(Submitted November 12, 1998)
Why is it that active galaxies today are substantially less luminous than
high redshift quasars (their presumed progenitor)?
This is a good question, and one which is at the frontier of current
research. So there is no definitive answer. It is possible that the apparent
shortage of high luminosity nearby AGNs is an artifact of incomplete
observations, and that more sensitive searches could reveal more low luminosity
objects at high redshift. More likely is that the high luminosity phase is
relatively short-lived, and that many low redshift galaxies harbor dormant
AGN. The AGN phenomena may be regulated by the supply of gas from the galaxy
to a massive black hole at the center, and a relatively uncommon and violent
event (such as a collision between galaxies or a burst of star formation) is
required in order to provide sufficient fuel.
Unlike most other objects massive black holes can't be destroyed, and at
some level should be detectable through their gravitational influence on
the stars in their host galaxies. As telescope technology improves these
searches are becoming more sensitive, and more evidence for massive compact
objects in otherwise inactive galaxies is emerging.
I hope this helps.
for Ask an Astrophysicist