e W i d e F i e l d Camera 3 ( W FC3), a new camera that is ready for installation on Hubble. T his camera provides a l a r g e f i e l d o f v i e w a t w a v e l e n g t h s l o n g e r t h a n t h e r e d l i m i t o f t h e A C S. U s i n g t h e s a m e t e c h n i q u e s that have been applied in the HUDF, the camera will allow a search for ionizing sources out to redshif ts as high as 10, less than 500 million years af ter the Big Bang. Based on estimated ages of some of the most distant galaxies in the HUDF, we expect to f ind at least a few young galaxies at this greater distance. Obser vations with W FC3 should help reveal how many galaxies star ted forming in this early era, and whether they provided enough energy to re-ionize the intergalactic gas. Toward the middle of the nex t decade, the James Webb Space Telescope will expand the f rontier t o r e d s h i f t s a s h i g h a s 3 0, g i v i n g u s t h e a b i l i t y t o d e t e c t s t i l l f a i n t e r a n d m o r e d i s t a n t g a l a x i e s w h e n they were for ming their ver y f irst stars. W

Ru t h Pe ter son is a rese arch astronomer af f iliated w ith t h e Uni ver si t y o f C ali f or ni a, S an t a C r u z an d A s t r op h y sic al A d v a n c e s, In c or p or a t e d. H e r studies of st ars and stellar clus ter s r ange f rom de t ailed s t udies o f a single chemic al e l e m e n t i n a s i n g l e s t a r, t o using vast collections of stars to deter mine the ages of e n t i r e g a l a x i e s . S h e is p r i n c i p a l investigator of the Hubble Treasur y program entitled MidUltraviolet Spectral Templates f o r O l d S t e l l a r S y s t e m s, w h i c h aims to p r ov id e t h e f o u n d a t io n f or more precise es tim a tes o f t he ages and s t ar-f or ming histories of galaxies.

Stellar Chronology
Ru th Pe ter son, peterson@ucolick.org

I

magine tr ying to tell time f rom the leaves on a tree. You might guess the time of year f rom w h e t h e r t h e l e a v e s a r e g r e e n o r b r o w n , o p e n i n g o r f a l l i n g . I n a t e m p e r a t e c l i m a t e, y o u c o u l d b e su r e o f t h e s e as o n a n d m ig h t b e a b l e to e s t i m a t e t h e d a t e to w i t h i n a m o n t h. I f y o u w e r e m o r e s c i e n t i f i c, a n d h a d a w h o l e f o r e s t o f t r e e s , y o u m i g h t b e a b l e t o p i n d o w n t h e d a t e t o within a week-- or even just a few days -- i f you understood the soil, weather histor y, the unique proper ties and behavior of each t ype of tree. So it is also with stars. Stars have chronologies, like trees. T hey are similarly complex to read, but they yield to science. A s t h e y b u r n t h e i r n u c l e a r f u e l o v e r e o n s, c h a n g e s i n t e m p e r a t u r e a n d c h e m i c a l c o m p o s i t i o n s ref lect the passage of time. Fur ther, because at bir th stars gather raw material f rom interstellar space w hich is detritus f rom earlier generations of stars, each star bespeaks not only its individual e v o l u t i o n d u r i n g i t s l i f e t i m e, b u t a l s o e v e n t s i n v o l v i n g m a n y o t h e r s t a r s l o n g p a s t . A s w e l e a r n t o i n t e r p r e t t h is s t e l l a r r e c o r d, w e o p e n a n d r e a d t h e h is t o r y o f g a l a x i e s -- b o t h o u r o w n a n d o t h e r s . For the past several years, the Hubble obser ving proposal solicitation has encouraged "treasur y" p r o p o s als -- p r o p o s als to o b t ai n l ar g e d a t a s e t s ai m e d a t an u n usu all y d i v e r s e s e t o f s cie n t i f i c q u e s t i o n s, o r p r o p o s a l s f o r o b s e r v a t i o n s t h a t l a y t h e g r o u n d w o r k f o r o t h e r r e s e a r c h . O n e o f t h e s e treasur y programs, a three -year sequence of spectroscopic obser vations, is providing a new foundation for our chronology of stars and galaxies. A sp e c t r o g r ap h b r e ak s t h e lig h t o f a s t ar i n to a h ig h l y d e t ail e d r e c o r d o f b r ig h t n e s s v e r sus w a v e l e n g t h. C h e m i c a l e l e m e n t s i n t h e o u t e r l a y e r s o f s t e l l a r g as a b s o r b a n d e m i t l ig h t a t p a r t i cu l a r waveleng ths, cor responding to the inter nal transitions of the par ticular atoms making up the star. T hus, spectra reveal a star 's chemical makeup and the temperature of the layer of its atmosphere where most starlight emerges, called the photosphere. S t e l l a r s p e c t r a c o n t a i n a v as t a m o u n t o f i n f o r m a t i o n -- a f a c t t h a t m a ke s t h e m b o t h v a l u a b l e a n d di f f icult to inter pret. Conditions in the stellar photosphere are di f f icult or impossible to reproduce in the laborator y, and the spectra involve millions of atomic transitions. Our k nowledge of atoms is b a s e d i n p a r t o n s t e l l a r s p e c t r a, w h i c h m e a n s t h e r e s e a r c h is r e c i p r o c a l . A s t r o p h y s i c is t s u s e the best available laborator y data and theoretical calculations to make a f irst at tempt at matching s t e l l a r s p e c t r a . W h e r e t h e r e a r e d i s c r e p a n c i e s, t h e y t r y t o a d j u s t t h e a t o m i c p h y si c s a n d c a l c u l a t e a n e w s p e c t r u m, m a k i n g s u r e t h e a d j us t m e n t s a r e Continued s t i l l c o nsis t e n t w i t h t h e l a b o r a t o r y d a t a. O b s e r v a t i o n s o f s t a r s o f d i f f e r e n t page 27 chemical compositions and temperatures also help ensure consistency.

