ENVIRONMENTAL DEPENDENCE OF ACTIVE GALACTIC NUCLEUS ACTIVITY. I. THE EFFECTS OF HOST GALAXY

Using a large sample of local galaxies (144,940) with -17.5 < M-r < -22 and 0.025 < z < 0.107, selected from Sloan Digital Sky Survey Data Release 5, we compare active galactic nucleus (AGN) host galaxies with non-AGN galaxies at matched luminosity, velocity dispersion, color, color gradient, or concentration index, to investigate how AGN activity is related to galaxy properties. The AGN sample is composed of Type II AGNs identified with flux ratios of narrow emission lines with signal-to-noise ratio > 6. We find that the fraction of galaxies hosting an AGN (f(AGN)) depends strongly on morphology together with color, and very weakly on luminosity or stellar velocity dispersion of host galaxies. In particular, f(AGN) of early-type galaxies is almost independent of luminosity or velocity dispersion when color is fixed. The host galaxy color preferred by AGNs is u - r approximate to 2.0 for early-type hosts and u - r = 2.0 similar to 2.4 for late-type hosts. This trend suggests that AGNs are dominantly hosted by intermediate-mass late-type galaxies because early-type galaxies with u - r approximate to 2.0 are very rare. We also investigate how the accretion power varies with galaxy properties. While the Eddington ratio ([O III] line luminosity normalized by black hole mass) ranges over three orders of magnitude for both morphological types, late-type galaxies are the dominant hosts over all AGN power. Among late-type galaxies, bluer color galaxies host higher power AGNs. These results are consistent with a scenario that more massive and redder galaxies are harder to host AGNs since these galaxies already consumed gas at the center or do not have sufficient gas supply to feed the black hole. In contrast, intermediate-mass, intermediate-color, and more concentrated late-type galaxies are more likely to host AGNs, implying that perhaps some fraction of low-mass, blue, and less concentrated late-type galaxies may not host massive black holes or may host very low power AGNs.