Letter to the Editor

Deep Herschel view of obscured star formation in the Bullet cluster^*

¹ Steward Observatory, University of Arizona, 933 N. Cherry Ave, Tucson, AZ 85721, USA e-mail: eegami@as.arizona.edu
² Department of Astronomy, University of Florida, Gainesville, 32611-2055, USA
³ NASA Herschel Science Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
⁴ Departamento de Astrofísica, Facultad de CC. Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁵ Herschel Science Centre, ESAC, ESA, PO Box 78, Villanueva de la Cañada, 28691 Madrid, Spain
⁶ California Institute of Technology, Pasadena, CA 91125, USA
⁷ Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁸ Jet Propulsion Laboratory, Pasadena, CA 91109, USA
⁹ Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 Av. Edouard Belin, 31400 Toulouse, France
¹⁰ Laboratoire d’Astrophysique de Marseille, CNRS – Université Aix-Marseille, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
¹¹ Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France
¹² Geneva Observatory, University of Geneva, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
¹³ UK Astronomy Technology Centre, Science and Technology Facilities Council, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁴ Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁵ Max-Planck-Institut für extraterrestrische Physik,
Postfach 1312, 85741 Garching, Germany
¹⁶ Department of Astronomy, University of Padova, Vicolo dell'Osservatorio 3, 35122 Padova, Italy
¹⁷ School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
¹⁸ Sterrewacht Leiden, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 31 March 2010
Accepted: 11 May 2010

Abstract

We use deep, five band (100–500 μm) data from the Herschel Lensing Survey (HLS) to fully constrain the obscured star formation rate, SFR_FIR, of galaxies in the Bullet cluster (z = 0.296), and a smaller background system (z = 0.35) in the same field. Herschel detects 23 Bullet cluster members with a total SFR_FIR = 144±14  yr^-1. On average, the background system contains brighter far-infrared (FIR) galaxies, with ~50% higher SFR_FIR (21 galaxies; 207± 9   yr^-1). SFRs extrapolated from 24 μm flux via recent templates (SFR_{24 µm}) agree well with SFR_FIR for ~60% of the cluster galaxies. In the remaining ~40%, SFR_{24 µm} underestimates SFR_FIR due to a significant excess in observed S₁₀₀/S₂₄ (rest frame S₇₅/S₁₈) compared to templates of the same FIR luminosity.