Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.naic.edu/~tghosh/oct07/gbt-sb-mol-v3.ps Äàòà èçìåíåíèÿ: Mon Oct 1 22:30:42 2007 Äàòà èíäåêñèðîâàíèÿ: Sun Apr 10 12:45:55 2016 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: arp 220

A GBT Cm­wave Search for Prebiotic & Other Molecules in Arp 220­like Galax­
ies
Scientific Justification
According to current views, interstellar organic species mostly form on the surface of dust
grains, and heating events, such as the formation of a protostar, release their icy mantles into
the gas phase 1,2 . Once released, these molecules may form amino acids by the combination
of organic species known as ``pre­biotic'' molecules. Methanimine (CH 2 NH) is one such
molecule 3 which can form the simplest amino acid, glycine 4 (NH 2 CH 2 COOH), either by
(i) first combining with hydrogen cyanide (HCN) to form aminoacetonitrile (H 2 NCH 2 CN),
with subsequent hydrolysis (Strecker Synthesis 5 ), or (ii) directly combining with formic acid
(HCOOH) 6 . Methanimine has been detected in the ISM 7,8 and tentatively in the nearby
galaxy, NGC 253 9 , but has never been seen beyond the neighborhood of our Galaxy before
(i.e. beyond #5 Mpc).
HCN is a well known indicator of high gas density. Recently, Gao & Solomon (2004) 10
demonstrated that a very strong linear correlation exists between the luminosities L IR and
LHCN for mm­wave transitions of HCN. This extends over a wide range of L IR running from
normal galaxies to Ultra Luminous IR Galaxies (ULIRGs). A similar L IR -- LCO correlation
has a less linear form, marking out LHCN as the best tracer of dense molecular gas mass
in galaxies, and hence of active star­forming gas. However, the smooth continuation of the
L IR -- LHCN correlation to (U)LIRGs has recently been contested by Papadopoulos et al.
(2007) 11 .
At Arecibo, we are well advanced towards completion of a spectral­line survey of Arp 220
between 1.1 and 10 GHz. Arp 220 is the prototypical megamaser/starburst galaxy, in which
both maser emission and absorption are seen in the four #18­cm transitions of the OH
radical 12,13 . Molecules such as CO (14) , formaldehyde 15 , ammonia 16 , and mm­transitions of
HCN (17) have been detected with large velocity widths in this galaxy. At a distance of
#77Mpc, it is also the nearest ULIRG, in which most of the IR luminosity arises from a
powerful, dust­enshrouded starburst, believed to be triggered by the merger of two gas rich
galaxies 18 . Evidence for this is provided by its huge supernova rate as found from recent
high resolution VLBI studies 19 .
Analysis of the Arecibo data taken so far on Arp 220 reveals a rich molecular spectrum,
including lines of methanimine, and three v2=1 direct l­type transitions of HCN (20) from
the J=4, 5 and 6 vibrational levels (Fig 1). Methanimine is seen in emission, while the
HCN lines appear in absorption against the continuum emission of Arp 220; we note that
these particular HCN transitions seem not to have been previously detected in any celestial
source. Within the frequency range of 4.4 to 5.2 MHz, in addition to methanimine and one
of the above HCN transitions, we also confirm previously reported formaldehyde emission,
and excited­OH absorption lines, and obtain the probable detection of emission lines of CH
and formamide (NH 2 CHO). In addition to these (and other spectral lines found in the bands
so far observed), we have detected a strong L­band absorption line that may be due to the
pre­biotic molecule, HCOOH, although there is ambiguity with a neighboring line of 18 OH.
The origin of life is still an open question. Discovery of ``pre­biotic'' molecules in a far­
o# galaxy with a high star formation rate adds a new dimension to considerations of this