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M104, the Sombrero Galaxy: A disk of relatively young stars, mixed with dust and gas, encircles a "bulge" of ancient stars. Ultraviolet spectroscopy helps astronomers decipher the history of such galaxies.

25


The Complex Spectra of Sun-like Stars
S p e c t r o s c o p y is o n e o f t h e m o s t p o w e r f u l t o o ls a v a i l a b l e t o a n astronomer. T h e di ag r am sh o w s a t in y p or t io n o f t h e u l t r av io l e t sp e c t r a o f f i v e stars. T he Hubble spectrograph stretches the light f rom each star s o t h a t w e c an s e e t h e ch an g e i n i n te nsi t y i n v e r y n ar r o w i n te r v als o f w a v e l e n g t h . T h e h o r i zo n t a l a x is i n t h is d i a g r a m is t h e w a v e l e n g t h of the light f rom each star. T he ver tical a xis shows the intensit y of lig h t r e c e i v e d a t e a ch w av e l e n g t h. T h e h e av y b l a ck cu r v e s sh o w the obser ved spectr um of each star. T he dips in intensit y are due to a b s o r p t i o n b y a to ms a n d m o l e cu l e s i n t h e o u t e r l a y e r s o f g as i n the stars. T housands of these features are blended together. T he co lor e d dips show t he de t aile d absor p t ion line pro f iles o f p ar t icul ar elements. Yellow is manganese and cobalt, red is y t trium and z i r c o n i u m, o r a n g e is g e r m a n i u m, a n d b l u e / p u r p l e a r e r a r e h e a v y e l e m e n t s . I n m e t a l - p o o r s t a r s, t h e U V i s t h e o n l y r e g i o n w h e r e m a n y o f t h e l a t t e r e l e m e n t s c a n b e m e asu r e d. T h e t hin cu r v e s su p e r im p o se d o n t h e s te ll ar sp e c t r a sh o w t h e r e su l t s o f a d e t ail e d t h e or e t ic al c al cu l a t io n o f w h a t t h e sp e c t r u m i s e x p e c t e d t o l o o k l i k e, a s s u m i n g a p a r t i c u l a r s t e l l a r t e m p e r a t u r e, sur f ace gravit y, turbulent velocit y within the emit ting gas, and ab u n d an c e o f ir o n an d o t h e r e l e m e n t s. T h e s t a r s i n t h is d i a g r a m a l l h a v e s i m i l a r s u r f a c e t e m p e r a t u r e s -- about 6,000 degrees Kelvin (10,000 degrees Fahrenheit)-- but have dif ferent chemical abundances. T he topmost star, HD 140283, is "metal poor." Heavier elements like carbon or ox ygen or iron are present in only about 1% of their abundance in the Sun. Working d o w n t h e d i a g r a m, t h e r e a r e s t a r s w i t h p r o g r e s s i v e l y l a r g e r a m o u n t s o f h e a v y e l e m e n t s, u n t i l w e g e t t o t h e S u n -- n o t o b s e r v e d b y Hubble -- at the bot tom of the diagram. As the abundances of heav y e l e m e n t s i n c r e a s e s, m o r e a n d m o r e o f t h e s p e c t r u m i s e a t e n a w a y by ab sor p t io n.

