FIRBACK

IV. Towards the nature of the 170 μm source population

¹ Institut d'Astrophysique de Paris, 98bis Boulevard Arago, 75014 Paris, France e-mail: dennefel@iap.fr
² Institut d'Astrophysique Spatiale, Bât. 121, Université Paris XI, 91405 Orsay, France
³ Johns Hopkins University, 3400 N. Charles Street, 21218 Baltimore, MD, USA
⁴ INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ Institute for Astronomy, University of Edinburg, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁶ Astrophysics Group, Blackett Laboratory, Imperial College, Prince Consort Road, SW7 2AZ London, UK

Received: 13 August 2004
Accepted: 14 March 2005

Abstract

We present a detailed study of the brighter (> detections) sources in the 170 μm FIRBACK northern N1 ISO survey, with the help of complementary data in the optical, radio, and mid-IR domain. For 82% of them, an optical galaxy counterpart is identified, either as the unique source of the IR emission, or as part of a multiple identification. With less than 15% of AGNs, these sources are essentially local, moderate starbursters with a dominating cold dust component. They are therefore very similar to the galaxies in the IRAS Very Faint Survey or the ISO 170 μm Serendipity Survey, and represent a population of cold galaxies rather neglected up to now. Their colours do not match those of the far-IR Cosmic IR Background (CIB), to which they contribute less than 5%. The bulk of the sources contributing to the CIB is thus to be searched for in more distant galaxies, possibly counterparts of the fainter FIRBACK sources still under study. These bright, local, galaxies however play an important role in the evolution of IR galaxies: they dominate the number counts at high 170 μm fluxes, and represent half of the contribution at 250 mJy. Although not particularly massive (typically M^*), they form more stars than a typical spiral galaxy and many are bulge dominated, that could represent the remnant of a former merger. The fainter part of this population may represent the missing link with the higher-z sources found in sub-mm observations.