Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.naic.edu/~tghosh/stuff/proposal.ps Äàòà èçìåíåíèÿ: Mon Mar 5 09:47:18 2012 Äàòà èíäåêñèðîâàíèÿ: Sun Apr 10 03:50:20 2016 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: arp 220

Going Deeper with the Arecibo 1 to 10 GHz Spectral Scan of Arp 220
1. Scientific Justification
Cm­wavelength studies of the molecular ISM in extragalactic objects have seen a resurgence recently. Molecular line
searches at these frequencies are relatively confusion free, and can be successful in finding heavier molecules.
Our recent Arecibo spectral survey of Arp 220 (Salter et al. 2008, AJ, 136, 389) included the first ever radio detections
outside of the Local Volume of the v 2 = 1 lines of HCN, the prebiotic molecule methanimine (CH 2 NH), and methanol
(CH 3 OH) (Figure 1). In addition, hydrogen radio recombination lines (RRLs) were detected down to 3 GHz (Figure
1), considerably lower in frequency than any previously measured in this object (e.g. Anantharamaiah et al. 2000,
ApJ, 537, 613) and constraining models of the ionized gas in the galaxy (Salter et al. 2012; in prep.) The spectrum
of the co­added RRLs between 3 and 4 GHz reached an RMS noise level of 50 µJy/beam.
4800 000 00 400 00 800 000
0 00
0 000
0 00
0 00
0 00
0 004
4500 5000 5500 6000
Heliocentric Velocity (km/s)
­0.010
­0.005
0.000
0.005
Fractional
Absorption
CH OH: 5 - 6 A +
1 0
4500 5000 5500 6000
Heliocentric Velocity (km/s)
0.000
0.200
0.400
0.600
Intensity
(mJy/beam)
Fig. 1.--- Examples of lines seen in Arp 220: (left) methanimine (CH 2 NH); (center) methanol (CH 3 OH); (right)
co­added radio recombination lines between H119# and H127# (3172.9 -- 3853.7 MHz)
A follow­up Arecibo study of 19 ``Arp220­like'' galaxies focussed on the frequency range of the C­band receiver,
allowing us to observe simultaneously one of the HCN (v 2 = 1) lines, CH 2 NH, H 2 CO, and the 6­cm excited­OH
transitions. Two galaxies, IC 860 and Zw 049.057, stand out from these observations for their great similarities to
Arp 220. Our observations of this pair of galaxies (Minchin et al. 2009, BAAS, 41, 328) demonstrate that both
contain all the lines listed above, plus confirming the H 2 CO 4829­MHz masers previously seen in these galaxies. We
also discovered a H 2 CO 4954­MHz maser in IC 860. As for Arp 220, the absorption line ratios of the excited­OH
lines appear consistent with that expected for thermal equilibrium. This project has also discovered a remarkable
spectral line/continuum outburst within the galaxy NGC 660.
In view of the exciting discoveries derived from our initial Arp 220 spectral scan and the above follow­up project, we
here propose improving the sensitivity of that survey by a factor of two in order to, (1) confirm several tentative (3 to
5#) detections found in the existing data, (2) explore the likely possibility of again detecting unexpected molecular
species, and (3) extend our RRL investigations to attempt detection of Hen# and Cn# lines, as well as of Hn#
emission, allowing the investigation of chemical abundances and physical conditions within the central region/s of
the galaxy.
The molecular line detections from our first spectral scan of Arp 220 are listed in Table 1. In addition, a number of
tentative detections were made which, while promising, were below the 5­# level that we require for a sure detection.
These tentative detections include; excited­CH main­lines in the 2 # 3/2 ladder at 4847 and 4870 MHz (recently also
detected by us in NGC 660); excited­CH emission in the 2 # 1/2 ladder at 7275 MHz; H 2 CO emission at 4954 MHz (as
we have detected in IC 860); and HC 3 N at 9097 MHz. All these would be confirmed (or rejected) by the proposed
new observations.
