Environmental dependence of AGN activity. I.: the effects of host galaxy
  Using a large sample of local galaxies (144,940) with -17.5< Mr <-22 and
0.025< z <0.107, selected from SDSS DR5, we compare 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
with galaxy properties. The AGN sample is composed of Type II AGNs identified
with flux ratios of narrow-emission lines with S/N > 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 velocity dispersion of
host galaxies. In particular, f_AGN of early-type galaxies is almost
independent of luminosity nor velocity dispersion when color is fixed. The host
galaxy color preferred by AGNs is u-r ~2.0 for early-type hosts and u-r=2.0-2.4
for late-type hosts. This trend suggests that AGNs are dominantly hosted by
intermediate-mass late-type galaxies. We also investigate how the accretion
power varies with galaxy properties. While the Eddington ratio ([OIII] 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 galaxiesalready 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 lat e-type galaxiesmay not
host massive black holes or may host very low-power AGNs.
