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Поисковые слова: arp 220
A GBT Cm-wave Search for Prebiotic & Other Molecules in Arp 220-like Galaxies Scientific Justification According to current views, interstellar organic sp ecies 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 phase1,2 . Once released, these molecules may form amino acids by the combination of organic sp ecies known as "pre-biotic" molecules. Methanimine (CH2 NH) is one such molecule3 which can form the simplest amino acid, glycine4 (NH2 CH2 COOH), either by (i) first combining with hydrogen cyanide (HCN) to form aminoacetonitrile (H2 NCH2 CN), with subsequent hydrolysis (Strecker Synthesis5 ), or (ii) directly combining with formic acid (HCOOH)6 . Methanimine has b een detected in the ISM7,8 and tentatively in the nearby galaxy, NGC 2539 , but has never b een seen b eyond the neighb orho o d of our Galaxy b efore (i.e. b eyond 5 Mp c). HCN is a well known indicator of high gas density. Recently, Gao & Solomon (2004) 10 demonstrated that a very strong linear correlation exists b etween the luminosities L IR and LHCN for mm-wave transitions of HCN. This extends over a wide range of LIR running from normal galaxies to Ultra Luminous IR Galaxies (ULIRGs). A similar LIR ­ LCO correlation has a less linear form, marking out LHCN as the b est tracer of dense molecular gas mass in galaxies, and hence of active star-forming gas. However, the smo oth continuation of the LIR ­ LHCN correlation to (U)LIRGs has recently b een contested by Papadop oulos et al. (2007)11 . At Arecib o, we are well advanced towards completion of a sp ectral-line survey of Arp 220 b etween 1.1 and 10 GHz. Arp 220 is the prototypical megamaser/starburst galaxy, in which b oth maser emission and absorption are seen in the four 18-cm transitions of the OH radical12,13 . Molecules such as CO(14) , formaldehyde15 , ammonia16 , and mm-transitions of HCN(17) have b een detected with large velo city widths in this galaxy. At a distance of 77 Mp c, it is also the nearest ULIRG, in which most of the IR luminosity arises from a p owerful, dust-enshrouded starburst, b elieved to b e triggered by the merger of two gas rich galaxies18 . Evidence for this is provided by its huge sup ernova rate as found from recent high resolution VLBI studies19 . Analysis of the Arecib o data taken so far on Arp 220 reveals a rich molecular sp ectrum, including lines of methanimine, and three v2=1 direct l-typ e transitions of HCN(20) from the J=4, 5 and 6 vibrational levels (Fig 1). Methanimine is seen in emission, while the HCN lines app ear in absorption against the continuum emission of Arp 220; we note that these particular HCN transitions seem not to have b een 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 ab ove HCN transitions, we also confirm previously rep orted formaldehyde emission, and excited-OH absorption lines, and obtain the probable detection of emission lines of CH and formamide (NH2 CHO). In addition to these (and other sp ectral lines found in the bands so far observed), we have detected a strong L-band absorption line that may b e due to the pre-biotic molecule, HCOOH, although there is ambiguity with a neighb oring line of 18 OH. The origin of life is still an op en question. Discovery of "pre-biotic" molecules in a faroff galaxy with a high star formation rate adds a new dimension to considerations of this


Methanimine multiplets in emission

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HCN J=4, direct l-type transition at 4488 MHz

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Fractional Absorption

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Flux Density (Jy)

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-0.000 -0.015 -0.001 5280 5285 5290 5295 Rest Frequency (MHz) for Vhel = 5373 km/s 5300

