Radio jets in galaxies with actively accreting black holes: new insights from the SDSS

Abstract

In the local Universe, the majority of radio-loud active galactic nuclei (AGN) are found in massive elliptical galaxies with old stellar populations and weak or undetected emission lines. At high redshifts, however, almost all known radio AGN have strong emission lines. This paper focuses on a subset of radio AGN with emission lines (EL-RAGN) selected from the Sloan Digital Sky Survey. We explore the hypothesis that these objects are local analogues of powerful high-redshift radio galaxies. The probability for a nearby radio AGN to have emission lines is a strongly decreasing function of galaxy mass and velocity dispersion and an increasing function of radio luminosity above 1025 W Hz−1. Emission-line and radio luminosities are correlated, but with large dispersion. At a given radio power, radio galaxies with small black holes have higher [O iii] luminosities (which we interpret as higher accretion rates) than radio galaxies with big black holes. However, if we scale the emission-line and radio luminosities by the black hole mass, we find a correlation between normalized radio power and accretion rate in Eddington units that is independent of black hole mass. There is also a clear correlation between normalized radio power and the age of the stellar population in the galaxy. Present-day EL-RAGN with the highest normalized radio powers are confined to galaxies with small black holes. High-redshift, high radio-luminosity AGN would be explained if big black holes were similarly active at earlier cosmic epochs. To investigate why only a small fraction of emission-line AGN become radio-loud, we create matched samples of radio-loud and radio-quiet AGN and compare their host galaxy properties and environments. The main difference lies in their environments; our local density estimates are a factor of 2 larger around the radio-loud AGN. We propose a scenario in which radio-loud AGN with emission lines are located in galaxies where accretion of both cold and hot gas can occur simultaneously. At the present day, these conditions are only satisfied for low-mass galaxies in dense environments, but they are likely to apply to most galaxies with massive black holes at higher redshifts.