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The Question
(Submitted March 30, 2001)
I recently saw a TV program exploring new theories about the relation
between black holes and galaxies. People thought that there was a black
hole in the center of every galaxy, and that it was possible that galaxies
are created by black holes. Is this true and if so, what do you think?
The Answer
It's great that you're interested in this subject, and that you got
your information from such a quality program.
It is true that observations show a tight relationship between the
masses of the central black holes and the masses of their host galaxies.
It is also true that many scientists believe there is a black hole at the
center of each large galaxy. (This doesn't apply to smaller galaxies,
by the way, like the Large and Small Magellanic Clouds, which are
satellites of the Milky Way Galaxy.)
Nobody knows for sure exactly how galaxies formed --- the details are very hard
to reconstruct. Whatever they turn out to be, one thing you need is a
region of enhanced density (once you start the process, it increases the
gravity, which in turn increases the density, so it builds up). Can the
central black hole be the seed that started the creation of the galaxy?
Possibly, although the galaxy is usually something like a hundred thousand
times more massive than the central black hole, so the numbers may not work
out. Other possibilities are that the black hole grows to "fit" the size of
the galaxy in some ways (i.e., the black hole came later) or that the two are
both the results of a single underlying cause (i.e., the two were created
at the same time). It's an exciting area of research --- check back with
us in a few years, and we may have a more definite answer by then.
Hope this helps.
Koji & Bish
for "Ask an Astrophysicist"
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