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

We use deepChandra X-ray imaging to measure the distribution of specific black hole accretion rates (LX 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 108.5 to 1011.5 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, λsBHAR. We identify a broad distribution of λsBHAR 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 λsBHAR, a steep cut-off at λsBHAR 0.1–1 (in Eddington equivalent units), and a turnover or flattening at λsBHAR 10−3 to 10−2. We find that the probability of a star-forming galaxy hosting a moderate λsBHAR AGN depends on stellar mass and evolves with redshift, shifting towards higher λsBHAR 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

X-rays across the galaxy population - III. The incidence of AGN as a function of star formation rate

We map the co-eval growth of galaxies and their central supermassive black holes in detail by measuring the incidence of active galactic nuclei (AGNs) in galaxies as a function of star formation rate (SFR) and redshift (to z ∼ 4). We combine large galaxy samples with deep Chandra X-ray imaging to measure the probability distribution of specific black hole accretion rates (LX relative to stellar mass) and derive robust AGN fractions and average specific accretion rates. First, we consider galaxies along the main sequence of star formation. We find a linear correlation between the average SFR and both the AGN fraction and average specific accretion rate across a wide range in stellar mass (M∗ ∼ 108.5−11.5 M ) and to at least z ∼ 2.5, indicating that AGNs in main-sequence galaxies are driven by the stochastic accretion of cold gas. We also consider quiescent galaxies and find significantly higher AGN fractions than predicted, given their low SFRs, indicating that AGNs in quiescent galaxies are fuelled by additional mechanisms (e.g. stellar winds). Next, we bin galaxies according to their SFRs relative to the main sequence. We find that the AGN fraction is significantly elevated for galaxies that are still star forming but with SFRs below the main sequence, indicating further triggering mechanisms enhance AGN activity within these sub-main-sequence galaxies. We also find that the incidence of high accretion rate AGN is enhanced in starburst galaxies and evolves more mildly with redshift than within the rest of the galaxy population, suggesting mergers play a role in driving AGN activity in such high-SFR galaxies

Observational constraints on the specific accretion-rate distribution of X-ray-selected AGNs

This paper estimates the specific accretion-rate distribution of AGNs using a sample of 4821 X-ray sources from both deep and shallow surveys. The specific accretion-rate distribution is used as a proxy of the Eddington ratio and is defined as the probability of a galaxy with a given stellar mass and redshift hosting an active nucleus with a certain specific accretion rate. We find that the probability of a galaxy hosting an AGN increases with decreasing specific accretion rate. There is evidence that this trend reverses at low specific accretion rates, λ ≲ 10^−4 –10^−3 (Eddington units). There is a break close to the Eddington limit, above which the probability of an accretion event decreases steeply. The specific accretion-rate distribution evolves such that the fraction of AGNs among galaxies drops towards lower redshifts. This decrease in the AGN duty cycle is responsible for the strong evolution of the accretion density of the Universe from redshift z ≈ 1–1.5 to the present day. Our analysis also suggests that this evolution is accompanied by a decoupling of accretion events on to black holes from the formation of stars in galaxies. There is also evidence that at earlier times the relative probability of high versus low specific accretion-rate events among galaxies increases. We argue that this differential redshift evolution of the AGN duty cycle with respect to λ produces the AGN downsizing trend, whereby luminous sources peak at earlier epochs compared to less luminous ones. Finally, we also find a stellar mass dependence of the specific accretion-rate distribution, with more massive galaxies avoiding high specific accretion-rate events

Observational constraints on the specific accretion-rate distribution of X-ray-selected AGNs

This paper estimates the specific accretion-rate distribution of AGNs using a sample of 4821 X-ray sources from both deep and shallow surveys. The specific accretion-rate distribution is used as a proxy of the Eddington ratio and is defined as the probability of a galaxy with a given stellar mass and redshift hosting an active nucleus with a certain specific accretion rate. We find that the probability of a galaxy hosting an AGN increases with decreasing specific accretion rate. There is evidence that this trend reverses at low specific accretion rates, λ ≲ 10^−4 –10^−3 (Eddington units). There is a break close to the Eddington limit, above which the probability of an accretion event decreases steeply. The specific accretion-rate distribution evolves such that the fraction of AGNs among galaxies drops towards lower redshifts. This decrease in the AGN duty cycle is responsible for the strong evolution of the accretion density of the Universe from redshift z ≈ 1–1.5 to the present day. Our analysis also suggests that this evolution is accompanied by a decoupling of accretion events on to black holes from the formation of stars in galaxies. There is also evidence that at earlier times the relative probability of high versus low specific accretion-rate events among galaxies increases. We argue that this differential redshift evolution of the AGN duty cycle with respect to λ produces the AGN downsizing trend, whereby luminous sources peak at earlier epochs compared to less luminous ones. Finally, we also find a stellar mass dependence of the specific accretion-rate distribution, with more massive galaxies avoiding high specific accretion-rate events