  Since the 1970s, astronomers have struggled with the issue of how matter can
be accreted to promote black hole growth. While low-angular-momentum stars may
be devoured by the black hole, they are not a sustainable source of fuel. Gas,
which could potentially provide an abundant fuel source, presents another
challenge due to its enormous angular momentum. While viscous torques are not
significant, gas is subject to gravity torques from non-axisymmetric potentials
such as bars and spirals. Primary bars can exchange angular momentum with the
gas inside corotation, driving it inward spiraling until the inner Lindblad
resonance is reached. An embedded nuclear bar can then take over. As the gas
reaches the black hole's sphere of influence, the torque turns negative,
fueling the center. Dynamical friction also accelerates the infall of gas
clouds closer to the nucleus. However, due to the Eddington limit, growing a
black hole from a stellar-mass seed is a slow process. The existence of very
massive black holes in the early universe remains a puzzle that could
potentially be solved through direct collapse of massive clouds into black
holes or super-Eddington accretion.