Spectral scans exist at #1.3 mm for Arp 220 (Martin et al. 2011, A&A, 527, 36) and Orion A (Sutton et al. 1985,
58, 341). We have considered the intensity distribution of molecular lines in these surveys at intensities well above
the ``line confusion level'', which is the regime applicable to cm­wave spectral scans. Given the 26 molecular­line
detections from our earlier Arp 220 survey, we would expect the proposed deeper survey to detect 35 ± 7 additional
molecular lines. The appropriately weighted co­addition of transitions of the same molecular species is expected to
add to this number.

Table 1: Molecular Transitions seen in Arp 220 (Salter et al. 2008)
Molecule Transition Rest Frequency Molecule Transition Rest Frequency
(MHz) (MHz)
OH v=0 J=3/2 #3/2 F=1--2 1612.23 18 OH (a) 2 # 3/2 J=3/2 F=2--2 1639.50
v=0 J=3/2 #3/2 F=1--1 1665.40 HCOOH (a) 1(1,0) -- 1(1,1) 1638.80
v=0 J=3/2 #3/2 F=2--2 1667.36 HCN v 2 =1 #J=0 J=3 2693.34
v=0 J=3/2 #3/2 F=2--1 1720.53 v 2 =1 #J=0 J=4 4488.47
2 # 1/2 J=1/2 F=0--1 4660.24 v 2 =1 #J=0 J=5 6731.91
2 # 1/2 J=1/2 F=1--1 4750.66 v 2 =1 #J=0 J=6 9423.33
2 # 1/2 J=1/2 F=1--0 4765.56 CH 3 OH 5 1 - 6 0 A + 6668.52
2 # 3/2 J=5/2 F=2--3 6016.75 CH 2 # 1/2 J=1/2 F=0--1 3263.79
2 # 3/2 J=5/2 F=2--2 6030.75 2 # 1/2 J=1/2 F=1--1 3335.48
2 # 3/2 J=5/2 F=3--3 6035.09 2 # 1/2 J=1/2 F=1--0 3349.19
2 # 3/2 J=5/2 F=3--2 6049.08 CH 2 NH l 10 --l 11 #F=0 ± 1 5289.81
2 # 1/2 J=3/2 F=1--1 7761.75 H 2 CO 1(1,0) -- 1(1,1) 4829.66
2 # 1/2 J=3/2 F=2--2 7820.12 6(2,4) -- 6(2,5) 4954.76
2 # 1/2 J=5/2 F=2--2 8135.87
(a) These entries correspond to the same spectral line, whose identification is ambiguous.
Crucial physical parameters for the interstellar media in this star­forming, IR­luminous galaxy could be derived from
lines detected through our proposed observations;
(1) Kinetic Temperature: The H 2 CO 4954­MHz transition detected in emission for IC860, and probably also for
Arp220, suggests that the 7(2,5) -- 7(2,6) line at 8884.82 MHz, and the 5(2,3)--5(2,4) line at 2483 MHz may also be
detectable. The ratio of these para lines to the para lines in the 0(0,0), 1(0,1), 2(0,2), etc. ladder (e.g. Martin et
al. 2011, A&A, 527, 36) can be used to derive the kinetic temperature of the emitting medium. (However, we note
that the frequency range around 2483 MHz is heavily interfered at Arecibo, leaving only the C and X­band lines
observable.)
(2) Age: Propadienylidene, l­H 2 C 3 , a linear version of c­C 3 H 2 , is a pre/postcursor of PAHs and possibly a molecular
clock. Likely transitions to be searched for are at 1.154, 2.308, 3.847, 5.770 and 8.078 GHz (Vrtilek et al. 1990,
ApJL, 364, L53).
