Date Published:

September 1, 201

Abstract:

We present an observationally motivated model to connect the activegalactic nucleus (AGN) and galaxy populations at 0.2 < z < 1.0 andpredict the AGN X-ray luminosity function (XLF). We start withmeasurements of the stellar mass function of galaxies (from the PrismMulti-object Survey) and populate galaxies with AGNs using models forthe probability of a galaxy hosting an AGN as a function of specificaccretion rate. Our model is based on measurements indicating that thespecific accretion rate distribution is a universal function across awide range of host stellar masses with slope γ₁ ≈-0.65 and an overall normalization that evolves with redshift. Wetest several simple assumptions to extend this model to high specificaccretion rates (beyond the measurements) and compare the predictionsfor the XLF with the observed data. We find good agreement with a modelthat allows for a break in the specific accretion rate distribution at apoint corresponding to the Eddington limit, a steep power-law tail tosuper-Eddington ratios with slope \gamma _2=-2.1^{+0.3}_{-0.5}, and ascatter of 0.38 dex in the scaling between black hole and host stellarmass. Our results show that samples of low luminosity AGNs are dominatedby moderately massive galaxies ( {M_*}\sim 10^{10}{--}10^{11} {M}_\odot)growing with a wide range of accretion rates due to the shape of thegalaxy stellar mass function rather than a preference for AGN activityat a particular stellar mass. Luminous AGNs may be a severely skewedpopulation with elevated black hole masses relative to their hostgalaxies and in rare phases of rapid accretion.

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