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The Arecibo Galactic Chemistry Survey
Introduction The study of complex molecules in space is crucial for understanding the physical and chemical processes in the interstellar medium (ISM), as well as the origins of life. To date, over 140 molecules have been identified in space. Some of these are quite complex (containing as many as 8 to 13 atoms) and are considered "pre-biotic" molecules ннspecies thought to be important as key precursors in the reaction chains that produce simple sugars and amino acids, essential in protein creation. It has been suggested that chemical processes in the ISM provided the necessary material that allowed the emergence of life on Earth (Maeda & Ohno 2006). A thorough chemical inventory of star forming regions is therefore needed to study the connection between life and space. Moreover, the detection and identification of complex molecules in different environments in the ISM are essential for better understanding their possible formation (and destruction) pathways, as well as for constraining chemical models (Ikeda et al. 2001). Unbiased spectral line surveys provide essential information needed for characterizing the physical and chemical conditions of a region, and the relatively unexplored spectral range between 1 and 10 GHz has the potential to provide new and exciting (as well as unexpected) discoveries. Many different kinds of complex molecules have spectral lines in the range between 1 and 10 GHz. Table 1 presents a sample of previously detected spectral lines between 1 and 10 GHz. Transitions from small polycyclic aromatic hydrocarbons (Thorwirth et al. 2007), pre-biotic molecules, and even the simplest amino acid, glycine, fall in this range. In addition, cm-wave observations trace different excitation conditions and types of transitions ннessential for our understanding of the column density, abundance and excitation of the detected species. Complete submm spectral line surveys of a few sources will be an important part of the Herschel Space Telescope guaranteed time key projects, and similar (sub)-mm spectral surveys are being planed with ALMA (e.g., Gerin et al. 2001; Guщlin et al. 2006). It is now clear that interpretation of these high-frequency observations will be challenging as line confusion (due to a forest of unidentified lines) will dominate certain regions. Sensitive, low-frequency, spectral line surveys do not suffer from line confusion and thus they are essential for obtaining a thorough chemical inventory and to provide complementary information that may not be accessible to the high-frequency telescopes. An example of the exciting (and unexpected) results that a cm-wave survey may provide was given by the recent Arecibo Observatory (AO) observations of the ultra luminous infra-red galaxy (ULIRG) Arp 220 taken by members of our team using (and commissioning) the dual-board mode of the WAPP spectrometer (Salter et al. 2008). The data reveal a spectrum rich in molecular transitions, including clear detections of the "pre-biotic" molecules methanimine (CH2NH) нн a molecule thought to be important for the formation of glycine (e.g., Dickerson 1978) нн and HCN. Since then, these and other pre-biotic species have been detected in other starbust galaxies as part of an on-going cm-wave survey at the Arecibo 305 m telescope and the GBT (Minchin et al. 2009). Just as exciting, the Arecibo observations by Kalenskii et al. (2004) and Araya et al. (2003) of the TMC1 cold dark cloud and the asymptotic giant branch carbon star IRC +10ђ216, respectively, produced detections of lines of several molecules, both complex and simple. These authors used their multi-molecular line data to investigate the chemical richness of the regions, estimate the temperature of the gas, and study the formation mechanism of molecules. These studies exhibit very interesting results and show the need for further (more sensitive) studies at 1-10 GHz for understanding the physical and chemical conditions in the ISM. Update on the Pilot Survey (A2350) Inspired by the success of the Salter et al. (2008) study, in October 2007 we proposed to carry out a pilot spectral survey of Galactic sources at the AO using the Mock spectrometers in single-pixel mode (with 1GHz bandwidth). The project (A2350) was given an A-grade. Unfortunately, these spectrometers were not in time for the Oct.-Nov. 2008 observations, and we had to use the WAPPs, providing a maximum bandwidth of about 340 MHz for the required frequency resolution. This forced us to substantially cut back on our integration time per band. We therefore did not achieve the required sensitivity to detect complex pre-biotic molecules (see below). Regardless of the set backs we have detected CH3OH, H2CO, H213CO, H3CN, and OH in the Galactic anti-center sources that formed the targets of our pilot search (see Figure 1).


