Evolution of the radio-loud galaxy population

Abstract

A catalogue of 14453 radio-loud 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 NVSS and FIRST radio surveys with the MegaZ-LRG catalogue of luminous red galaxies derived from Sloan Digital Sky Survey imaging data. New techniques were developed for extending the cross-correlation algorithm to FIRST detections that are below the nominal 1 mJy S/N limit of the catalogued sources. We estimate a reliability of ~98.3%, and completeness level (for LRGS) of about 95% 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 SDSS main survey. The comoving number density of radio AGN with luminosities less than 10^{25} W Hz^{-1} increases by a factor ~1.5 between z=0.1 and z=0.55. At higher lumiosities, this factor increases sharply, reaching values of more than 10 at radio luminosities larger than 10^{26} 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.