Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.stsci.edu/ftp/science/hdf/project/paris.ps Äàòà èçìåíåíèÿ: Thu Jan 11 20:18:36 1996 Äàòà èíäåêñèðîâàíèÿ: Sat Dec 22 22:40:48 2007 Êîäèðîâêà: IBM-866 Ïîèñêîâûå ñëîâà: zodiacal light

THE HUBBLE DEEP FIELD OBSERVATIONS
R.E. Williams, B.S. Blacker, M. Dickinson, H.C. Ferguson, A.S. Fruchter,
M. Giavalisco, R.L. Gilliland, R.A. Lucas, D.B. McElroy, L.D. Petro, and M. Postman
Space Telescope Science Institute
Baltimore, MD 21218
ABSTRACT
The Hubble Deep Field (HDF) is a Director's Discretionary program on HST in
Cycle 5 to image an indistinguished field at high galactic latitude in four wavelength
passbands as deeply as reasonably possible. In order to optimize observing in the time
available, a field in the northern continuous viewing zone has been selected and images
will be taken for 10 consecutive days, or approximately 150 orbits. Shorter 1í2 orbit
images will also be obtained of the fields immediately adjacent to the primary HDF in
order to facilitate spectroscopic followíup by groundíbased telescopes. The observations
are to be made from 18í30 December 1995, and both raw and reduced data will
immediately be put in the public domain as a community service.
1. INTRODUCTION
The HDF program is an outgrowth of the successful imaging of distant clusters
that was performed with HST by Dressler et al. (1994) for 0939+4713 at z = 0.41, and by
Dickinson et al. (1995) for the cluster(s) associated with the radio galaxy 3C 324 at z =
1.21. Both of these programs demonstrated the ability of the refurbished HST to resolve
galaxy structure at moderate to high redshift in a way that made morphological
classification and a quantitative study of various parameters possible. Cluster 0939+4713
does not look entirely unlike nearby clusters insofar as it is populated largely by apparent
spiral and elliptical galaxies, albeit somewhat disturbed and with evidence for tidal
interactions. It also shows the ButcheríOemler effect. The galaxies associated with 3C
324, on the other hand, are not representative of present day clusters inasmuch as no
spiral galaxies are discernible. A large fraction of amorphous objects populate the
cluster, together with apparent elliptical galaxies. The latter have been measured by
Dickinson to have r 1/4ílaw radial light distributions, commensurate with their being
dynamically relaxed systems.
Since the first servicing mission, HST has imaged a number of other distant
galaxies out to redshifts of z > 3 (cf. Giavalisco et al. 1995), and several things have
become clear. First, HST can indeed resolve galaxyísized systems out to high redshift.
Second, the Universe at high redshift looks rather different than it does at the current
epoch. The fact that HST can image galaxies back at epochs when they were apparently
forming and evolving rapidly is of fundamental importance to our understanding of
galaxy evolution, and it is imperative that this capability be fully exploited.
Based on the current excellent performance of the telescope a decision was made
to devote a substantial fraction of the Director's Discretionary time in Cycle 5 to the study
of distant galaxies. A special Institute Advisory Committee was convened which
recommended to the Director that deep imaging of one 'typical' field at high galactic
latitude be done with WFPC2 in several filters, and that the data be made available
immediately to the community for study. Following this recommendation a working