Portions of the UV spectra of five stars
HD 140283

HD 184499

Intensity
1.0 .9 .8 .7 .6 .5 .4 .3 .2 .1 .0

HD 157466

Procyon

SUN - FLUX

3060

3061

3062

3063

3064

3065

3066

3067

3068

3069

3070

3071

Wavelength in ångstroms

26


Stellar Chronology from page 24

T h e b o l d a m b i t i o n o f r e a d i n g t h e h is t o r y o f g a l a x i e s i n v o l v e s f o u r d e p e n d e n t a n d m u t u a l l y r e i n f o r c i n g l a y e r s o f r e s e a r c h: i n t o t h e s t r u c t u r e o f a t o m s a s i n f e r r e d f r o m s p e c t r a l l i n e s , i n t o t h e c o m p o s i t i o n a n d t e m p e r a t u r e o f i n d i v i d u a l s t a r s a s i n f e r r e d f r o m t h e i r s p e c t r a, i n t o e v o l u t i o n a r y m o d e l s o f s t a r s b a s e d o n t h e i r c o m p o s i t i o n a n d t e m p e r a t u r e, a n d i n t o t h e a g e s a n d c h e m i c a l m a ke u p o f g a l a x i e s as i n f e r r e d f r o m t h e c o m p o si t e sp e c t r a o f m a n y s t a r s. T h e ill us t r a t io n o n p ag e 2 6 sh o w s a t i n y p o r t io n o f t h e u l t r a v io l e t sp e c t r a o f f i v e s t ar s. T h e picket-fence appearance is due to thousands of blended absor ption lines. T he topmost star is a metal-poor star, made mostly of hydrogen and helium. Heavier elements like carbon or ox ygen or iron are present with only about 1% of their abundance in the Sun. Dow n the diagram come s t a r s w i t h p r o g r e s s i v e l y l a r g e r a m o u n t s o f h e a v y e l e m e n t s , u n t i l w e g e t t o t h e S u n (n o t o b s e r v e d by Hubble). T he colored cur ves show the contributions of speci f ic elements, like magnesium a n d c o b a l t . T h e t h i n b l a c k c u r v e s s h o w t h e b e s t e f f o r t a t m o d e l i n g t h e s p e c t r a, u s i n g a l l t h e atomic physics, estimates of the stellar temperatures, chemical abundances, rotation rates, and atmospheric turbulence. T he agreement is good, but cer tainly not per fect. In the process of get ting the theoretical model to agree this well, we have ref ined our k nowledge of atomic transitions and improved our understanding of the temperatures and chemical abundances of these stars. T he Hubble obser vations focus on the ultraviolet por tion of the spectr um, because it gives us our b e s t h a n d l e o n t h e a g e s o f t h e o l d e s t g a l a x i e s . C o n s i d e r t h e c l u s t e r o f s t a r s o n p a g e 2 9. A c l o s e l o o k r e v e a l s t h a t b o t h t h e b r i g h t e s t s t a r s a n d t h e f a i n t e s t s t a r s t e n d t o b e r e d d is h . I n b e t w e e n, most of the stars look bluer. T his is not an optical illusion. W hen measured caref ully, the colors of s t a r s i n c l us t e r s s h o w d is t i n c t p a t t e r ns t h a t i n d i c a t e t h e o v e r a l l a g e a n d c h e m i c a l c o m p o si t i o n o f the cluster. T he best way to estimate the age of a cluster is to measure the brightness of its bluest main-sequence stars (see sidebar on pages 28 ­29). We can make an even bet ter age estimate i f w e m e asu r e t h e sp e c t r a o f t h e s e s t a r s a n d d e t e r m i n e t h e i r ex a c t t e m p e r a t u r e s a n d ch e m i c a l composition. A p r o b l e m f o r m o r e d is t a n t c l us t e r s a n d g a l a x i e s is t h a t w e c a n n o t s e e Continued i n d i v i d u a l s t a r s . I n s t e a d , w e h a v e t o m e a s u r e t h e m i x e d l i g h t-- t h e i n t e g r a t e d page 28 s p e c t r a -- o f a l l t h e s t a r s . At u l t r a v i o l e t w a v e l e n g t h s t h e b l u e s t m a i n -

