Nuclear Spiral Shocks and Induced Gas Inflows in Weak Oval Potentials
  Nuclear spirals are ubiquitous in galaxy centers. They exist not only in
strong barred galaxies but also in galaxies without noticeable bars. We use
high-resolution hydrodynamic simulations to study the properties of nuclear gas
spirals driven by weak bar-like and oval potentials. The amplitude of the
spirals increases toward the center by a geometric effect, readily developing
into shocks at small radii even for very weak potentials. The shape of the
spirals and shocks depends rather sensitively on the background shear. When
shear is low, the nuclear spirals are loosely wound and the shocks are almost
straight, resulting in large mass inflows toward the center. When shear is
high, on the other hand, the spirals are tightly wound and the shocks are
oblique, forming a circumnuclear disk through which gas flows inward at a
relatively lower rate. The induced mass inflow rates are enough to power black
hole accretion in various types of Seyfert galaxies as well as to drive
supersonic turbulence at small radii.