Goals and Methodology Here we propose to carry out the first set of observations of a radio line survey, which we name the Arecibo Galactic Chemistry Survey. Our main goals are to: 1) produce a molecular line inventory of the observed regions; 2) study the physical and chemical conditions of the sources; and 3) search for new interstellar molecules and for previously undetected transitions. We will use the wideband Mock spectrometers, which are scheduled to be operational in single-pixel mode within a few months. These spectrometers are capable of providing 5.2 kHz-wide channels over a 1 GHz bandwidth. Thus, they make highly sensitive, complete 1-10 GHz line searches of Galactic sources feasible at the AO, even in sources requiring high-velocity resolution (<1 km s-1). The unmatched sensitivity and angular resolution of the world's largest single-dish radio telescope, along with its new wide-band, high-resolution, spectrometers makes the AO the perfect instrument to carry out our proposed unprecedented project. In this first run we aim to study one hot core, W51 IRS1. Hot cores are compact (d < 0.1pc), dense (n >106 cm-3) and hot (T ~ 200 K) regions that harbor massive protostars and are very chemically rich (Kurtz et al. 2000). The W51 region has been the target of successful mm-wave searches for large molecules (e.g., Remijan et al. 2003; 2004). This source is at ~7 kpc and thus can also be used to study foreground clouds through absorption lines along the lines of sight, against its bright continuum emission (e.g., Neufeld et al. 2002). This region is an important target in the Herschel Space Telescope key project PRISMAS: PRobing InterStellar Molecules with Absorption line Studies. Our cm-wave line survey will complement and help in the interpretation of the Herschel sub-mm study. A strong continuum sources is also favorable for our study as it can slightly amplify low-energy transition lines of complex molecules through "weak masing" (e.g., Chengalur & Kanekar 2003). We note that W51 was included in our original (A-graded) proposal, but the ASAC did not allow us to observe it because it is in the Galactic center quadrant ннa region committed to PALFA and I-GALFA. The latter of these two projects will be completed this summer, so time will become available for us to be able to observe the proposed source. We will use our data to build a molecular line catalog of the source, and will search for previously undetected complex (and pre-biotic) ISM molecules, such as glycine and small PAHs, that are hard to detect in the mm regime due to line confusion or because they yet lack a measured mm wavelength transition (e.g., Thorwirth et al. 2007). We will detect many radio recombination lines, which we will use for studying systematic trends between molecular species and the ionized gas (e.g., infall signatures). Rotational diagrams using several line transitions of the same molecules will be used to obtain their column density (and relative abundance) and rotational temperature (e.g. Goldsmith & Langer 1999). By comparing our results to chemical models we will also be able to study the formation mechanism of complex molecules in our regions of interest (e.g., Ikeda et al. 2001; Pardo et al. 2007). We will expand our target list in up-coming calls for proposals in order to build a statistically sound sample of chemically active galactic sources. Technical Details The molecule HCOOH is a complex species found in hot cores, with a typical column density of ~ 1015 to 1016 cm-2 (other complex molecules have similar abundances in chemically active regions, see, e.g., Remijan et al. 2004; Requena-Torres et al. 2006). Assuming a temperature of 100K, the expected line strength of the HCOOH (211-212) at 4.9 GHz would be about 0.03 to 0.1 K. Using HCOOH as an example, we would then need to achieve an rms of about 6 to 10 mK to convincingly detect complex (pre-biotic) molecules. The HCOOH line at 4.9 GHz was detected in Sgr B2 by Winnewisser & Churchwell (1975) who measured an antenna temperature of ~ 0.040 K and a line width of ~ 390 kHz (24 km s-1). This suggests that searching for this transition would mean examining spectra as smoothed in frequency by a number of steps from the chosen 5-kHz resolution of the spectrometer to about 200-kHz. The hot core, W51, is a strong continuum emitter, and we plan to employ modified Double Position Switched (DPS) observations, as optimized for the recent Arp 220 observations (A2234) to minimize baseline ripples. For this, each 5-min ON/OFF position-switched observation on a target is preceded (or followed) by a 1-min ON/OFF on a nearby strong (here >>1 Jy) continuum source, which acts both as a band-pass calibrator, and provides an independent spectral check for RFI and system artifacts such as trapped modes.


This strategy has been vital for validating detections in Arp 220. One DPS cycle requires ~ 14 min (including 1 min for waiting between ON and OFF of target and reference and ~1 min for the calibration cycle.) For these observations, we propose employing the new "Mock" spectrometer in its "single-pixel" mode. For each "Mock box" (14 in total) there are ~14450 independent channels across a bandwidth of 300/2n MHz, where 0 n < 10. We present a time budget based on n=2 for > 4 GHz (5-kHz channels), n=3 for 1.8 н 4 GHz (2.5 kHz channels) and n=4 for <1.8 GHz (1.25-kHz channels). We note that this is likely to be the first regular proposal to be scheduled using the single-pixel Mock mode, and we would expect the experiment to shake down/commission the new capability and its integration with the CIMA user interface. Having a number of Arecibo staff members on this proposal facilitates this aspect of the project. Table 2 lists the observational parameters for each frequency band. Allowing for the continuum brightness of W51 (from project A2334 this is 19 Jy/beam at 5 GHz) the required "on+off" duration on W51 to obtain a 5- detection of HCOOH would be 5.1 hr (7.2 hr of DPS) for half the width given by Winnewisser & Churchwell (1975). Requesting a similar integration time for each frequency setting of the spectrometer, a total of 112 hr of observing is requested, including 10% for slewing, set-up, and other overheads.