Spectrum of a globular cluster
M31 G1 Mid-UV E(B-V) = 0.06

Best Fit: Cool BHB + EHB [Fe/H] = ­0.8

2300

2400

2500

2600

2700

2800

2900

3000

3100

T his diagram shows the spectr um of a globular star cluster in the nearby A ndromeda galax y. T he orange cur ve is the obser ved spectrum, while the b l u e c u r v e is t h e t h e o r e t i c a l m o d e l t h a t p r o v i d e s t h e c l o s e s t m a t ch t o t h e o b s e r v e d sp e c t r u m .

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Stellar Chronology from page 27

se qu e n c e s t ar s pr o du c e m o s t o f t h e lig h t , so t h e b e s t c o ns t r ain t s o n ag e c o m e f r o m sp e c t r a in the ultraviolet. T h e s p e c t r o s c o p i c t r e asu r y p r o g r a m is a p p l y i n g t h is t e ch n i q u e t o e s t i m a t e t h e a g e s o f d e n s e s t a r clusters (k now n as globular clusters) in the nearby A ndromeda galax y. T his galax y is close enough that Hubble can detect individual stars, but f ar enough away that astronomers can measure the composite spectra of its clusters. A ndromeda's clusters provide a par ticularly good test for the age - dating technique, because we can dissect the clusters star by star (eventually) to test whether t h e i n f e r e n c e s f r o m t h e c o m p o s i t e s p e c t r a a r e c o r r e c t . F u r t h e r m o r e, t h e A n d r o m e d a c l u s t e r s a r e relatively metal-rich compared to globular clusters in the Milk y Way. T he A ndromeda clusters p r ov id e a b e t te r m a t ch to t h e ch e m i c al c o m p o si t io ns o f t h e c e n te r s o f g al a x ie s. T h e d i ag r am o n p ag e 27 sh o w s t h e sp e c t r u m o f o n e o f t h e A n d r o m e d a g l o b u l ar cl us te r s. A m o d e l s p e c t r u m is s h o w n f o r c o m p a r is o n . T h is m o d e l a l l o w s us t o c o n v e r t t h e i n f o r m a t i o n c o n t a i n e d i n t h e sp e c t r u m i n to a p h y si c al u n d e r s t an d i n g o f t h e s te ll ar p o p u l a t io n. F r o m t h e i n f e r r e d te m p e r a t u r e o f t h e b l u e s t m a i n - s e q u e n c e s t a r s, w e i n f e r t h a t t h e c l u s t e r i s a s o l d a s t h e o l d e s t s t a r s i n t h e M i l k y Way. T he chemical abundances infer red f rom the spectrum agree well with the abundances infer red f r o m i n d i v i d u a l s t a r s. A n d t h e sp e c t r u m g i v e s us a n i n d i c a t i o n o f t h e t e m p e r a t u r e d is t r i b u t i o n o f t h e helium-bur ning stars in the cluster (an additional piece of in for mation that may be helpf ul in pinning dow n the behavior of this poorly understood phase of stellar evolution). Our Hubble Treasur y program is providing calibrated templates of stellar spectra to decipher the i n f o r m a t i o n i n c o m p l e x s p e c t r a o f s t a r s a n d g a l a x i e s . S t a r s w i l l n e v e r b e e a s y t o u s e a s c l o c k s, b u t with concer ted ef for t, we can improve their accuracy. W

