Date Published:

February 1, 2012

Abstract:

We present evidence that the incidence of active galactic nuclei (AGNs)and the distribution of their accretion rates do not depend on thestellar masses of their host galaxies, contrary to previous studies. Weuse hard (2-10 keV) X-ray data from three extragalactic fields (XMM-LSS,COSMOS, and ELAIS-S1) with redshifts from the Prism Multi-object Surveyto identify 242 AGNs with L _{2-10 keV} = 10^42-44 ergs^-1 within a parent sample of ~25,000 galaxies at 0.2 < z< 1.0 over ~3.4 deg² and to i ~ 23. We find that althoughthe fraction of galaxies hosting an AGN at fixed X-ray luminosity risesstrongly with stellar mass, the distribution of X-ray luminosities isindependent of mass. Furthermore, we show that the probability that agalaxy will host an AGN can be defined by a universal Eddington ratiodistribution that is independent of the host galaxy stellar mass and hasa power-law shape with slope -0.65. These results demonstrate that AGNsare prevalent at all stellar masses in the range 9.5 and that the samephysical processes regulate AGN activity in all galaxies in this stellarmass range. While a higher AGN fraction may be observed in massivegalaxies, this is a selection effect related to the underlying Eddingtonratio distribution. We also find that the AGN fraction drops rapidlybetween z ~ 1 and the present day and is moderately enhanced (factor ~2)in galaxies with blue or green optical colors. Consequently, while AGNactivity and star formation appear to be globally correlated, we do notfind evidence that the presence of an AGN is related to the quenching ofstar formation or the color transformation of galaxies.

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