The Host Galaxies of Micro-Jansky Radio Sources

Abstract

We 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 ~ 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 ~ 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 ~ 2, galaxies with stellar masses > 10^(11.5) M⊙ 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.

Additional Information

© 2015 The American Astronomical Society. Received 2014 November 16; accepted 2015 June 29; published 2015 August 25. We thank the referee for a careful reading of the manuscript. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This work made extensive use of topcat (Taylor 2005) for catalog matching and analysis, and the Virtual Observatory SAMP protocol for communication between applications. This paper also made use of the SERVS Lockman data holdings in the Infrared Science Archive, ADS/IRSA.Atlas#2014/1025/091233_11822. J.A. gratefully acknowledges support from the Science and Technology Foundation (FCT, Portugal) through the research grant PTDC/FIS-AST/2194/2012 and PEst-OE/FIS/UI2751/2014.

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Submitted - 1507.01144v1.pdf

Published - Luchsinger_2015p87.pdf

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