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In the Beginning

Published: 02-10-2010
Source: JILA Scientific Communications

Artist’s conception of a supermassive black hole at the center of a galaxy. Fellow Mitch Begelman and Isaac Shlosman of the University of Kentucky are developing a theory that not only explains how these massive structures formed in the early Universe, but also the origin of quasars. Credit: NASA/JPL-Caltech

Before there were galaxies with black holes in their centers, there were vast reservoirs of dark matter coupled to ordinary matter, mostly hydrogen gas. These reservoirs were sprinkled with the Universe’s early stars born in pregalactic dark matter halos. But according to Fellow Mitch Begelman, another population of atypical stars formed millions of years later during the creation of galaxies. These stars grew to truly colossal sizes — a million times more massive than the Sun. These Titans of the early Universe burned out in less than two million years, a blink of an eye in a Universe whose years now number nearly 14 billion. But in their short time, they sowed the seeds for the black holes that grew to power mighty quasars and became the behemoths that now reside at the center of every galaxy in the Universe.

Begelman and his colleague Isaac Shlosman of the University of Kentucky recently analyzed the rapid accumulation of gas needed to form these supermassive stars and discovered why the inflowing gas didn’t fragment into stars during the process. Rather, as gas was being funneled into the centers of massive protogalaxies, it would stop flowing inward and form a disk that was violently unstable. In simulations done by other researchers, this disk did not fragment into stars. Instead, the rotating disk formed a bar, which reduced the turbulence and allowed the gas to resume falling in toward the center, where a single supermassive star was forming. Begelman and Shlosman suggested that it was the turbulence that inhibited other stars from forming. Every time the gas became unstable and grew turbulent (stopping the infall again), another bar formed. Every cycle of this “bars within bars” process funneled more gas into the center of the protogalaxies. Half the known galaxies in the Universe show evidence of this process.

The bars-within-bars process rapidly created dense, self-gravitating cores so optically thick, even photons couldn’t leave them. Begelman has also studied the evolution of these cores, which were surrounded by stable envelopes that contained most of the the accumulated mass. The cores soon collapsed into black holes. Once the black hole appeared, the black hole and its surrounding gaseous envelope became a quasistar. Deep inside the massive quasistars, the black holes rapidly enlarged by sucking in matter from their bloated envelopes. The matter about to fall into the black holes released large amounts of energy, which puffed up the quasistars. However, the quasistars evaporated in relatively short order, leaving behind “seed” black holes with masses of a hundred thousand suns at the centers of billions of young galaxies.The seed black holes were already mini-quasars, churning out light and radiation as they gorged on matter falling into them.

These monster black holes are still with us. In fact, they lurk in the center of every galaxy. They're just quieter now, at least most of the time.   - Julie Phillips


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