Letter to the Editor

Herschel observations of FIR emission lines in brightest cluster galaxies^*

¹ Institute for Computational Cosmology, Department of Physics, Durham University, Durham, DH1 3LE, UK e-mail: alastair.edge@durham.ac.uk
² Leiden Observatory, Leiden University, PB 9513, Leiden 2300 RA, The Netherlands
³ Chester F. Carlson Center for Imaging Science, Rochester Institute of Technology, Rochester, NY 14623, USA
⁴ Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall, Stanford, CA 94305-4085, USA
⁵ Max-Planck-Institut für extraterrestrische Physik, 85748 Garching, Germany
⁶ University of Michigan, Dept. of Astronomy, Ann Arbor, MI 48109, USA
⁷ H H Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL, UK
⁸ Observatoire de Paris, LERMA, CNRS, 61 Av. de l'Observatoire, 75014 Paris, France
⁹ Institute of Astronomy, Madingley Rd., Cambridge, CB3 0HA, UK
¹⁰ Michigan State University, Physics and Astronomy Dept., East Lansing, MI 48824-2320, USA
¹¹ Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA
¹² Department of Physics, University of Kentucky, Lexington KY 40506, USA
¹³ School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, UK
¹⁴ Department of Physics & Astronomy, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1, Canada
¹⁵ Department of Physics, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY 14623-5603, USA
¹⁶ Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA
¹⁷ Department of Astronomy, University of Virginia, PO Box 400325, Charlottesville, VA 22904-4325, USA
¹⁸ School of Physics, University of Melbourne, Victoria 3010, Australia
¹⁹ ASTRON, Netherlands Institute for Radio Astronomy, PO Box 2, 7990 AA Dwingeloo, The Netherlands

Received: 30 March 2010
Accepted: 3 May 2010

Abstract

The question of how much gas cools in the cores of clusters of galaxies has been the focus of many, multiwavelength studies in the past 30 years. In this letter we present the first detections of the strongest atomic cooling lines, [Cii], [Oi] and [Nii] in two strong cooling flow clusters, A1068 and A2597, using Herschel-PACS. These spectra indicate that the substantial mass of cold molecular gas (> 10⁹ ) known to be present in these systems is being irradiated by intense UV radiation, most probably from young stars. The line widths of these FIR lines indicate that they share dynamics similar but not identical to other ionised and molecular gas traced by optical, near-infrared and CO lines. The relative brightness of the FIR lines compared to CO and FIR luminosity is consistent with other star-forming galaxies indicating that the properties of the molecular gas clouds in cluster cores and the stars they form are not unusual. These results provide additional evidence for a reservoir of cold gas that is fed by the cooling of gas in the cores of the most compact clusters and provide important diagnostics of the temperature and density of the dense clouds this gas resides in.