Gas flows in an Active galactic nuclei. I. Two-phase gas inflow
  We perform two-dimensional hydrodynamic simulations to study the non-rotating
gas flow in the region from 1-1000 parsec. This region connects the galaxy
scale and the accretion disk scale. The gas is irradiated by the photons
generated by the accretion disk system near the central black hole with $10^8$
solar mass. We assume that the luminosity of the central accretion system is
constant and equals to $0.3$ Eddington luminosity. Gas with density higher than
$10^{-24} {\rm g \ cm^{-3}}$ is found to be thermally unstable. Two phase, cold
and hot, gas flow will form due to thermal instability. We calculated the ratio
of cold gas accretion rate to hot gas accretion rate. This ratio is a function
of gas density and generally increases with the increase of gas density. The
maximum value of this ratio is $16$, when gas density is $10^{-21} {\rm g \
cm^{-3}} $. Gas with density lower than $10^{-24} {\rm g \ cm^{-3}}$ is found
to be thermally stable and the gas flow is one-phase. The applications of the
results are briefly discussed.
