Large amounts of gas are sometimes funneled to a galaxy’s nuclear regions, with profound consequences. The gas triggers starburst activity and can also feed the supermassive black hole, converting it into an active galactic nucleus (AGN); indeed the supermassive black holes in AGN are thought to gain most of their mass in these accretion events. Eventually, outward pressure from supernovae, shocks, and/or AGN activity terminate the inflow. Galaxy mergers are thought to be one mechanism capable of triggering these massive inflows by disrupting the medium. A less dramatic cause may result from gas flows induced by a combination of galactic rotation and the gravitational instabilities generated by galactic bars, the elongated central structures (composed of stars) found in numerous spiral galaxies including the Milky Way.
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Source: Phys.org