Ages and Stellar Temperatures
T h e i m a g e t o t h e r i g h t is o f a n o l d c l us t e r o f s t a r s i n t h e Milk y Way Galax y, NGC 6791, located about 17,000 light-years f r o m E a r t h. U n d e r t h e as su m p t i o n t h a t t h e s t a r s a l l f o r m e d a t t h e s a m e t i m e, c l u s t e r s l i k e t h i s p r o v i d e v a l u a b l e i n f o r m a t i o n a b o u t s t e l l a r e v o l u t i o n . F o r e x a m p l e, l o o k i n g c l o s e l y a t t h e image you will notice that the brightest stars and the f aintest s t a r s t e n d t o b e r e d . A t i n t e r m e d i a t e b r i g h t n e s s, t h e s t a r s a r e bluer. T he brightest blue stars do not belong to the cluster, and are located bet ween us and the cluster. T he hal f- dozen blue, point-like stars are the helium-bur ning stars noted above. T he bluest of the remaining f ainter, yellowish swar m o f s t a r s a r e t h e ke y : t h e b l u e r t h e b r i g h t e n d o f t h is s w a r m, the younger the cluster, for the color of the stars is mostly an i n d i c a t io n o f t h e ir te m p e r a t u r e s. T h e b l u e s t ar s ar e h o t te r than the red stars, and bur n out sooner. W h e n m e a s u r e d i n d e t a i l, t h e c o l o r s o f s t a r s i n c l u s t e r s like NGC 6791 for m a distinct pat ter n, show n in the diagram (bot tom, right). T he lower par t of the pat ter n, below the kink, is called the "main sequence." Stars spend most of their lives o n t h e m a i n s e q u e n c e, p r o d u c i n g e n e r g y v i a n u c l e a r f u s i o n i n t h e ir c o r e s. W h e n t h e y h a v e us e d u p m o s t o f t h e ir n u cl e ar f uel, they "peel away" f rom the main sequence and evolve to l ar g e r si ze s an d l o w e r te m p e r a t u r e s. T h e o r e t i c al c al cu l a t io ns tell us that the average temperature of the brightest "mains e q u e n c e" s t a r s i s a v e r y g o o d i n d i c a t o r o f t h e a g e o f a c l u s t e r o f s t ar s. W h e n w e o b s e r v e m o r e d i s t a n t c l u s t e r s o r g a l a x i e s, t h e stars are all jumbled together. T he only way to measure t h e a g e is t o t r y t o m o d e l t h e s p e c t r u m o f t h is c o m p o si t e p o p u l a t io n o f s t ar s. T h e c o n t r ib u t io n f r o m h o t m ai n - s e q u e n c e s t a r s i s g r e a t e s t i n t h e u l t r a v i o l e t-- h e n c e t h e m o t i v a t i o n f o r obser ving f rom space with Hubble.

28


The stars that used to be here evolved into RGB stars

Above, the old open cluster, NGC 6791, located 17,000 light-years f rom Ear th, in the Milk y Way G U n d e r t h e as su m p t i o n t h a t t h e s t a r s a l l f o r m e d a t t h e t i m e, c l u s t e r s l i k e t h i s p r o v i d e v a l u a b l e i n f o r m a t i o n stellar evolution.

abou t alax y. s ame abou t

LUMINOSITY

Main-sequence turnoff

C O L O R / T E M P E R AT U R E

Red Giant branch

At left, a schematic H-R diagram for old clusters like NGC 6791: T he colors of stars for m a distinct pat ter n, as s h o w n h e r e.

Unevolved main sequence


Hubble IllumInates

"Dorian Gray " Galaxy

N

http://hubblesite.org/newscenter/archive/releases/2007/35/ Image Credit: NASA , ESA , and A . Aloisi (Space Telescope Science Institute and European Space Agency, Baltimore, Md.)

ASA's Hubble Space Telescope quashed the possibilit y that what was previously believed to be a toddler galax y in the nearby universe may actually be considered an adult. Called I Zwick y 18, this galax y has a youthful appearance that resembles galaxies t ypically found only in the early universe. Hubble has now found faint, older stars within this galax y, suggesting that the galax y may have formed at the same time as most other galaxies. For the first time, Hubble data also allowed astronomers to identif y Cepheid variable stars in I Zwick y 18. T hese flashing stellar mile-markers were used to determine that I Zwick y 18 is 59 million light-years f rom Ear th, almost 10 million light-years more distant than previously believed.


