OBSERVATIONS OF Arp 220 USINGHERSCHEL-SPIRE: AN UNPRECEDENTED VIEW OF THE MOLECULAR GAS IN AN EXTREME STAR FORMATION ENVIRONMENT
Year: 2011
Type: article
Abstract: 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 μm, 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 ∼2 × 10 8 L ☉ ; very high- J transitions of HCN ( J = 12–11 to 17–16) in absorption; strong absorption features of rare species such as OH + , H 2 O + , 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 τ dust ∼ 5 at 100 μm. The total far-infrared luminosity of Arp 220 is L FIR ∼ 2 × 10 12 L ☉ . 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 ∼ 50 K and warm molecular gas at T ∼ 1350 +280 − 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 CO / L FIR ∼ 10 −4 (the most luminous lines, such as J = 6–5, have L CO, J = 6–5 / L FIR ∼ 10 −5 ). The temperature of the warm gas is in excellent agreement with the observations of H 2 rotational lines. At 1350 K, H 2 dominates the cooling (∼20 L ☉ M −1 ☉ ) in the interstellar medium compared to CO (∼0.4 L ☉ M −1 ☉ ). 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 ∼20 L ☉ M −1 ☉ . Analysis of the very high- J lines of HCN strongly indicates that they are solely populated by infrared pumping of photons at 14 μm. 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 + , H 2 O + , and H 2 O. The outflow has a mass ≳ 10 7 M ☉ and is bound to the nuclei with velocity ≲ 250 km s −1 . The large column densities observed for these molecular ions strongly favor the existence of an X-ray luminous AGN (10 44 erg s −1 ) in Arp 220.
Source: The Astrophysical Journal
Authors: Naseem Rangwala, Philip R. Maloney, Jason Glenn, Christine D. Wilson, Adam Rykala, Kate Isaak, Maarten Baes, George J. Bendo, Alessandro Boselli, Charles M. Bradford, D. L. Clements, Asantha Cooray, Trevor Fulton, Peter Imhof, Julia Kamenetzky, Suzanne C. Madden, Erin Mentuch, Nicola Sacchi, Marc Sauvage, Maximilien R. P. Schirm, M. W. L. Smith, Luigi Spinoglio, Mark Wolfire
Institutions: University of Colorado Boulder, McMaster University, Cardiff University, European Space Research and Technology Centre, Ghent University, University of Manchester, Centre National de la Recherche Scientifique, Laboratoire d’Astrophysique de Marseille, Jet Propulsion Laboratory, Imperial College London, University of California, Irvine, Commissariat à l'Énergie Atomique et aux Énergies Alternatives, Astrophysique, Instrumentation et Modélisation, Institut de Recherche sur les Lois Fondamentales de l'Univers, National Institute for Astrophysics, University of Maryland, College Park
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: bronze
Funders: National Aeronautics and Space Administration, California Institute of Technology, UK Space Agency, Imperial College London, Centre National d’Etudes Spatiales, Centre National de la Recherche Scientifique, Science and Technology Facilities Council, National Astronomical Observatories, Chinese Academy of Sciences
Awards: G4081850257, G5913683366
OpenAlex ID: W1993384070