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 c s , 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 ☉ yr −1 when c s is large, providing a potential explanation of nuclear activity in Seyfert galaxies.