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 to 1000 pc. This region connects the galaxy scale and the accretion disc scale. The gas is irradiated by the photons generated by the accretion disc 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) 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) g cm(-3). Gas with density lower than 10(-24) g cm(-3) is found to be thermally stable and the gas flow is one-phase. The applications of the results are briefly discussed.