A sample of radio-loud AGN in the Sloan Digital Sky Survey

Abstract

A sample of 2712 radio-luminous galaxies is defined from the second data release of the Sloan Digital Sky Survey (SDSS) by cross-comparing the main spectroscopic galaxy sample with two radio surveys: the NRAO VLA Sky Survey (NVSS) and the Faint Images of the Radio Sky at Twenty centimetres (FIRST) survey. The comparison is carried out in a multi-stage process and makes optimal use of both radio surveys by exploiting the sensitivity of the NVSS to extended and multi-component radio sources in addition to the high angular resolution of the FIRST images. A radio source sample with 95% completeness and 98.9% reliability is achieved, far better than would be possible for this sample if only one of the surveys was used. The radio source sample is then divided into two classes: radio-loud AGN and galaxies in which the radio emission is dominated by star formation. The division is based on the location of a galaxy in the plane of 4000Ang break strength versus radio luminosity per unit stellar mass and provides a sample of 2215 radio-loud AGN and 497 star forming galaxies brighter than 5mJy at 1.4GHz. A full catalogue of positions and radio properties is provided for these sources. The local radio luminosity function is then derived both for radio-loud AGN and for star-forming galaxies and is found to be in agreement with previous studies. By using the radio to far-IR correlation, the radio luminosity function of star forming galaxies is also compared to the luminosity function derived in the far-infrared. It is found to agree well at high luminosities but less so at lower luminosities, confirming that the linearity of the radio to far-IR correlation breaks down below about 10^22 W/Hz at 1.4GHz.