From Gas to Starburst and AGN

Although it is clear that the circumnuclear regions of galaxies are intimately related to their host galaxies, most directly through their bars, it remains unclear what exactly initiates and fuels nuclear stellar (starburst) and non-stellar (AGN) activity. Deviations from axisymmetry in the gravitational potential of a galaxy, set up by a bar or an interaction, are known to cause gas in°ow, and must at some scale and level be related to the fueling of AGN and starbursts. We review the observed relations between bars and interactions on the one, and nuclear activity of the Seyfert and starburst variety on the other hand, and conclude that none of these relations is particularly signi¯cant in a statistical sense, except in extreme and rare cases, such as ultra-luminous infrared galaxies. Nuclear rings of star formation, however, are not only related directly to non-axisymmetries in the potential, but also, it seems, to the occurrence of nuclear activity. Their role as potential tracers of the fueling process must be further explored. 1