ISO OBSERVATIONS OF FAINT RADIO SOURCES ⋆ JOSÉ AFONSO Astrophysics Group, Imperial College of Science, Technology and Medicine, Blackett Laboratory, Prince Consort Rd, London SW7 2BZ, UK; E-mail: j.afonso@ic.ac.uk BAHRAM MOBASHER ⋆⋆ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD 21218, USA ANTONIS GEORGAKAKIS School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK LAWRENCE CRAM School of Physics, University of Sydney, Sydney NSW 2006, Australia ANDREW HOPKINS Department of Physics and Astronomy, University of Pittsburgh, 3941 O’Hara Street, Pittsburgh, PA 15260, USA Abstract. Deep radio surveys have revealed, below the millijansky level, a population of actively starforming galaxies, undergoing strong luminosity evolution as found from the observed radio source counts. A random sample of 65 sub-mJy radio sources, selected from the Phoenix Deep Survey, was observed at mid and far-infrared wavelengths using the Infrared Space Observatory. Nine sources were detected at both far-infrared and at least one of the mid-infrared wavelengths. A preliminary analysis of the infrared properties of these faint radio sources is carried out by modeling their infrared Spectral Energy Distribution. The radio vs. far-infrared relation, for these faint radio objects, is also presented. Keywords: galaxies: evolution – infrared: galaxies – radio continuum: galaxies – surveys 1. Introduction Very deep radio (1.4 GHz) surveys have revealed a new population of faint radio sources different from those detected at brighter flux levels. Their presence was first indicated by the flattening of the radio source counts below ∼5 mJy (e.g. Windhorst et al., 1985). Follow-up spectroscopic and photometric studies (Benn et al., 1993; Windhorst et al., 1994; Thuan et al., 1992; Richards et al., 1998; Georgakakis et al., 1999) have shown that, instead of the classical giant ellipticals harboring an AGN, faint radio sources are predominantly a population of actively starforming ⋆ Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA. ⋆⋆ Also affiliated with the Space Sciences Departpent of the European Space Agency. Astrophysics and Space Science 277 (Suppl.): 527–530, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.