IN OUR NEXT ISSUE:

A

An Atlas of Interacting Galaxies

stronomy tex tbooks t ypically present galaxies as staid, solitar y, and majestic island worlds o f g l i t t e r i n g s t a r s . B u t g a l a x i e s h a v e a d y n a m i c a l s i d e. T h e y h a v e c l o s e e n c o u n t e r s t h a t s o m e t i m e s e n d i n g r a n d m e r g e r s a n d o v e r f l o w i n g s i t e s o f n e w s t a r b i r t h as t h e c o l l i d i n g galaxies morph into wondrous new shapes. In celebration of the Hubble Space Telescope's 18th launch anniversar y, 59 views of colliding galaxies-- constituting the largest collection of Hubble images ever-- were released to the public. T his new Hubble atlas dramatically illustrates how galax y collisions produce a remarkable variet y of intricate structures in never-before-seen detail. http://hubblesite.org/newscenter/archive/releases/2008/16/ Image Credit: NASA , ESA , the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A . Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

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The Institute's website is: http://www.stsci.edu Assistance is available at help@stsci.edu or 800-544-8125. International callers can use 1-410-338-1082. For current Hubble users, program information is available at: http://presto.stsci.edu/public/propinfo.html. The current members of the Space Telescope Users Committee (STUC) are: Pat McCarthy (chair), Carnegie Observatories, pmc2@ociw.edu Martin Barstow, U. of Leicester Peter Garnavich, U. of Notre Dame Jim Green, U. of Colorado Jean-Paul Kneib, OAMP David Koo, UCSC Lori Lubin, UCD Mario Mateo, U. of Michigan The Space Telescope Science Institute Newsletter is edited by Robert Brown, rbrown@stsci.edu, who invites comments and suggestions. Technical Lead: Christian Lallo, clallo@stsci.edu Contents Manager: Sharon Toolan, toolan@stsci.edu Design: Kathy Cordes, cordes@stsci.edu To record a change of address or to request receipt of the Newslet ter, please send a message to address-change@stsci.edu. Phil Nicholson, Cornell U. Robert O'Connell, U. of Virginia Alvio Renzini, INAF Abi Saha, NOAO Tommaso Treu, UCSB Marianne Vestergaard, U. of Arizona

T

h p a g b and s t a te Telescope

e Sp a c e Te l e s c o p e ­ E u r o p e an C o o r d in a t in g F a cili t y u b l is h e s a n e w s l e t t e r w h i c h, a l t h o u g h a i m e d p r i n ci p a l l y t European Space Telescope users, cont ains ar ticles o f ener al interest to the HST communit y. I f you w ish to e i n cl u d e d i n t h e m a i l i n g l is t , p l e as e c o n t a c t t h e e d i to r yo u r a f f ili a t io n an d sp e ci f ic in vo l v e m e n t in t h e Sp a c e P roject.

ST-E CF Newsletter

Richard Hook (Editor) Space Telescope­European Coordinating Facility Karl Schwarzschild Str. 2 D-85748 Garching bei MÝnchen Germany E-Mail: rhook@eso.org

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Contents:
Synergy Between Webb and Hubble.................. 1 ACS Report ..................................... 3 NICMOS: Improving Science by Removing Bright-Earth Persistence ................ 4 STIS Update .................................... 5 Improving Hubble's Pointing and Astrometry............ 7 WFC3 Status .................................... 9 COS Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Testing the MIRI Verification Model.................. 12 Institute Honors John Bahcall . . . . . . . . . . . . . . . . . . . . . . 14 Introducing John Mather to the Science Operations Center .................. 15 Plumbing the Depth of the Hubble Ultra Deep Field ..... 16 The Hubble Ultra Deep Field ....................... 20 Stellar Chronology ............................... 24

Calendar
May Symposium, A Decade of Dark Energy, http://www.stsci.edu/institute/conference/spring2008 ........ 5­8 May HST Cycle 17 Panels and TAC meeting .......................... 12­16 May MAST Senior Review proposal deadline ............................ 16 May Youth for Astronomy & Engineering (YAE) Program, Women's Science Forum for Middle Schools (STScI Auditorium) ..................... 17 May IIVC .................................................. 19­20 May HST Cycle 17 TAC results released ................................ 28 May Youth for Astronomy & Engineering (YAE) Program, Women's Science Forum for Middle Schools (STScI Auditorium) ..................... 31 May AAS meeting (St. Louis) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1­5 June STIC (Garching) ............................................ 17­18 June NASA Data Centers' Senior Review ............................ 17­19 June SPIE, Astronomical Instrumentation (Marseille) ................... 23­28 June HST Cycle 17 Phase II deadline ..................................... 3 July JWST SWG (Noordwijk) ....................................... 9­10 July STS 125 launch (KSC) ......................................... 8 October STUC meeting (STScI)................................... 13­14 November 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008

Contact STScI .................................. 31 Calendar ...................................... 32

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