Evolution of the radio-loud galaxy population

Abstract

A catalogue of 14 453 radio-loud active galactic nuclei (AGN) with 1.4 GHz fluxes above 3.5 mJy in the redshift range 0.4 < z < 0.8, has been constructed from the cross-correlation of the National Radio Astronomy Observatory Very Large Array Sky Survey (NVSS) and Faint Images of the Radio Sky at Twenty centimetres (FIRST) radio surveys with the MegaZ-luminous red galaxy (MegaZ-LRG) catalogue of LRGs derived from Sloan Digital Sky Survey imaging data. The NVSS provides accurate flux measurements for extended sources, while the angular resolution of FIRST allows the host galaxy to be identified accurately. New techniques were developed for extending the cross-correlation algorithm to FIRST detections that are below the nominal 1 mJy signal-to-noise ratio limit of the catalogued sources. The matching criteria were tested and refined using Monte Carlo simulations, leading to an estimated reliability of ∼98.3 per cent and completeness level (for LRGs) of about 95 per cent for our new catalogue. We present a new determination of the luminosity function of radio AGN at z∼ 0.55 and compare this to the luminosity function of nearby (z∼ 0.1) radio sources from the Sloan Digital Sky Survey main survey. The comoving number density of radio AGN with luminosities less than 1025 W Hz−1 increases by a factor of ∼1.5 between z= 0.1 and 0.55. At higher luminosities, this factor increases sharply, reaching values of more than 10 at radio luminosities larger than 1026 W Hz−1. We then study how the relation between radio AGN and their host galaxies evolves with redshift. Our main conclusion is that the fraction of radio-loud AGN increases towards higher redshift in all massive galaxies, but the evolution is particularly strong for the lower mass galaxies in our sample. These trends may be understood if there are two classes of radio galaxies (likely associated with the ‘radio’ and ‘quasar mode’ dichotomy) that have different fuelling/triggering mechanisms and hence evolve in different ways.