X-rays across the galaxy population – II. The distribution of AGN accretion rates as a function of stellar mass and redshift

Abstract

We use deep Chandra X-ray imaging to measure the distribution of specific black hole accretion rates (L X relative to the stellar mass of the galaxy) and thus trace active galactic nucleus (AGN) activity within star-forming and quiescent galaxies, as a function of stellar mass (from 10 8.5 to 10 11.5 M ) and redshift (to z 4). We adopt near-infrared-selected samples of galaxies from the CANDELS and UltraVISTA surveys, extract X-ray data for every galaxy, and use a flexible Bayesian method to combine these data and to measure the probability distribution function of specific black hole accretion rates, sBHAR . We identify a broad distribution of sBHAR in both star-forming and quiescent galaxies -likely reflecting the stochastic nature of AGN fuellingwith a roughly power-law shape that rises towards lower sBHAR , a steep cut-off at sBHAR 0.1-1 (in Eddington equivalent units), and a turnover or flattening at sBHAR 10 -3 to 10 -2 . We find that the probability of a star-forming galaxy hosting a moderate sBHAR AGN depends on stellar mass and evolves with redshift, shifting towards higher sBHAR at higher redshifts. This evolution is truncated at a point corresponding to the Eddington limit, indicating black holes may self-regulate their growth at high redshifts when copious gas is available. The probability of a quiescent galaxy hosting an AGN is generally lower than that of a starforming galaxy, shows signs of suppression at the highest stellar masses and evolves strongly with redshift. The AGN duty cycle in high-redshift (z 2) quiescent galaxies thus reaches 20 per cent, comparable to the duty cycle in star-forming galaxies of equivalent stellar mass and redshift.