Formation of Nuclear Spirals in Barred Galaxies

We have performed smoothed particle hydrodynamics (SPH) simulations for theresponse of the gaseous disk to the imposed moderately strong non-axisymmetricpotentials. The model galaxies are composed of the three stellar components(disk, bulge and bar) and two dark ones (supermassive black hole and halo)whose gravitational potentials are assumed to be invariant in time in the framecorotating with the bar. We found that the torques alone generated by themoderately strong bar that gives the maximum of tangential-to-radial forceratio as $(F_{Tan}/F_{Rad})_{max}= 0.3$ are not sufficient to drive the gasparticles close to the center due to the barrier imposed by the inner Lindbladresonances (ILRs). In order to transport the gas particles towards the nucleus($r<100$ pc), a central supermassive black hole (SMBH) and high sound speed ofthe gas are required to be present. The former is required to remove the innerinner Lindblad resonance (IILR) that prevents gas inflow close to the nucleus,while the latter provides favourable conditions for the gas particles to losetheir angular momentum and to spiral in. Our models that have no IILR show thetrailing nuclear spirals whose innermost parts reach close to the center in acurling way when the gas sound speed is $ c_{s} \gtrsim 15$ km s$^{-1}$. Theyresemble the symmetric two-armed nuclear spirals observed in the centralkiloparsec of spiral galaxies. We found that the symmetric two-armed nuclearspirals are formed by the hydrodynamic spiral shocks caused by thegravitational torque of the bar in the presence of a central SMBH that canremove IILR when the sound speed of gas is high enough to drive a large amountof gas inflow deep inside the ILR. However, the detailed morphology of nuclearspirals depends on the sound speed of gas.Comment: 38 pages, 10 figures, accepted for publication in Ap