The host galaxies of radio-loud active galactic nuclei: mass dependences, gas cooling and active galactic nuclei feedback

The properties of the host galaxies of a well-defined sample of 2215 radio-loud active galactic nuclei (AGN) with redshifts 0.03 < z < 0.3, defined from the Sloan Digital Sky Survey (SDSS), are investigated. These are predominantly low radio-luminosity sources, with 1.4-GHz luminosities in the range 10(23)-10(25) W Hz(-1). The fraction of galaxies that host radio-loud AGN with L-1.4GHz > 10(23) W Hz(-1) is a strong function of stellar mass, rising from nearly zero below a stellar mass of 10(10) M-circle dot to more than 30 per cent at stellar masses of 5 x 10(11) M-circle dot In contrast to the integrated [O in] luminosity density from emission-line AGN, which is mainly produced by black holes with masses below 108 M-circle dot the integrated radio luminosity density comes from the most massive black holes in the Universe. The integral radio luminosity function is derived in six ranges of stellar and black hole masses. Its shape is very similar in all of these ranges and can be well fitted by a broken power law. Its normalization varies strongly with mass, as M-*(2.5) or M-BH(1.6); this scaling only begins to break down when the predicted radio-loud fraction exceeds 20-30 per cent. There is no correlation between radio and emission-line luminosities for the radio-loud AGN in the sample and the probability that a galaxy of given mass is radio loud is independent of whether it is optically classified as an AGN. The host galaxies of the radio-loud AGN have properties similar to those of ordinary galaxies of the same mass, with a tendency for radio-loud AGN to be found in larger galaxies and in richer environments. The host galaxies of radio-loud AGN with emission lines match those of their radio-quiet counterparts. All of these findings support the conclusion that the optical AGN and low radio-luminosity AGN phenomena are independent and are triggered by different physical mechanisms. Intriguingly, the dependence on black hole mass of the radio-loud AGN fraction mirrors that of the rate at which gas cools from the hot atmospheres of elliptical galaxies. It is speculated that gas cooling provides a natural explanation for the origin of the radio-loud AGN activity, and it is argued that AGN heating could plausibly balance the cooling of the gas over time.