EVOLUTION OF A NUCLEAR GAS DISK AND GAS SUPPLY TO THE GALACTIC CENTER. I. MIGRATION OF MASSIVE GAS CLUMPS

Abstract

We investigate the gas supply process from a nuclear gas disk to the Galactic center by performing two-dimensional hydrodynamic simulations including self-gravity and radiative cooling. We assume that the gas is supplied to the nuclear gas disk by the host galaxy disk. We show that many gas clumps are formed in the nuclear gas disk after the nuclear gas disk becomes massive enough to become gravitationally unstable. Coalescence of the gas clumps leads to the formation of more massive gas clumps. Gravitational encounters between these massive gas clumps induce angular momentum transfer between the gas clumps. Massive gas clumps that lose their angular momentum due to gravitational encounters move to the galactic center. In this way, the gas supply to the galactic center is realized. We also perform a numerical simulation with twice the spatial resolution to check numerical resolution effects. Time evolution of azimuthally averaged surface density distributions in both models agree well with each other. The high-mass part of the clump mass function in the low-resolution model is in good agreement with that of the high-resolution model, and lower mass clumps are well resolved in the high-resolution model. Therefore, we conclude that massive gas clumps play a major role in the angular momentum transfer. The gas supply rate to the center is about half of the gas supply rate from the host galaxy when the whole region of the nuclear gas disk becomes self-gravitationally unstable. More massive gas clumps are formed and migrate to the Galactic center in the models of the higher gas supply rate from the host galaxy.