2018MNRAS.474.1225A

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, λ_sBHAR. We identify a broad distribution of λ_sBHAR in both star-forming and quiescent galaxies - likely reflecting the stochastic nature of AGN fuelling - with a roughly power-law shape that rises towards lower λ_sBHAR, a steep cut-off at λ_sBHAR >= 0.1-1 (in Eddington equivalent units), and a turnover or flattening at λ_sBHAR ≲10^–3^-2^. We find that the probability of a star-forming galaxy hosting a moderate λ_sBHAR AGN depends on stellar mass and evolves with redshift, shifting towards higher λ_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 star-forming 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.