"Supermassive Black Holes at Galaxies Cores"

“Astronomers have been studying the largest thing in the galaxy, and despite what you might think, it required very careful and accurate work to find it. At the center of the Milky Way galaxy lies Sagittarius A-star, a supermassive black hole, which undoes any lovely notions of light or creation: because our entire galaxy is held together by an engine of destruction.

A sixteen year study by scientists at the Max Planck Institute tracked the orbits of central stars orbiting the black hole. Yes, sixteen years, and that's the very smallest of the dimensions involved here; the black hole itself has a mass of 3.7 MegaSuns, and the unit of distance measurement in the study was the "light month". That's 777 Terameters. Over such scales, entire stars are viewed as "test particles", which is a level of gigantic which only God, and now these scientists, can even conceive of. The observations also prove that no "dark matter" is needed in the center of the galaxy - it's all regular matter. Or at least it was until it got crunched up in the cosmic trash compactor that is a black hole singularity.

Scientist believe that most galaxies, certainly all the spiral ones, have such supermassive black holes in the center. Several have been observed or inferred by their radiation signatures, polar jets or other massively energetic effects. While the exact mechanics of such singularity-centrality have to be worked out, a simple picture can explain why they should be there. Of all the places in the universe, a galactic core is the most likely place for a vast star to form (and then collapse into a black hole), or for a smaller black hole to consume enough matter to become supermassive.

Once the hole passes a certain size its gravitational attraction will shape the orbits of everything it doesn't eat. It no longer matters if it was exactly at the center before - it's made itself the center now. Does this mean that asymmetric dwarf galaxies or globular clusters will eventually develop their own black hole pivot points? It seems inevitable: all black holes below a certain size evaporate, and above a certain size they just keep growing. Wait long enough and one will turn up. And eat you.

There is another special case: Galaxy 0402+379, which is kind of an unimposing name for the greatest unexploded bomb in existence. It doesn't have one supermassive black hole - it has two, and is thought to be the result of a truly massive collision between two galaxies, each with only the standard "one mega-ultra-huge black hole per galaxy". A galactic collision is the second most amazingly violent event you can think of - the first will be when those two black holes eventually hit each other. The resulting merger will release energy on an utterly unprecedented scale, and emit gravity waves which will bend spacetime itself.”

Posted by Luke McKinney,
http://www.dailygalaxy.com/my_weblog/2008/12/are-supermassiv.html#more