2012MNRAS.421.3060S

We present spectroscopic and 11-band photometric redshifts for galaxies in the 100-µJy Subaru/XMM–NewtonDeep Field radio source sample. We find good agreement between our redshift distribution and that predicted by the Square Kilometre Array (SKA) Simulated Skies project. We find no correlation between K-band magnitude and radio flux, but show that sources with 1.4-GHz flux densities below ∼ 1 mJy are fainter in the near-infrared than brighter radio sources at the same redshift, and we discuss the implications of this result for spectroscopically incomplete samples where the K–z relation has been used to estimate redshifts. We use the infrared–radio correlation to separate our sample into radio-loud and radio-quiet objects and show that only radio-loud hosts have spectral energy distributions consistent with predominantly old stellar populations, although the fraction of objects displaying such properties is a decreasing function of radio luminosity. We calculate the 1.4-GHz radio luminosity function (RLF) in redshift bins to z= 4 and find that the space density of radio sources increases with lookback time to z ≈ 2, with a more rapid increase for more powerful sources. We demonstrate that radio-loud and radio-quiet sources of the same radio luminosity evolve very differently. Radio-quiet sources display strong evolution to z ≈ 2 while radio-loud active galactic nuclei below the break in the RLF evolve more modestly and show hints of a decline in their space density at z > 1, with this decline occurring later for lower-luminosity objects. If the radio luminosities of these sources are a function of their black hole spins then slowly rotating black holes must have a plentiful fuel supply for longer, perhaps because they have yet to encounter the major merger that will spin them up and use the remaining gas in a major burst of star formation.