5280 5285 5290 5295 5300
Rest Frequency (MHz) for Vhel = 5373 km/s
­0.001
­0.000
0.001
0.002
0.003
0.004
Flux
Density
(Jy)
Methanimine multiplets in emission
4500 5000 5500 6000
Heliocentric Velocity (km/s)
­0.015
­0.010
­0.005
0.000
0.005
Fractional
Absorption
HCN J=4, direct l-type transition at 4488 MHz
Figure 1: Newly detected C­band transitions of Methanimine (left) and HCN (right) in Arp 220. The
methanimine spectrum is plotted against rest frequency for z = 0.018126 with the 6 hyperfine lines of this
transition marked as vertical lines. The HCN v2=1, J=4 line is plotted against radial velocity. The presence
of at least 3 components in this HCN line is also seen in the HCN v2=1, J=5 and 6 lines.
problem. Hence, we now plan to extend our Arp 220 study to other ``Arp 220­like'' galaxies,
aiming particularly at investigating the occurrence of pre­biotic molecular species in such
objects. For this, we have selected a sample of sources from the lists of (i) all OH­Megamaser
sources (as compiled by Chen et al. 21 ), (ii) starburst sources where formaldehyde has been
detected via maser emission or absorption in the C­band 15 , (iii) ULIRGs with direct evidence
of nuclear starbursts 22 and (iv) FIR­luminous merging galaxies 23 . We note that Arp 220 is
common to all these lists. Applying the criteria of z < 0.043, and S 1.4GHz > 50 mJy, our
sample consists of 40 sources. Of these, 21 are visible from Arecibo and, were recently granted
telescope time (Jun07 proposal deadline). In the present proposal, we request observing time
to cover the other 19 galaxies that are outside the ``Arecibo sky''. In our source selection, the
redshift and the flux­density restrictions were imposed to allow the detection of emission lines
having similar strength to those we find in Arp 220, and to also have su#cient continuum
flux density against which to detect absorption lines such as we see in Arp 220.
Our aim here is to conduct a complimentary GBT search in the (rest­frame) frequency range
of 4200 -- 5400 MHz, covering transitions of HCO + , HCN, NH 2 CHO, excited­OH, H 2 CHO,
CH, and CH 2 NH which lie at rest frequencies of 4234, 4489, 4619, 4660--4765, 4829, 4848,
and 5289 MHz respectively in the other nineteen, Arp220­like galaxies.
Any HCN and HCO + detections will be followed up with further proposals to observe the
J=2, 3, 5 and 6, v2=1 direct l­type lines using other receivers. This will enable us to estimate
densities and temperatures in the emitting gas. In addition, we will search for correlations
of the molecular constituents we measure with megamaser activity, total radio emission, the
presence of an AGN, etc.
The lines we have detected in Arp 220 in our spectral survey are similar to those detected
from hot cores in Giant Molecular Clouds in our own Galaxy. However, we know of no
previous cm­wave detection of the v2=1 direct l­type transitions of HCN in such sources.
Also, while methanimine has mostly been detected in the mm­wave range, only very few
measurements have so far been made, and the C­band multiplet has only been seen towards
Sgr B2 7 . Hence, for a direct comparison with the measurements of this sample of starburst

galaxies that we propose to observe here, we would also like to make an identical (2.5 hr,
see below) observation of the Sgr B2 (R.A.=17 h 47 m 20.4 s ; Dec=­28 # 23 # 05 ## ) region. In the
companion Arecibo proposal, we will be observing the W51 region, since Sgr B2 is not visible
from Arecibo.
Technical Details
For the criteria detailed above, our ``GBT sample'' contains 19 galaxies, plus Sgr­B2 (Ta­
ble 1). We will be using the C­band receiver, and the ACS spectrometer in frequency switch­
ing mode. The required spectrometer configuration will be ``nwin=2, bandwidth=800 MHz,
nchannel = high, spectral level =3''. With this, we will have a basic velocity resolution of
#12 km/s (for a fictitious line at 4.9 GHz), and 4096 channels per frequency window. With
frequency switching and a total integration time of 2.5 hrs per source (including both the
phases of a frequency switching cycle), we will have rms noise per channel of 0.2 mJy/beam.
For Arp 220, we found linewidths of a few hundreds of km/s, indicating large velocities in its
ISM. Hence it was possible to smooth the spectra to velocity resolutions of up to 40 km/s
without loss of information. Application of similar smoothing will result in noises down to
#0.1 mJy/beam, similar to that for the objects in the Arecibo sample. This will allow detec­
tion at the 3­# level of absorption with optical depth of <1% in those targets that have the
weakest continuum emission (40 mJy at C­band). Including a slewing overhead and set­up
time of 10%, we request a total observing time of 55 hr.
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Table 1: The Source Sample
Name R.A.(B1950) Dec.(B1950) Redshif t(z) S 6cm (mJy) Ref.
NGC 0034 00 08 33.40 --12 23 08.9 0.019617 45 2
NGC 253 00 45 05.74 --25 33 39.6 0.000811 2500 3
IC 1623 01 05 19.9 --17 46 26 0.020067 96 2
NGC 1068 02 40 07.05 --00 13 31.64 0.003793 1900 3
NGC 1365 03 31 41.802 --36 18 26.55 0.005457 200 3
NGC 1614 04 31 35.8 --08 40 57 0.015938 63 1,2
UGC 03351 05 41 24.59 +58 40 52.3 0.01486 46 3
UGC 05101 09 32 04.78 +61 34 37.0 0.039367 76 2,3
M 82 09 51 42.4 +69 54 59 0.000677 3900 1
Mrk 171 11 25 42.1 +58 50 18 0.010411 398 1,2
Fairall 1151 11 50 39.87 --38 51 07.1 0.010781 56 3
NGC 4194 12 11 41.3 +54 48 10 0.008342 40 1
Mrk 231 12 54 05.01 +57 08 38.2 0.04217 400 1,2,3
Arp 238 13 13 39.6 +62 23 26 0.030831 45 1,2
NGC 5256 13 36 14.73 +48 31 50.1 0.027863 48 2
Mrk 273 13 42 51.7 +56 08 13.59 0.03778 99 1,2
NGC 5861 15 06 32.62 --11 07 54.1 0.006174 400 3
Arp 293 16 57 44.64 +59 00 37.6 0.018349 50 2
IRAS 17208­0014 17 20 47.87 --00 14 15.87 0.042810 60 3
1: ``Arp 220­like'' FIR galaxies, Armus et al. 1990
2: Compact Starburst in ULIRGs, Condon et al. 1991
3: OH­Megamaser sources, Chen et al. 2007