Abstract
This paper estimates the specific accretion-rate distribution of AGN using a sample of 4821 X-ray sources from both deep and shallow surveys. The specific accretion-rate distribution 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, $\lambda<10^{-4}-10^{-3}$ (in Eddington units). There is also 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 AGN among galaxies drops toward lower redshifts. This decrease in the AGN duty cycle is responsible for the strong evolution of the accretion density of the Universe from redshift $z\approx1-1.5$ to the present day. Our analysis also suggests that this evolution is accompanied by a decoupling of accretion events onto black holes from the formation of stars in galaxies. There is also evidence that at earlier times the relative probability of high vs low specific accretion-rate events among galaxies increases. We argue that this differential redshift evolution of the AGN duty cycle with respect to $\lambda$ 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.
Highlights
X-ray surveys in the last 15 years have provided an excellent census of the Active Galactic Nuclei (AGN) population and describe with increasing accuracy how the space density of these sources depends on redshift, accretion luminosity and amount of line-of-sight obscuration (e.g., Brandt & Alexander 2015)
This behaviour is manifested by a threshold in the specific accretion-rate below which the width of the hatched regions is comparable to the full range of the P (λ, z) prior
The non-parametric approach for the determination of the specific accretion-rate distribution of AGN recovers a shape that is characterised by an increase toward low specific accretion rates, a break close to the Eddington limit, above which the probability of a galaxy hosting an AGN drops steeply, and a turnover at very low specific accretion rates, log λ
Summary
X-ray surveys in the last 15 years have provided an excellent census of the Active Galactic Nuclei (AGN) population and describe with increasing accuracy how the space density of these sources depends on redshift, accretion luminosity and amount of line-of-sight obscuration (e.g., Brandt & Alexander 2015). One approach is to study the accretion events onto supermassive black holes in relation to their host galaxies to explore which galaxy types and environments are conducive to black-hole growth. The level of star-formation in AGN hosts is used as a proxy of gas availability to explore the necessary conditions for black-hole accretion events (e.g., Santini et al 2012; Rovilos et al 2012; Rosario et al 2013; Mullaney et al 2015). The position of AGN on the cosmic web and their clustering properties relative to galaxies may hold important clues on the diversity of black-hole fuelling modes and the role of Mpc-scale environment on accretion events (Allevato et al 2011; Fanidakis et al 2013; Allevato et al 2014)
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