  We examine the relationship between galaxies, supermassive black holes and
AGN using a sample of 22,000 narrow-emission-line AGN drawn from a a sample of
122,000 galaxies from the Sloan Digital Sky Survey. We have studied how AGN
host properties compare with those of normal galaxies and how they depend on
the luminosity of the active nucleus. We find that AGN reside in massive
galaxies and have distributions of sizes and concentrations that are similar to
those of the early-type galaxies in our sample. The host galaxies of
low-luminosity AGN have stellar populations similar to normal early-types. The
hosts of high-luminosity AGN have much younger mean stellar ages and a
significant fraction have experienced recent starbursts. High-luminosity AGN
are also found in lower density environments. We use the stellar velocity
dispersions of the AGN hosts to estimate black hole masses and their
[OIII]$\lambda$5007 emission line luminosities to estimate black hole accretion
rates. We find that the volume averaged ratio of star formation to black hole
accretion is ~1000, in remarkable agreement with the observed ratio of stellar
mass to black hole mass in nearby bulges. Our estimated accretion rates imply
that low mass black holes are growing on a timescale that is comparable to the
age of the Universe. The growth timescale increases by more than an order of
magnitude for the most massive black holes in our sample. We conclude that the
evolution of the AGN luminosity function is driven by a decrease in the
characteristic mass scale of actively accreting black holes.