(3) Evolutionary State: Isocyanic Acid (HNCO) abundances in galaxies have been shown (Martin et al. 2009,
ApJ, 694, 610) to be a good tracer of the evolutionary stage of the nuclear starburst regardless of the presence or
absence of an AGN in the system. At the early stage of the star­burst phase, a large amount of HNCO is injected
into the gas phase due to shocks, but with the onset of the massive UV flux from the newly formed stars, these
molecules get photodissociated (giving rise to CS molecules). Within the requested frequency range, there are a
number of multiplets of HNCO that could be employed for the above purpose. In addition, the HCN/HNC line ratio
is considered to be an indicator of starburst evolution (Aalto et al. 2002, A&A, 381, 783), and four HNC (v 2 = 1)
transitions occur within the frequency band of our search. As we have already detected a number of transitions of
HCN (v 2 = 1) in Arp 220 (Figure 2), the equivalent HNC lines (of somewhat lower excitation energy!) are clearly an
important set of transitions to search for. Co­addition of transitions could be highly relevant for this HNC search.
2. Technical Details
As all radio lines detected by us in Arp 220 (molecular lines and RRLs) are at least 200 km s -1 wide, we intend to
employ the WAPP spectrometer in ``8­board mode'' with 100­MHz bandwidth per board over the entire spectrum.
With Hanning smoothing, this gives a velocity resolution for 13 to 1.5 kms -1 between 1.1 and 10.0 GHz respectively.
For the new observations we intend to use the same WAPP set­up as for our earlier Arp­220 survey. This means
that we can combine the new observations with the previous set, saving 25% observing time. We would use dual

4500 5000 5500 6000
Heliocentric Velocity (km/s)
­0.015
­0.010
­0.005
0.000
0.005
Fractional
Absorption
4500 5000 5500 6000
Heliocentric Velocity (km/s)
­0.020
­0.015
­0.010
­0.005
0.000
0.005
Fractional
Absorption
4500 5000 5500 6000
Heliocentric Velocity (km/s)
­0.030
­0.020
­0.010
0.000
0.010
Fractional
Absorption
4500 5000 5500 6000
Heliocentric Velocity (km/s)
­0.003
­0.002
­0.001
0.000
0.001
0.002
0.003
Fractional
Absorption
HCN v =1, J=0
D
2 J = 2
J = 5
J = 4
J = 6
Fig. 2.--- The first astronomical detections of the v 2 = 1 direct l­type absorption lines of HCN with vibrational levels
J=4, 5 and 6 (at 4488, 6731 and 9423 MHz respectively). Although not shown here, the J=3 line is also detected.
Non­detection of the J=2 level (at 1346 MHz) is believed to be due to foreground free­free absorption. The velocity
resolution is #30 km s -1 .
polarization on each board, permitting 4096 channels per 100­MHz bandwidth, a channel width of 24 kHz. To permit
recording of the maximum number of RRLs possible, while also making a full molecular line search, we propose a
full spectral coverage of the bands of the LBW, SBW, SBH, CB, CBH and XB receivers. In total, we will need
14 frequency settings to cover the frequency range 1.1 to 10 GHz. Experience has shown that the Double Position
Switched (DPS; Ghosh & Salter 2002, ASP Conf. Proc., Vol. 278, 521) observing mode needs to be used to avoid
standing­wave e#ects on the spectral baselines due to the continuum emission from Arp 220 itself. We plan to spend
6 hr per frequency setting, in additional to the previous average of 2 hr per setting. Table 2 presents the velocity
resolution per spectrometer channel, the anticipated rms noise after Hanning smoothing, and the rms noise after
smoothing to a velocity resolution of 30 km s -1 for selected frequencies across the total frequency range to be covered.
The table shows that for a 30 km s -1 velocity resolution, noise levels between 60 and 120 µJy beam -1 are expected
over the entire frequency range covered.
Including 15% overhead for slewing, set­up and calibration, we request a total of 96 hours.
Table 2: Observational Details
Freq Rx name Vel. per Chan. # Hanning # 30 km s -1
(MHz) (km s -1 ) (mJy beam -1 ) (mJy beam -1 )
1175 LBW 6.2 0.19 0.12
1400 LBW 5.2 0.14 0.08
1666 LBW 4.4 0.13 0.07
2600 SBW 2.1 0.20 0.09
3500 SBH 2.1 0.19 0.07
5000 CB 1.5 0.19 0.06
7000 CBH 1.1 0.29 0.08
9000 XB 0.8 0.40 0.09