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

Abstract

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 1023–1025 W Hz−1. The fraction of galaxies that host radio‐loud AGN with L1.4 GHz > 1023 W Hz−1 is a strong function of stellar mass, rising from nearly zero below a stellar mass of 1010 M⊙ to more than 30 per cent at stellar masses of 5 × 1011 M⊙. In contrast to the integrated [O iii] luminosity density from emission‐line AGN, which is mainly produced by black holes with masses below 108 M⊙, 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 M2.5* or M1.6BH; 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.

Radio-AGN feedback: when the little ones were monsters

“…We confirmed this using the spectroscopy-based AGN/SF separation in the local sample (Best et al 2005b). Moreover, the star formation rate for this radio power is in excess of 25 M yr −1 (Condon 1992) so we expect AGN to continue to dominate even at higher redshifts.…”

“…The SMFs we derive for our optical galaxy samples (for both the SDSS and COS-MOS samples) are consistent with those of Muzzin et al (2013b) who use a more sophisticated maximum-likelihood analysis to derive the SMFs in several redshift bins. As expected (Best et al 2005b), ρ Rad differs significantly from ρ Opt -the hosts of radio sources are biased towards more massive systems. Interestingly, while the comoving number density of all galaxies decreases with redshift, that of the radio source hosts with log(M * /M ) 11.0 increases.…”

“…The normalization of the LF requires knowledge of the precise intersection area of all input surveys. The sky area for the SDSS-NVSS sample is taken to be 2.17 sr (Best et al 2005b). For the VLA-COSMOS sample, the sky area is taken to be 1.62 deg 2 for all sources above 100 µJy.…”

“…It is also well known that, within the local Universe (z 0.3), the fraction of galaxies which host a radio source, i.e. the radio-loud fraction, is a very steep function of host galaxy stellar mass (f radio−loud ∝ M 2.5 * ; Best et al 2005b), increasing to >30 per cent at stellar masses above 5 × 10 11 M for radio luminosities >10 23 W Hz −1 . This pervasiveness of radio-loudness among high-mass galaxies in the local Universe, combined with the dramatic increase in density of radio-loud sources at earlier times suggests that there must be an increase in the prevalence of radio activity among galaxies of lower mass.…”

Star formation properties of sub-mJy radio sources

“…The first is related to selection effects since we are sensitive to low sSFR only for sources whose radio emission is enhanced by AGN activity, i.e., RL AGNs. On the other hand, the probability of hosting a RL AGN has been observed to be higher in massive passive galaxies (e.g., Best et al 2005) even if its physical reason is still unclear.…”