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Black holes' spin rates

New research sheds light on how black holes form and evolve.
M31
Scientists theorize most galaxies, such as the Andromeda Galaxy shown here, contain a black hole at their center.
Paolo Candy
May 30, 2007
Tuesday morning at the American Astronomical Society meeting in Honolulu marked the emergence of a significant release of news on black holes. First and foremost, a group of astronomers from the University of Maryland has made the first quantitative measurements of how several supermassive black holes spin. This seemingly innocuous information is revealing a great deal about how the black holes formed and evolved.

Graduate student Laura Brenneman and astronomer Christopher Reynolds employed observations made with the European Space Agency's XMM-Newton X-ray Telescope to observe iron spectral lines emitted from the black holes' accretion disks, noting their relativistically-altered shapes. Comparing these new data with sophisticated new computer models estimating how this telltale iron line should appear, they produced analyses of the angular momentums, or spin rates, of the black holes. Amazingly, the best quality example of data connects with the black hole centrally located in the galaxy MCG-06-30-15, for which Brenneman and Reynolds calculated a spin rate of 98.7 percent of the maximum allowed by Einstein's general theory of relativity.

Curiously, Brenneman and Reynolds found that spin rates vary considerably even among similar types of black holes. "We really know very little about how supermassive black holes form and grow," says Reynolds. "We have models for how it can happen, but being able to determine spin rate is critical to our understanding of the process by which it actually happens."

One important way black holes can spin faster is by growing, and particularly by merging. "In merging galaxies like the Antennae (NGC 4038-9)," says Reynolds, "eventually the central black holes find each other, producing ripples in space-time as they merge." The resulting black hole can recoil, or kick, after the merger, in some cases being ejected from the new galaxy. "We would think that 10 percent or more of merging galaxies eject the central black hole from the galaxy," says Reynolds. "But nearly all major galaxies have central black holes. Why?"

Brenneman and Reynolds will continue their research to attempt to answer the question. Perhaps major mergers are rare. (But astronomers see merging galaxies all over the place.) Maybe most black holes spin slowly. But that doesn't match many observational lines of evidence. Something else seems to come into play.

The solution may have arrived from another Maryland researcher, astronomer Tamara Bogdanovic, who worked with Reynolds on another project, investigating why galaxies without black holes seem to be rare. Looking at merging pairs of galaxies, the researchers tackled the question of why no so-called empty-nest galaxies, those without central black holes, have yet been found.

Bogdanovic found that merged black holes probably are kicked out far less often than previously thought because torques from the accretion disks of material surrounding the black holes act to preferentially align their spins in a way that reduces the kick force.

"In gas-rich mergers," says Bogdanovic, "torques from the gas can act on spinning black holes to cause the alignment in a million years or less, which is quick considering the whole merger process of galaxies takes much longer than that." Bogdanovic noted that the radio galaxy 0402+379 may provide the first observational evidence of a spin alignment in the process of occurring.

At the same press conference, Erin Bonning of the Paris Observatory and University of Texas astronomers Gregory Shields and Sarah Salviander presented ideas from their research on black-hole kicks. The Paris-Texas team has gone searching for runaway black holes in quasars, the energetic centers of young galaxies containing supermassive black holes. The team searched the database of the Sloan Digital Sky Survey (SDSS), looking carefully at the spectra of about 2,600 quasars.

"There are a couple of these quasars whose spectra raise suspicions," says Shields. None showed definitive evidence of being a kicked-out black hole. "We didn't find nearly as many quasars with high-velocity shifts as we thought we would," says Bonning, "and even those quasars that were 'shifty' didn't show any other evidence of being absconded black holes. They were more likely to be stationary black holes in the middle of a slightly more complicated quasar than usual."

The negative results of this survey are just the kind astronomers are looking for: ones that tie in to the work of the Maryland astronomers. It seems that perhaps we are seeing the first overall picture come together of how black-hole spins tell us a lot more than astronomers, just a few years ago, thought they could.
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