  Both fuelling of a central black hole and the build-up of bulges are -- at
least theoretically -- facilitated by non-axisymmetric potentials. It is argued
here that the recently established surface brightness division between
bulge-dominated galaxies that dominate the local type 2 AGN luminosity
function, and disk-dominated galaxies that host (at best) weak AGN can be
understood in terms of a simple bar-instability criterion.

Astrophysics