The Relative Role of Bars and Environments in AGN Triggering
  We quantify the relative role of galaxy environment and bar presence on AGN
triggering in face-on spiral galaxies using a volume-limited sample with $0.02
< z < 0.055$, $M_r < 19.5$, and $\sigma > 70 \rm ms s^{-1}$ selected from SDSS
Data Release 7. To separate their possible entangled effects, we divide the
sample into bar and non-bar sample, and each sample is further divided into
three environment cases of isolated galaxies, interacting galaxies with a pair,
and cluster galaxies. The isolated case is used as a control sample. For these
six cases, we measure AGN fractions at a fixed central star formation rate and
central velocity dispersion, $\sigma$. We demonstrate that the internal process
of the bar-induced gas inflow is more efficient in AGN triggering than the
external mechanism of the galaxy interactions in groups and cluster outskirts.
The significant effects of bar instability and galaxy environments are found in
galaxies with a relatively less massive bulge. We conclude that from the
perspective of AGN-galaxy co-evolution, a massive black hole is one of the key
drivers of spiral galaxy evolution. If it is not met, a bar instability helps
the evolution, and in the absence of bars, galaxy interactions/mergers become
important. In other words, in the presence of a massive central engine, the
role of the two gas inflow mechanisms is reduced or almost disappears. We also
find that bars in massive galaxies are very decisive in increasing AGN
fractions when the host galaxies are inside clusters.
