CENTRAL REGIONS OF BARRED GALAXIES: TWO-DIMENSIONAL NON-SELF-GRAVITATING HYDRODYNAMIC SIMULATIONS

The inner regions of barred galaxies contain substructures such as off-axis shocks, nuclear rings, and nuclear spirals. These substructures may affect star formation, and control the activity of a central black hole (BH) by determining the mass inflow rate. We investigate the formation and properties of such substructures using high-resolution, grid-based hydrodynamic simulations. The gaseous medium is assumed to be infinitesimally thin, isothermal, and non-self-gravitating. The stars and dark matter are represented by a static gravitational potential with four components: a stellar disk, a bulge, a central BH, and a bar. To investigate various galactic environments, we vary the gas sound speed, c(s), as well as the mass of the central BH, M-BH. Once the flow has reached a quasi-steady state, off-axis shocks tend to move closer to the bar major axis as c(s) increases. Nuclear rings shrink in size with increasing cs, but are independent of M-BH, suggesting that the ring position is not determined by the Lindblad resonances. Rings in low-c(s) models are narrow since they are occupied largely by gas on x(2)-orbits and well decoupled from nuclear spirals, while they become broad because of large thermal perturbations in high-c(s) models. Nuclear spirals persist only when either c(s) is small or M-BH is large; they would otherwise be destroyed completely by the ring material on eccentric orbits. The shape and strength of nuclear spirals depend sensitively on c(s) and M-BH such that they are leading if both c(s) and M-BH are small, weak trailing if c(s) is small and M-BH is large, and strong trailing if both c(s) and M-BH are large. While the mass inflow rate toward the nucleus is quite small in low-c(s) models because of the presence of a narrow nuclear ring, it becomes larger than 0.01 M-circle dot yr(-1) when cs is large, providing a potential explanation of nuclear activity in Seyfert galaxies.