Building Galaxies

StarDate: April 29, 2009

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Which came first -- the chicken, or the egg?

The astronomers who study the birth of galaxies are facing that same type of puzzle. Most large galaxies -- and perhaps all of them -- have supermassive black holes in their cores. And there's a relationship between the mass of the black hole and the mass of the "bulge" of stars and gas around it. But that raises the chicken-and-egg question of which came first -- the black hole or the bulge?

Earlier this year, a team of astronomers found a hint that the answer may be the black hole.

Over the last decade or so, astronomers have found that the bulge of stars and gas around a black hole is about 600 times as massive as the black hole itself.

That means that the growth of the black hole and its galaxy are related. The gravity of the black hole probably pulls in gas and dust, building up the bulge. Some of the material forms a hot disk around the black hole. When the disk gets hot enough, it blows away the remaining material, shutting down the growth of the bulge around it.

Astronomers recently looked at several galaxies with quasars -- extremely bright disks around their central black holes. The galaxies are so far away that we see them as they looked when the universe was very young.

The black holes in these galaxies are quite massive, but the bulges around them are fairly small. That suggests that at least in these galaxies, the black holes came first -- and the galaxies built up around them.

Script by Damond Benningfield, Copyright 2009

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