THE RADIO LUMINOSITY FUNCTION AND GALAXY EVOLUTION IN THE COMA CLUSTER

Abstract

We investigate the radio luminosity function and radio source population for two fields within the Coma cluster of galaxies, with the fields centered on the cluster core and southwest infall region and each covering about half a square degree. Using VLA data with a typical rms sensitivity of 28 μJy per 44 beam, we identify 249 radio sources with optical counterparts brighter than r = 22. For cluster galaxies, these correspond to L1.4 = 1.7 × 1020 W Hz−1(for a 5σ source) and Mr = −13. Comprehensive optical spectroscopy identifies 38 of these as members of the Coma cluster, evenly split between sources powered by an active nucleus and sources powered by active star formation. The radio-detected star-forming galaxies are the dominant population only at radio luminosities between about 1021 and 1022 W Hz−1, an interesting result given star formation dominates field radio luminosity functions for all luminosities lower than about 1023 W Hz−1. The majority of the radio-detected star-forming galaxies have characteristics of starbursts, including high specific star formation rates and optical spectra with strong emission lines. In conjunction with prior studies on post-starburst galaxies within the Coma cluster, this is consistent with a picture in which late-type galaxies entering Coma undergo a starburst prior to a rapid cessation of star formation. Optically bright elliptical galaxies (Mr ⩽ −20.5) make the largest contribution to the radio luminosity function at both the high (≳3×1022 W Hz−1) and low (≲1021 W Hz−1) ends. Through a stacking analysis of these optically bright ellipticals we find that they continue to harbor radio sources down to luminosities as faint as 3 × 1019 W Hz−1. However, contrary to published results for the Virgo cluster we find no evidence for the existence of a population of optically faint (Mr ≈ −14) dwarf ellipticals hosting strong radio AGNs.