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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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