The role of bars in AGN fueling in disk galaxies over the last seven
  billion years
  We present empirical constraints on the influence of stellar bars on the
fueling of active galactic nuclei (AGNs) out to z=0.84 using a sample of
X-ray-selected AGNs hosted in luminous non-interacting face-on and moderately
inclined disk galaxies from the Chandra COSMOS survey. Using high-resolution
Hubble Space Telescope imaging to identify bars, we find that the fraction of
barred active galaxies displays a similar behavior as that of inactive spirals,
declining with redshift from 71% at z~0.3, to 35% at z~0.8. With active
galaxies being typically massive, we compare them against a mass-matched sample
of inactive spirals and show that, while at face value the AGN bar fraction is
slightly higher at all redshifts, we cannot rule out that the bar fractions of
active and inactive galaxies are the same. The presence of a bar has no
influence on the AGN strength, with barred and unbarred active galaxies showing
equivalent X-ray luminosity distributions. From our results, we conclude that
the occurrence and the efficiency of the fueling process is independent of the
large scale structure of a galaxy. The role of bars, if any, may be restricted
to providing the suitable conditions for BH fueling to occur, i.e., bring a
fresh supply of gas to the central 100 pc. At the high-redshift end, we find
that roughly 60% of active disk galaxies are unbarred. We speculate this to be
related with the known dynamical state of disks at higher redshifts---more
gas-rich and prone to instabilities than local spirals---which could also lead
to gas inflows without the need of bars.
