Letter to the Editor

Herschel observations of water vapour in Markarian 231 ^*

¹ Universidad de Alcalá de Henares, Departamento de Física, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain e-mail: eduardo.gonzalez@uah.es
² Naval Research Laboratory, Remote Sensing Division, Washington, DC 20375, USA
³ ESA Astrophysics Missions Div/ Research and Scientific Support Dept ESTEC/SRE-SA Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
⁴ Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁵ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands
⁶ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁷ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁸ University of Crete, Department of Physics, 71003, Heraklion, Greece
⁹ Laboratoire d'Études du Rayonnement et de la Matière en Astrophysique (LERMA), UMR 8112 du CNRS, OP, ENS, UPMC, UCP, 61 Av. de l'Observatoire, 75014 Paris, France
¹⁰ Onsala Space Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden
¹¹ MPIfR, Auf dem Hügel 69, 53121 Bonn, Germany
¹² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹³ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
¹⁴ Centro de Astrobiología (CSIC-INTA), Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹⁵ Space Astronomy Division, Institute for Space Imaging Science, Department of Physics and Astronomy, University of Lethbridge, Lethbridge, Alberta Canada, T1K 3M4, Canada
¹⁶ Istituto di Fisica dello Spazio Interplanetario, CNR via Fosso del Cavaliere 100, 00133 Roma, Italy
¹⁷ Department of Astronomy, University of Maryland, College Park, MD 20742 USA
¹⁸ IPAC, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
¹⁹ Institute of Astronomy and Astrophysics, National Observatory of Athens, P. Penteli, GR-15236 Athens, Greece
²⁰ University of Hawaii, Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
²¹ Laboratoire AIM, CEA/DSM, CNRS, Université Paris Diderot, Irfu/Service d'Astrophysique, CEA Saclay, Orme des Merisiers, 91191 Gif sur Yvette Cedex, France
²² Instituto Radioastronomia Milimetrica (IRAM), Av. Divina Pastora 7, Nucleo Central, 18012 Granada, Spain
²³ Argelander Institut fuer Astronomy, Auf dem Huegel 71, 53121, Germany
²⁴ Department of Astronomy, Cornell University, Ithaca, NY, USA
²⁵ Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, VA 22904, USA
²⁶ Purple Mountain Observatory, Chinese Academy of Sciences, 2 West Beijing Road, Nanjing 210008, PR China

Received: 31 March 2010
Accepted: 28 April 2010

Abstract

The Ultra luminous infrared galaxy (ULIRG) Mrk 231 reveals up to seven rotational lines of water (H₂O) in emission, including a very high-lying (E_upper = 640 K) line detected at a 4σ level, within the Herschel/SPIRE wavelength range (190 < λ (μm) < 640), whereas PACS observations show one H₂O line at 78 μm in absorption, as found for other H₂O lines previously detected by ISO. The absorption/emission dichotomy is caused by the pumping of the rotational levels by far-infrared radiation emitted by dust, and subsequent relaxation through lines at longer wavelengths, which allows us to estimate both the column density of H₂O and the general characteristics of the underlying far-infrared continuum source. Radiative transfer models including excitation through both absorption of far-infrared radiation emitted by dust and collisions are used to calculate the equilibrium level populations of H₂O and the corresponding line fluxes. The highest-lying H₂O lines detected in emission, with levels at 300–640 K above the ground state, indicate that the source of far-infrared radiation responsible for the pumping is compact (radius = 110–180 pc) and warm (T_dust = 85–95 K), accounting for at least 45% of the bolometric luminosity. The high column density, N(H₂O) ~ 5×10¹⁷ cm^-2, found in this nuclear component, is most probably the consequence of shocks/cosmic rays, an XDR chemistry, and/or an “undepleted chemistry” where grain mantles are evaporated. A more extended region, presumably the inner region of the 1-kpc disk observed in other molecular species, could contribute to the flux observed in low-lying H₂O lines through dense hot cores, and/or shocks. The H₂O 78 μm line observed with PACS shows hints of a blue-shifted wing seen in absorption, possibly indicating the occurrence of H₂O in the prominent outflow detected in OH (Fischer et al. 2010, A&A, 518, L41). Additional PACS/HIFI observations of H₂O lines are required to constrain the kinematics of the nuclear component, as well as the distribution of H₂O relative to the warm dust.