group of scientists and technical staff at the Institute was formed to develop and carry
out the project.
2. THE FIELD
It had been suggested to the Advisory Committee by the Institute that we think of
utilizing one of the continuous viewing zones of HST for the field selection in order to
gain a factor of two in observing efficiency. The working group focused our attention on
the northern CVZ, thereby constraining the HDF location to a declination of +62o.
Furthermore, to facilitate studies at other wavelengths a field was selected that had no
other bright objects that had previously been detected at any wavelength, nor contained
nearby galaxy clusters. A location of low extinction, low H I column density, small farí
IR flux, and having no radio sources brighter than 1 mJy at 3.6 cm was identified in the
constellation of Ursa Major since this part of the northern CVZ is farthest from the
Galactic plane.
The exact position of the HDF within this general area has been dictated by the
availability of two acceptable guide star pairs for the HST Fine Guidance Sensors. In
order to be conservative in safeguarding the entire sequence of observations we have
required an independent pair of backíup guide stars, and they are scarce at this high
galactic latitude. The precise location of the HDF has therefore been determined by this
requirement. The location and characteristics of the resulting field are given in Table 1,
and a 500 sec Ríband image of the field obtained by P. Eisenhardt with the KPNO 4m
telescope is shown in Fig. 1 with the imprint of the WFPC2 superposed which outlines
the HDF.
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TABLE 1
CHARACTERISTICS OF THE HUBBLE DEEP FIELD
Location: 12h 36m 49.4s
+62o 12' 58" (Epoch J2000.0 / WFPC2 'WFALL FIX' position)
E (BíV) = 0.000
HI column density, N(HI) = 1.7 x 1020 cmí2
In 100² IR cirrus minimum
Low 2² DIRBE flux, < 0.14 MJy/ster
Radio quiet, no sources with flux > 1 mJy at 3.6 cm
No interfering bright stars or nearby galaxy clusters
ííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííí
3. THE OBSERVATIONS
The selection of the filters to be used has been mandated by the belief that as
broad a wavelength interval as possible should be used without sacrificing throughput
inordinately. Also, spatially resolved color information of objects was deemed highly
desirable even though global colors for each object could be obtained from the ground.
Since the faintest 2í3 magnitudes of the images must remain beyond the reach of

groundí based spectroscopy for the foreseeable future, color information is doubly
important in understanding the faintest populations in the images, and perhaps even in
enabling crude redshifts to be determined for them from broadíband colors. We have
therefore selected to image the HDF in four passbands, using the broadíband filters
F300W, F450W, F606W, and F814W. These filters have good throughput while
providing broad color information, and depth may be obtained by combining the images
of the longer wavelength filters.
The number of exposures and total integration time in each of the filters has been
determined partly by conditions that prevail in the CVZ and partly by the desire to
achieve a similar limiting magnitude in all of the passbands. The line of sight in the CVZ
is never far from the earth's limb and therefore the daylight half of the HST orbit
experiences higher scattered background light, compromising those exposures. However,
the lower throughput of F300W always causes images with this filter to be readínoise
limited in any event, even in the bright part of the orbit, and so the observations taken in
bright sun are devoted almost entirely to periodic dark frames and the images in F300W.
The images obtained in earth shadow are fairly evenly divided among the three other
filters. Thus, the use of the CVZ with its higher scattered light background, especially in
the daylight half of the orbit, enables images which are readínoise limited to be obtained
gratis since that part of the orbit could not be used to improve upon the S/N of images
obtained in the other filters.
As a result of detailed study of the possible distribution of exposures among the
various filters and the consequent S/N ratios of the images, an observing schedule has
been established. Table 2 lists the equivalent number of orbits to be devoted to each of
the four filters, and the total number of exposures in each filter. We also list the
approximate limiting AB magnitude (defined as a flux 10s > 20 pixels of sky) achievable
in each of the passbands if all of the images in that filter are stacked together.
______________________________________________________________________
TABLE 2
WFPC2 HDF EXPOSURES
Filters: F300W F450W F606W F814W
No. Orbits 49 36 35 35
No. Exposures: 100 62 77 49
Limiting AB mag: 27.6 28.1 28.7 28.0
ííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííííí
A dithering scheme will be implemented in which the exposures for each
passband are to be obtained at 9 different x,y positions within a 2 arcsec square, with the
separations being of noníinteger pixel size. At each of the 9 positions more than five
separate exposures will generally be taken so cosmic ray rejection can be accomplished
satisfactorily. The dithering permits noníuniformities in the CCD's with spatial scales
less than the dither interval to be corrected for, and it also allows critical sampling of the
data at subípixel scales so that higher spatial resolution may be achieved by image
reconstruction.
The advisory committee had called attention to the wisdom of obtaining short
WFPC2 images of the sky immediately adjacent to the HDF in order to support
spectroscopic study of the field, inasmuch as most of the groundíbased follow up will be

