OBSERVATIONS OF Arp 220 USING HERSCHEL-SPIRE: AN UNPRECEDENTED VIEW OF THE MOLECULAR GAS IN AN EXTREME STAR FORMATION ENVIRONMENT
- Center for Astrophysics and Space Astronomy, University of Colorado, 1255 38th street, Boulder, CO 80303 (United States)
- Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, L8S 4M1 (Canada)
- School of Physics and Astronomy, Cardiff University, Queens Buildings The Parade, Cardiff CF24 3AA (United Kingdom)
- ESA Astrophysics Missions Division, ESTEC, PO Box 299, 2200 AG Noordwijk (Netherlands)
- Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, B-9000 Gent (Belgium)
- UK ALMA Regional Centre Node, Jordell Bank Center for Astrophysics, School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL (United Kingdom)
- Laboratoire d'Astrophysique de Marseille, UMR6110 CNRS, 38 rue F. Joliot-Curie, F-13388 Marseille (France)
- JPL, Pasadena, CA 91109 (United States)
- Astrophysics Group, Imperial College, Blackett Laboratory, Prince Consort Road, London SW7 2AZ (United Kingdom)
- Department of Physics and Astronomy, University of California, Irvine, CA 92697 (United States)
- Blue Sky Spectroscopy Inc, Suite 9-740 4th Avenue South, Lethbridge, Alberta T1J 0N9 (Canada)
- CEA, Laboratoire AIM, Irfu/SAp, Orme des Merisiers, F-91191 Gif-sur-Yvette (France)
- Istituto di Fisica dello Spazio Interplanetario, INAF, Via del Fosso del Cavaliere 100, I-00133 Roma (Italy)
We present Herschel Spectral and Photometric Imaging Receiver Fourier Transform Spectrometer (Herschel SPIRE-FTS) observations of Arp 220, a nearby ultra-luminous infrared galaxy. The FTS provides continuous spectral coverage from 190 to 670 {mu}m, a wavelength region that is either very difficult to observe or completely inaccessible from the ground. The spectrum provides a good measurement of the continuum and detection of several molecular and atomic species. We detect luminous CO (J = 4-3 to 13-12) and water rotational transitions with comparable total luminosity {approx}2 Multiplication-Sign 10{sup 8} L{sub Sun }; very high-J transitions of HCN (J = 12-11 to 17-16) in absorption; strong absorption features of rare species such as OH{sup +}, H{sub 2}O{sup +}, and HF; and atomic lines of [C I] and [N II]. The modeling of the continuum shows that the dust is warm, with T = 66 K, and has an unusually large optical depth, with {tau}{sub dust} {approx} 5 at 100 {mu}m. The total far-infrared luminosity of Arp 220 is L{sub FIR} {approx} 2 Multiplication-Sign 10{sup 12} L{sub Sun }. Non-LTE modeling of the extinction corrected CO rotational transitions shows that the spectral line energy distribution of CO is fit well by two temperature components: cold molecular gas at T {approx} 50 K and warm molecular gas at T {approx} 1350{sup +280}{sub -100} K (the inferred temperatures are much lower if CO line fluxes are not corrected for dust extinction). These two components are not in pressure equilibrium. The mass of the warm gas is 10% of the cold gas, but it dominates the CO luminosity. The ratio of total CO luminosity to the total FIR luminosity is L{sub CO}/L{sub FIR} {approx} 10{sup -4} (the most luminous lines, such as J = 6-5, have L{sub CO,J=6-5}/L{sub FIR} {approx} 10{sup -5}). The temperature of the warm gas is in excellent agreement with the observations of H{sub 2} rotational lines. At 1350 K, H{sub 2} dominates the cooling ({approx}20 L{sub Sun} M{sup -1}{sub Sun }) in the interstellar medium compared to CO ({approx}0.4 L{sub Sun} M{sup -1}{sub Sun }). We have ruled out photodissociation regions, X-ray-dominated regions, and cosmic rays as likely sources of excitation of this warm molecular gas, and found that only a non-ionizing source can heat this gas; the mechanical energy from supernovae and stellar winds is able to satisfy the large energy budget of {approx}20 L{sub Sun} M{sup -1}{sub Sun }. Analysis of the very high-J lines of HCN strongly indicates that they are solely populated by infrared pumping of photons at 14 {mu}m. This mechanism requires an intense radiation field with T > 350 K. We detect a massive molecular outflow in Arp 220 from the analysis of strong P Cygni line profiles observed in OH{sup +}, H{sub 2}O{sup +}, and H{sub 2}O. The outflow has a mass {approx}> 10{sup 7} M{sub Sun} and is bound to the nuclei with velocity {approx}< 250 km s{sup -1}. The large column densities observed for these molecular ions strongly favor the existence of an X-ray luminous AGN (10{sup 44} erg s{sup -1}) in Arp 220.
- OSTI ID:
- 22004508
- Journal Information:
- Astrophysical Journal, Vol. 743, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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