The FIRST radio survey: Panchromatic properties of FIRST radio sources identified in the Boötes and Cetus fields

In this paper the availability of multi-wavelength optical/infrared information of FIRST (Faint Images of the Radio Sky at 20 cm) radio sources counterparts over ̃9.2 deg² in the Boötes field and ̃2.4 deg² in the Cetus field is exploited to infer the physical properties of the faint radio population. The radio sources optically identified have been divided into resolved galaxies and stellar-like objects finding that the faint radio population is mainly composed of early-type galaxies with very red colour (Bw-R̃ 4.6). A total number of 57 counterparts of FIRST radio sources have extremely red colour (R-K≥ 5). Photometric redshift from {Hyperz} implies that the Extremely Red Objects (EROs) counterparts to FIRST radio sources are mostly located in the range z=0.7-2 , with the bulk of the population at z̃ 1. Taking advantage of the near infrared imaging with FLAMEX (FLAMINGOS Extragalactic Infrared Survey), the EROs counterparts to FIRST radio sources are separated into passively-evolving and dusty star-forming galaxies using their {RJK} colours; the relatively blue J-K of these galaxies suggest that most are old elliptical galaxies (18/25) rather than dusty starburst galaxies (7/25). A total of 15 Distant Red Galaxy (DRGs) have been identified as counterparts to FIRST radio sources in the Cetus field and 3 DRGs in the Boötes field with J-K>2.3.