I saw a television program on the Discovery-Times Channel about
the role of massive black holes in the formation of galaxies.
The show asserted that very recent studies have shown EVERY galaxy
has a massive black hole at its center. They then asserted something
that seems even more far-fetched to me. They asserted that a
black hole can become dormant, and then re-awaken. They asserted
that massive black holes have periods in their existence when
they become very active and become quasars. Are these notions
common in astronomy today? If so, why would a massive black hole
ever become dormant?
Although "every" is a strong word, yes, where we have been able to look for supermassive black holes in the cores of galaxies, they always seem to be there. For instance, such a black hole has been detected at the center of the Milky Way, and Andromeda may have two in a binary system. Indeed, wherever we have had the resolution to detect a black hole at the core of a galaxy, we have seen one.
The phrases "active" and "dormant" in this
context refer to the amount of material falling into the holes.
As matter streams into a black hole, friction and viscosity produce
large amounts of heat, which we see as radio and X-Ray emission.
As the heated infalling plasma interacts with the magnetic fields,
some of it is slingshotted around the hole at high velocity in
"jets" directed away from the poles of the black hole.
These are the characteristics of quasars and active galactic nuclei.
The more matter that falls in, the more emission we see and the
stronger the jets. So if a large amount of matter happens to be
falling in at any particular time, we say the black hole is "active."
If little to no matter happens to be falling in, we call it "dormant."
Answers provided by HSU Astronomy Professor David Kornreich.