THE HOST GALAXIES OF MICRO-JANSKY RADIO SOURCES

International audienceWe combine a deep 0.5 deg(2), 1.4 GHz deep radio survey in the Lockman Hole with infrared and optical data in the same field, including the Spitzer Extragalactic Representative Volume Survey (SERVS) and UKIDSS near-infrared surveys, to make the largest study to date of the host galaxies of radio sources with typical radio flux densities similar to 50 mu Jy. 87% (1274/1467) of radio sources have identifications in SERVS to AB approximate to 23.1 at 3.6 or 4.5 mu m, and 9% are blended with bright objects (mostly stars), leaving only 4% (59 objects), which are too faint to confidently identify in the near-infrared. We are able to estimate photometric redshifts for 68% of the radio sources. We use mid-infrared diagnostics to show that the source population consists of a mixture of star-forming galaxies, rapidly accreting (cold mode) active galactic nuclei (AGNs) and low accretion rate (hot mode) AGNs, with neither AGNs nor star-forming galaxies clearly dominating. We see the breakdown in the K-z relation in faint radio source samples, and show that it is due to radio source populations becoming dominated by sources with radio luminosities similar to 10(23) WHz(-1). At these luminosities, both the star-forming galaxies and the cold mode AGNs have hosts with stellar luminosities of about a factor of two lower than those of hot mode AGNs, which continue to reside in only the most massive hosts. We show that out to at least z similar to 2, galaxies with stellar masses \textgreater 10(11.5) M-circle dot have radio-loud fractions up to similar to 30%. This is consistent with there being a sufficient number of radio sources for radio-mode feedback to play a role in galaxy evolution