2015AJ....150...87L

We combine a deep 0.5 deg², 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 ∼50 µJy. 87% (1274/1467) of radio sources have identifications in SERVS to AB ≈ 23.1 at 3.6 or 4.5µm, 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 ∼ 10²³. 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∼ 2, galaxies with stellar masses > 10^11.5M_☉ have radio-loud fractions up 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.