Nuclear spirals: gas in asymmetric galactic potential with a massive   black hole
  Nuclear spirals can provide a wealth of information about the nuclear potential in disc galaxies. They form naturally as a gas response to non-axisymmetry in the gravitational potential, even if the degree of this asymmetry is very small. Linear wave theory well describes weak nuclear spirals, but stronger asymmetries in the potential induce waves beyond the linear regime, which appear as spiral shocks. If a central massive black hole (MBH) is present, spiral shocks can extend all the way to its immediate vicinity, and generate gas inflow up to 0.03 Msun/yr. This coincides with the accretion rates needed to power local Active Galactic Nuclei. 
Witold Maciejewski (Jagiellonian University)