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The Question

(Submitted May 09, 2001)

How do the super-massive black holes alleged to be found at the centers of galaxies form? They would have to be much powerful than the average black hole formed by stars to keep billions of stars in orbit.

The Answer

This is a very good question. You might want to read:

for the current evidence for supermassive black holes, and what it implies for the formation mechanism. To summarize:

Recent studies indeed show that perhaps all galaxies with "bulges" have a supermassive black hole, which contains about 0.2% of the mass of the bulge. Depending on the size of the galaxy, that might translate to a few million to a few billion times the mass of the sun. This fraction, 0.2%, is too small for the supermassive black hole to control the motion of all the stars in the bulge. On the other hand, these masses are indeed too large to have formed from a single star.

So how did they form? We don't have a detailed answer yet. We do know that gravity can act in a catastrophic way. If you have a region of enhanced density (say more stars per volume than in surrounding regions), its gravity will be stronger, which tends to increase the density more ... .

The recent finding, that the mass of the supermassive black holes is closely related to that of the bulge, shows that the formation of supermassive black holes is also closely linked to that of the host galaxy. That is, they (the galaxies and the supermassive black holes) probably grow together. This is a really exciting area of research --- check back with us in a few years' time, and we may have a more definitive answer for you by then.

Hope this helps,

Koji & Bish
for "Ask an Astrophysicist"

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