performed using either long slits or fiber bundles which could, as a byíproduct of study of
the HDF, coincidentally acquire the spectra of objects immediately surrounding the HDF
proper. We have therefore created a mosaic of WFPC2 positions that will be used to
image the area of the sky adjacent to the HDF. Eight 'flanking fields' will each be imaged
for 1í2 orbits in filter F814W as part of the HDF 13íday campaign, with the intent of
achieving in each image a limiting flux, mr = 26, that is roughly the limit for which an 8í
10m telescope can do spectroscopy.
Parallel observations are being made with the Faint Object Spectrograph during
the primary WFPC2 observations of the HDF. The FOS observations are being made as
part of a Cycle 5 TACíapproved program GO 5968 to use deep WFPC2 images to
measure the extragalactic background light, using simultaneous FOS observations of the
scattered solar spectrum in the Mg I b feature at 5175 A to subtract out the contribution to
the EBL from the zodiacal light. The FOS data are also being used by Institute staff to
model the scattered light in the CVZ, which will be important to correct for when
performing longíslit spectroscopy with STIS.
4. CALIBRATION AND DATA REDUCTION
The Institute plans to provide both raw and calibrated data for the HDF to the
community as a service. Calibration frames necessarily include biases, darks, and flat
fields. Because biases and flats are quite stable over time for the WFPC2, HDF
'superbias' and earth 'superflat' calibration frames are being assembled from calibration
frames that have been acquired over the period prior to the HDF campaign, including
some that will be taken immediately prior to the commencement of the HDF
observations. The flat fields will consist of earth flats, which have high signalítoínoise
and correct for large spatial variations in the CCDs, and preílaunch data from thermal
vacuum tests which are valid for correction of pixelítoípixel variations. Sky flats would
be even better to use to flatten the HDF data, however they do not have sufficient signalí
toínoise to improve upon the earth flats. Dark frames, by contrast, show changes with
time due to the emergence of hot pixels which are caused by cosmic ray hits. The
characterization of hot pixels therefore requires contemporaneous dark frames, and these
are planned periodically during the observations. A 'superdark' calibration, which will be
appropriate for subtraction of that component of the dark current which is invariant over
shorter time scales, is being assembled from darks obtained in the months prior to the
HDF campaign.
Each HDF image will be reduced in a manner that is similar to that used in the
normal STScI pipeline. Cosmic rays will be rejected by median filtering via a version of
the 'CRREJ' routine. All of the images that have been taken at the different dither
positions in each passband will be registered and combined to produce one deep image
for each of the filters. These final images will be made available over the internet as
soon as possible after the campaign has been completed, as will the resultant color image
of the HDF.
5. FOLLOW UP STUDIES
Based on number counts of galaxies from previous studies, of the order of 500
galaxies per WF chip are expected to be seen in the HDF down to about mr ~ 29.
Interpretation of the HDF data will benefit greatly from follow up studies at other
wavelengths and from groundíbased spectroscopy. Already, observations of the HDF
from space are being considered by ISO and ROSAT, and the science team of NICMOS
has made IR study of the field an important component of their GTO program when this

instrument is installed in HST. Groundíbased IR observations have already been
scheduled on three large telescopes, and an extensive spectroscopic program is being
undertaken on the HDF with the Keck 10m telescope. Time variability of objects in the
HDF will be studied with further HST observations in Cycle 6 as a TACíapproved GO
program, and the VLA plans to fully map the field to low radio flux limits.
The fact that objects within the faintest 2í3 magnitude interval of the HDF are not
likely to be reachable with spectroscopy signifies that for many (most?) of the objects in
the images, distances may have to come from some other means than the determination
of the radial velocity of the object. This fact should give impetus to the determination of
approximate redshifts of galaxies from broadíband colors in the future, which will require
a better knowledge of the evolution of the spectral energy distributions of galaxies than
we have today.
The Hubble Deep Field observations will undoubtedly capture images of faint
objects that populate the solar system, the halo of the Galaxy, and distant galaxies.
Subsequent study of such objects should cause this data set to be invaluable to our
understanding of phenomena that occurred at early epochs in the formation of the solar
system and galaxies.
EPILOGUE
Inasmuch as the HDF observations were executed soon after the HST Science
conference was held, we are able to present in these proceedings in Fig. 2 the combined
image of part of the field that results from coíadding the final registered frames in filter
F606W.
REFERENCES
Dickinson, M.E., et al. 1995, in preparation
Dressler, A., Oemler, A., Sparks, W.B., & Lucas, R.A. 1994, ApJL, 435, L23
Giavalisco, M., Macchetto, F.D., Madau, P., & Sparks, W.B. 1995, ApJL, 441, L13
________________________________________________________________________
Fig. 1 íí A 500ísec red image of the HDF taken by P. Eisenhardt with the KPNO
4m telescope. The outline of the WFPC2 is shown in its orientation
when the HDF is imaged in December 1995.
Fig. 2 íí The combined image of the HDF obtained with the HST WF3 chip by coíadding
all 77 exposures of the field taken with filter F606W.