A Novel Method to Identify AGNs Based on Emission Line Excess and the
  Nature of Low-luminosity AGNs in the Sloan Digital Sky Survey: II - Nature of
  Low-luminosity AGNs
  We develop a novel method to identify active galactic nuclei (AGNs) and study
the nature of low-luminosity AGNs in the Sloan Digital Sky Survey. This is the
second part of a series of papers and we study the correlations between the AGN
activities and host galaxy properties. Based on a sample of AGNs identified
with the new method developed in Paper-I, we find that AGNs typically show
extinction of tau_V=1.2 and they exhibit a wide range of ionization levels. The
latter finding motivates us to use [OII]+[OIII] luminosity as an indicator of
AGN power. We find that AGNs are preferentially located in massive, red,
early-type galaxies. By carefully taking into account a selection bias of the
Oxygen-excess method, we show that strong AGNs are located in actively star
forming galaxies and rapidly growing super-massive black holes are located in
rapidly growing galaxies, which clearly shows the co-evolution of super-massive
black holes and the host galaxies. This is a surprising phenomenon given that
the growths of black holes and host galaxies occur at very different physical
scales. Interestingly, the AGN power does not strongly correlate with the host
galaxy mass. It seems that mass works like a 'switch' to activate AGNs. The
absence of AGNs in low-mass galaxies might be due the absence of super-massive
black holes in those galaxies, but a dedicated observation of nuclear region of
nearby low-mass galaxies would be necessarily to obtain deeper insights into
it.
