Letter to the Editor

Black hole accretion and star formation as drivers of gas excitation and chemistry in Markarian 231

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: pvdwerf@strw.leidenuniv.nl
² School of Physics & Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
³ ESA Astrophysics Missions Division, ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands
⁴ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁵ Blue Sky Spectroscopy, Lethbridge, Alberta, Canada
⁶ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁸ Spitzer Science Center, California Institute of Technology, MS 220-6, Pasadena, CA 91125, USA
⁹ Naval Research Laboratory, Remote Sensing Division, Washington, DC 20375, USA
¹⁰ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
¹¹ Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹² Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA
¹³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹⁴ Universidad de Alcalá Henares, Departamente de Física, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
¹⁵ Department of Physics, University of Lethbridge, 4401 University Drive, Lethbridge, Alberta, T1J 1B1, Canada
¹⁶ Department of Radio and Space Science, Onsala Observatory, Chalmers University of Technology, 43992 Onsala, Sweden
¹⁷ University of Crete, Department of Physics, 71003 Heraklion, Greece
¹⁸ Service d'Astrophysique, CEA Saclay, Orme des Merisiers, 91191 Gif sur Yvette Cedex, France
¹⁹ Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, VA 22904, USA
²⁰ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
²¹ Purple Mountain Observatory, Chinese Academy of Sciences, 2 West Beijing Road, Nanjing 210008, PR China
²² Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
²³ Instituto Radioastronomie Millimetrica (IRAM), Av. Divina Pastora 7, Nucleo Central, 18012 Granada, Spain
²⁴ Departamento de Astrofisica Molecular e Infrarroja-Instituto de Estructura de la Materia-CSIC, Calle Serrano 121, 28006 Madrid, Spain
²⁵ Argelander Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
²⁶ University of Hawaii, Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
²⁷ Istituto di Fisica dello Spazio Interplanetario, CNR, via Fosso del Cavaliere 100, 00133 Roma, Italy
²⁸ Department of Astronomy, Cornell University, Ithaca, NY 14853, USA
²⁹ Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France
³⁰ Institute of Astronomy and Astrophysics, National Observatory of Athens, P. Penteli, 15236 Athens, Greece

Received: 31 March 2010
Accepted: 27 April 2010

Abstract

We present a full high resolution SPIRE FTS spectrum of the nearby ultraluminous infrared galaxy Mrk 231. In total 25 lines are detected, including CO J = 5–4 through J = 13–12, 7 rotational lines of H₂O, 3 of OH⁺ and one line each of H₂O⁺, CH⁺, and HF. We find that the excitation of the CO rotational levels up to J = 8 can be accounted for by UV radiation from star formation. However, the approximately flat luminosity distribution of the CO lines over the rotational ladder above J = 8 requires the presence of a separate source of excitation for the highest CO lines. We explore X-ray heating by the accreting supermassive black hole in Mrk 231 as a source of excitation for these lines, and find that it can reproduce the observed luminosities. We also consider a model with dense gas in a strong UV radiation field to produce the highest CO lines, but find that this model strongly overpredicts the hot dust mass in Mrk 231. Our favoured model consists of a star forming disk of radius 560 pc, containing clumps of dense gas exposed to strong UV radiation, dominating the emission of CO lines up to J = 8. X-rays from the accreting supermassive black hole in Mrk 231 dominate the excitation and chemistry of the inner disk out to a radius of 160 pc, consistent with the X-ray power of the AGN in Mrk 231. The extraordinary luminosity of the OH⁺ and H₂O⁺ lines reveals the signature of X-ray driven excitation and chemistry in this region.