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Figure 1: Newly detected C-band 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 220-like" galaxies, aiming particularly at investigating the o ccurrence of pre-biotic molecular sp ecies in such ob jects. For this, we have selected a sample of sources from the lists of (i) all OH-Megamaser sources (as compiled by Chen et al.21 ), (ii) starburst sources where formaldehyde has b een detected via maser emission or absorption in the C-band15 , (iii) ULIRGs with direct evidence of nuclear starbursts22 and (iv) FIR-luminous merging galaxies23 . We note that Arp 220 is common to all these lists. Applying the criteria of z < 0.043, and S1.4GH z > 50 mJy, our sample consists of 40 sources. Of these, 21 are visible from Arecib o and, were recently granted telescop e time (Jun07 prop osal deadline). In the present prop osal, we request observing time to cover the other 19 galaxies that are outside the "Arecib o sky". In our source selection, the redshift and the flux-density restrictions were imp osed to allow the detection of emission lines having similar strength to those we find in Arp 220, and to also have sufficient 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 of 4200 ­ 5400 MHz, covering transitions of HCO CH, and CH2 NH which lie at rest frequencies of and 5289 MHz resp ectively in the other nineteen, search in the (rest-frame) frequency range , HCN, NH2 CHO, excited-OH, H2 CHO, 4234, 4489, 4619, 4660­4765, 4829, 4848, Arp220-like galaxies.
+

Any HCN and HCO+ detections will b e followed up with further prop osals to observe the J=2, 3, 5 and 6, v2=1 direct l-typ e lines using other receivers. This will enable us to estimate densities and temp eratures 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 sp ectral survey are similar to those detected from hot cores in Giant Molecular Clouds in our own Galaxy. However, we know of no previous cm-wave detection of the v2=1 direct l-typ e transitions of HCN in such sources. Also, while methanimine has mostly b een detected in the mm-wave range, only very few measurements have so far b een made, and the C-band multiplet has only b een seen towards Sgr B27 . Hence, for a direct comparison with the measurements of this sample of starburst


galaxies that we prop ose to observe here, we would also like to make an identical (2.5 hr, see b elow) observation of the Sgr B2 (R.A.=17h 47m 20.4s ; Dec=-28 23 05 ) region. In the companion Arecib o prop osal, we will b e observing the W51 region, since Sgr B2 is not visible from Arecib o. Technical Details For the criteria detailed ab ove, our "GBT sample" contains 19 galaxies, plus Sgr-B2 (Table 1). We will b e using the C-band receiver, and the ACS sp ectrometer in frequency switching mo de. The required sp ectrometer configuration will b e "nwin=2, bandwidth=800 MHz, nchannel = high, sp ectral level =3". With this, we will have a basic velo city resolution of 12 km/s (for a fictitious line at 4.9 GHz), and 4096 channels p er frequency window. With frequency switching and a total integration time of 2.5 hrs p er source (including b oth the phases of a frequency switching cycle), we will have rms noise p er channel of 0.2 mJy/b eam. For Arp 220, we found linewidths of a few hundreds of km/s, indicating large velo cities in its ISM. Hence it was p ossible to smo oth the sp ectra to velo city resolutions of up to 40 km/s without loss of information. Application of similar smo othing will result in noises down to 0.1 mJy/b eam, similar to that for the ob jects in the Arecib o sample. This will allow detection at the 3- level of absorption with optical depth of <1% in those targets that have the weakest continuum emission (40 mJy at C-band). Including a slewing overhead and set-up time of 10%, we request a total observing time of 55 hr. References: 1 Namura H., Millar T.J., 2004, A&A, 414, 409 2 Caselli P., Hasegawa T.J., Herbst E., 1993, ApJ, 408, 548 3 Kirchoff, W.H., Johnson, D. R., & Lovas, F.J., 1973, J. Phys. Chem. Ref. Data, 2, 1 4 Dickerson, R.E., 1978, Sci. Am., Septemb er, 62 5 Xu S., Tobin C., Peslherb e, G.H., Hynes, J.T., APS, 2004, Abst#D34.005 6 Feldman, M.T., Widicus, S.L., Blake, G.H., Kent IV, D.R., Go ddard I I I, W.A., 2005, J. Chem. Phys., 123, 034304 7 Go dfrey, P.D., Brown, R.D., Robinson, B.J., Sinclair, M.W., 1973, Astrophys. Lett., 13, 119 8 Dickens, J.E., Irvine, W.M., DeVries, C.H., Ohishi, M., 1997, ApJ, 479, 307 9 Martin, S., Mauersb erger, R., Martin-Pintado, J., Henkel, C., Garcia-Burillo, S., 2006 ApJS, 164, 450 10 Gao, Yu., Solomon, P. M., 2004, ApJS, 152, 63 11 Papadop oulos, P.P., Greve, T.R., van der Werf, P., Mehle, S., Isaak, K., Gao, Yu., 2007, Astro-ph, 0701829 12 Baan W.A., Wo o d P.A.D., Haschick A.D., ApJ, 260, 1982, L49 13 Ghosh, T., Kavars, D.W., Robinson, P.E., Saintonge, A., Strasser, S.T., Salter, C.J., 2003, BAAS, 20311508 14 Scoville, N.Z., Sanders, D.B., Sargent, A.I., Soifer, B.T., Scott, S.L., Lo, K.Y., 1986, ApJ, 311, L47 15 Araya, E., Baan, W.A., Hofner, P., 2004, ApJ, 154, 541 16 Takano, S., Nakanishi, K., Nakai, N., Takano, T., 2005, PASJ, 57, L29 17 Aalto, S., Spaans, M., Weidner, M.C., Huttemeister, S., 2007, A&A., 464, 193 18 Sanders, D.B., Mirab el, I.F., 1996, ARAA, 34, 749 19 Lonsdale, C.J., Diamond, P.J., Thrall, H., Smith, H. E., Lonsdale, C.J., 2006, ApJ, 647,


