Thursday, August 26, 2010

When galaxies collide.

Thanks to science fiction, most people know that our universe is a dangerous place. If you happen to cross the event horizon of a black hole (which is invisible, by the way), your body will immediately be ripped to shreds by tidal forces, and every bit of matter that ever made you "you" will be crushed to an infinite density. In case that doesn't scare you, try this on for size: black holes exist at the center of many galaxies, including our own. That's right. 20,000 light years away, there is an enormously energetic, spectacularly dense, churning pit of doom just waiting for its next victim to wander one step too close. Supermassive black holes (SMBHs) fuel objects called quasars, which are some of the most powerful and luminous sources of radiation in the universe. Quasars can be detected from billions of light years away due to the extraordinary amounts of energy that SMBHs provide them with. But how did these black holes form in the first place?

Artist rendering of dust in the quasar wind. Image courtesy of NASA/JPL.

The SMBHs that power quasars were born when the universe was still very young, mere millions of years after the Big Bang. This means that they must have formed very quickly, and in an environment that contained very few metals. A metal-rich atmosphere would have given rise to normal stars, not black holes; however, even a metal-poor atmosphere would not have been able to yield such massive black holes in such a short time. Now, researchers from Ohio State University believe they have solved the problem. In a paper published in this week's Nature, they suggest that SMBHs formed during the collisions of protogalaxies in the early universe. According to their theory, SMBHs are created when two massive, orbiting galaxies collide and merge into one large, spinning disk of gas. As the gas swirls, it collects in the center of the disk and condenses under the influence of gravity. In only 10,000 years, this process creates a gas clump that weighs hundreds of millions of times the mass of our sun! Eventually, the clump becomes dense enough to create the precursor to a SMBH, or a "seed." This seed gathers more material over time, until it becomes one of the powerful and ancient black holes we observe today.

Artist's rendering of a supermassive black hole. Image courtesy of Phil Plait.

Of course, this mechanism can only explain the formation of SMBHs by colliding galaxies above a certain mass. The OSU team plans to further their research by surveying the masses of SMBHs at different distances, and thus different times in the past. (Since light from faraway stars always travels to us at a fixed rate, looking into the distant universe actually allows scientists to see the universe as it was billions and billions of years ago.) If they can back their model with observational evidence, the OSU team will be that much closer to understanding the complexities of stellar evolution.

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