The Unified Model

These observations and arguments all suggest that active galaxies are powered by material accreting onto a massive central black hole in the nuclei of galaxies.

What does the nucleus look like?

Not to scale!

Accretion disk: hot, luminous gas falling into the black hole
Jets: charged particles moving at relativistic speeds out of the nucleus
Broad-line clouds: Gas clouds near the accretion disk, turbulent motions at high speed.
Dusty torus: a ring of denser gas and dust surrounding the nucleus.
Narrow-line clouds: Gas clouds further out, moving more slowly.

To achieve the necessary luminosity, ~ 1-10 Msun/yr must be accreted onto the black hole.

Under this model, Seyfert galaxies, radio galaxies, and quasars are similar objects -- accretion powered active nuclei. They may differ in total luminosity (quasars vs seyferts) or in radio power (seyferts vs quasars/radio galaxies) or in host galaxies (seyferts vs radio galaxies).

Question: how does this model "unify" Seyfert 1 and Seyfert 2 galaxies?
Hint: think viewing angle.

Is there direct observational support for this?

What triggers this activity?


1. What do the host galaxies of quasars look like?

These disturbed hosts suggest that quasars form when galaxies are experiencing interactions or gravitational perturbations.


2. When were quasars most active?

We can look at the distribution in redshift of quasars. They occur much more frequently at high redshift, meaning they are very far away, so we are seeing them as they were long ago. We are actually looking at the early universe.



(courtesy Bill Keel, University of Alabama)


Quasars formed when the universe was young, probably coincident with the violence associated with galaxy formation. They quickly died out after that -- only a very few remain.
Question: If quasars are associated with galaxy formation, what should we find in the centers of nearby galaxies?