185 20 Thorwirth S., Wyrowski F., Schilke P., Menten K.M., Brunken S., Muller H.S.P. Winnewisser G., 2003, ApJ, 586, 338 21 Chen, P.S., Shan, H.G., Gao, Y.F., 2007, AJ, 133, 496 22 Armus, L., Heckman, T.M., Miley, G.K., 1990, ApJ, 364, 471 23 Condon, J.J., Huang, Z,-P., Yin, Q.F., Thuan, T.X., 1991, ApJ, 378, 65

Table 1: The Source Sample Name NGC 0034 NGC 253 IC 1623 NGC 1068 NGC 1365 NGC 1614 UGC 03351 UGC 05101 M 82 Mrk 171 Fairall 1151 NGC 4194 Mrk 231 Arp 238 NGC 5256 Mrk 273 NGC 5861 Arp 293 IRAS 17208-0014 R.A.(B1950) 00 08 33.40 00 45 05.74 01 05 19.9 02 40 07.05 03 31 41.802 04 31 35.8 05 41 24.59 09 32 04.78 09 51 42.4 11 25 42.1 11 50 39.87 12 11 41.3 12 54 05.01 13 13 39.6 13 36 14.73 13 42 51.7 15 06 32.62 16 57 44.64 17 20 47.87 Dec.(B1950) ­12 23 08.9 ­25 33 39.6 ­17 46 26 ­00 13 31.64 ­36 18 26.55 ­08 40 57 +58 40 52.3 +61 34 37.0 +69 54 59 +58 50 18 ­38 51 07.1 +54 48 10 +57 08 38.2 +62 23 26 +48 31 50.1 +56 08 13.59 ­11 07 54.1 +59 00 37.6 ­00 14 15.87 Redshif t(z ) 0.019617 0.000811 0.020067 0.003793 0.005457 0.015938 0.01486 0.039367 0.000677 0.010411 0.010781 0.008342 0.04217 0.030831 0.027863 0.03778 0.006174 0.018349 0.042810 S (mJy) 45 2500 96 1900 200 63 46 76 3900 398 56 40 400 45 48 99 400 50 60 Ref. 2 3 2 3 3 1,2 3 2,3 1 1,2 3 1 1,2,3 1,2 2 1,2 3 2 3

6cm

1: "Arp 220-like" FIR galaxies, Armus et al. 1990 2: Compact Starburst in ULIRGs, Condon et al. 1991 3: OH-Megamaser sources, Chen et al. 2007