Seyfert Galaxies (2) ...
The adjacent right figure
shows closeups of some Seyfert galaxies imaged
by the Hubble Space Telescope.
(The arrows indicate the direction to north in each image.) The images are
strongly overexposed to emphasize the structure near the nucleus, causing diffraction spikes and other image
artifacts. Although all of these are Seyfert galaxies, they exhibit clear differences in
optical appearance.
The differences among these images may be a function of the
different ways in which the central black hole that we believe supplies a Seyfert's power
is acquiring matter in these galaxies.
A Unified Picture of Seyfert Galaxies
There appear to be two kinds of Seyfert galaxies, type 1 and type 2, with rather different
spectral characteristics. Furthermore, the time variability of their large luminosities
implies an enormously powerful but extremely compact energy source residing in the core.
We may further infer that there must be a very large mass in the core of a Seyfert galaxy,
because the high
velocities implied by the broad spectral lines seen in Seyfert 1s
only makes sense if there is a huge
gravitational mass within the region where the broad lines are created. Simple estimates
place this mass in the 100 million to 10 billion solar mass range. Can we assemble these facts
into a model that might account for what we observe for Seyfert galaxies?
The answer appears to be "Yes"! Although all details are certainly not understood,
these facts can be correlated by a model in which the central energy source of a Seyfert galaxy
is a supermassive (of order a billion solar mass) spinning black hole. Let us describe the
basics of this model.