Arecibo Observatory molecular line detections in the high-mass star forming region NGC2264
0.016

0.014

H213CO

CH3OH

HC3N

0.012

0.010

0.008

4592.5

4593.0 Frequency (MHz)

4593.5 Frequency (MHz)

Figure1: Sample molecular lines detected during the Oct.-Nov. 2008 Arecibo Observatory o bserving run of pro ject A2350. The lines sho wn here were detected to wards NGC2264, a high-mass star-forming regio n.

Table 1 н Sub-sample of Detected Molecular Lines in the 1-10 GHz Range
Molecule Name CH3CHO CH2CHCN NH2 CHO CH3OCHO HCOOH OH HC5N H2CS CH3CHO CH NH2 CHO OH H2CO HCOOH CH3OH CH2NH HC5N OH Res t Fr eq. Q uantum [M Hz] Tr ans ition 1065.076 1(1,0) - 1(1,1) A-+ 1371.722 2(1,1)-2(1,2) F=1-1 1539.832 1(1,0)-1(1,1) F=2-2 1610.247 1(1,0)-1(1,1) A 1638.805 1(1,0)-1(1,1) 1665.401 23/2 J=3/2 F=1-1 2661.610 1-0 F=1-1 3139.404 2(1,1)-2(1,2) 3195.162 2(1,1) - 2(1,2) A-+ 3335.481 21/2 J=1/2 F=1-1 4618.967 2(1,1)-2(1,2) F=3-3 4660.242 21/2 J=1/2 F=0-1 4829.659 1(1,0)-1(1,1) F=2-2 4916.312 2(1,1)-2(1,2) 5005.320 3(1,2)-3(1,3) A-+ 5289.813 1(1,0)-1(1,1) F=2-2 5324.058 2-1 F=2 -2 6016.746 23/2 J=5 /2 F=2-3 M olecule Nam e H2CS CH3CHO CH3OH OH HC7N HC5N OH CH3NH2 HC7N HCCCN CH3OCH3 NH2 CHO C4H C6H HC9N CCCN CH3OH CH3OH Res t Fr eq. Q uantum [MHz] Tr ansition 6278.628 3(1,2)-3(1,3) 6389.933 3(1,2) - 3(1,3) A-+ 6668.519 5(1,6)-6(0,6) A++ 7820.125 21/2 J=3/2 F=2-2 7896.023 7-6 F=8-7 7987.782 3-2 F=2 -1 8189.587 21/2 J=5/2 F=3-3 8778.200 2(0,2)-1(0,1) F=2-2 9024.009 8-7 9097.034 1-0 F=1-1 9118.823 2(0,2)-1(1,1) 9235.119 3(1,2)-3(1,3) F=3-3 9493.061 3 /2-1 /2 F=1-0 9703.508 23/2J=3.5 -2.5 F=4 -3 9877.606 17-16 9885.890 1-0 J=3 /2-1 /2 F=5 /2-3 /2 9936 .202 9(-1,9)-8(-2,7) E 9978 .686 4(3,2)-5(2,3)


Table Notes: Sub-sample of the more than 100 prev iously detected molecular lines in the 1н10 GHz range fro m the Lovas catalo g. This list does not include many o ther astro physical impo rtant transitions (e.g, OH 1720 MHz) the hyperfine structure of several lines that have been used as astro physical diagnostics (e.g., the H2CO 4.8 GHz hyperfine structure), isoto pic species, no r the hundreds of hy dro gen, helium and carbon radio recombinatio n lines in the 1 н 10 GHz range. Several tho usand molecular lines in the 1н10 GHz range are listed in the JPL and Cologne molecular spectroscopy catalogs. Other (yet undetected) transitions of important molecules (like gly cine and small PAHs) lie in this range, but are not listed in the table.

Table 2- Observing parameters with the Mock spectrometer Vb Receiver Reference Freq.a Bandwidth [MHz] [MHz] [km/sec] L 1.4 0.27 250 S-low 2.6 0.29 500 S-high 3.5 0.21 500 C 5.0 0.30 1000 C-high 7.0 0.21 1000 X 9.0 0.16 1000
a b c d e

c

N

settings

d

3 2 2 2 2 2

(60 min)e [mJy/beam] 1.7 1.6 1.5 1.1 1.7 2.3

Reference Frequency at which V is calculated. Channel width in km/s fo r cho sen spectro meter bandwidth. To tal bandwidth fo r a single frequency setting o f the Mock spectro meter. Number of frequency settings needed to cover the band of this receiver. The ex pected rms for 1-ho ur o f integration time fo r the selected channel, and fo r a so urce with zero flux density .

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