PRIMUS: THE DEPENDENCE OF AGN ACCRETION ON HOST STELLAR MASS AND COLOR

Abstract

We present evidence that the incidence of active galactic nuclei (AGNs) and\nthe distribution of their accretion rates do not depend on the stellar masses\nof their host galaxies, contrary to previous studies. We use hard (2-10 keV)\nX-ray data from three extragalactic fields (XMM-LSS, COSMOS and ELAIS-S1) with\nredshifts from the Prism Multi-object Survey to identify 242 AGNs with L_{2-10\nkeV}=10^{42-44} erg /s within a parent sample of ~25,000 galaxies at 0.2<z<1.0\nover ~3.4 deg^2 and to i~23. We find that although the fraction of galaxies\nhosting an AGN at fixed X-ray luminosity rises strongly with stellar mass, the\ndistribution of X-ray luminosities is independent of mass. Furthermore, we show\nthat the probability that a galaxy will host an AGN can be defined by a\nuniversal Eddington ratio distribution that is independent of the host galaxy\nstellar mass and has a power-law shape with slope -0.65. These results\ndemonstrate that AGNs are prevalent at all stellar masses in the range 9.5<log\nM_*/M_sun<12 and that the same physical processes regulate AGN activity in all\ngalaxies in this stellar mass range. While a higher AGN fraction may be\nobserved in massive galaxies, this is a selection effect related to the\nunderlying Eddington ratio distribution. We also find that the AGN fraction\ndrops rapidly between z~1 and the present day and is moderately enhanced\n(factor~2) in galaxies with blue or green optical colors. Consequently, while\nAGN activity and star formation appear to be globally correlated, we do not\nfind evidence that the presence of an AGN is related to the quenching of star\nformation or the color transformation of galaxies.\n