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spacer spacer spacer spacer September 18, 2008

Tracking the path of black holes


A supermassive black hole is about to mangle a doomed star, indicated by the orange circle. Gravity stretches the star and pulls it into the black hole, disrupting the star’s mass and tearing it apart.

Image provided by NASA/CXC/M. Weiss

A supermassive black hole that has been kicked out of the center of its galaxy after colliding with another has to go somewhere. But finding a massive dark object in intergalactic space is especially challenging if the black hole has left without the glowing disk of gas it wears as a belt and as a signpost.

A paper published in the Aug. 10 issue of The Astrophysical Journal, by David Merritt at RIT and colleague S. Komossa at the Max-Planck-Institute in Germany, shows how brightly flaring stars can be used to track evicted black holes.

“It’s a very obvious idea,” says Merritt, a professor in the Department of Physics and the Center for Computational Relativity and Gravitation. “I think it’s just that we were the first to work it out—the idea that black holes would carry stars with it. There’s always going to be stars near the center of a black hole at the time it’s kicked out. And you don’t see the black hole, you see the stars around it.

“We’re talking about a strange new object that’s never been talked about before. A black hole of millions of billions of solar masses surrounded by maybe a million stars. Almost all the mass is going to be in the black hole.”

Merritt conducted groundbreaking research predicting the rate at which gravitational recoil or kick can knock a black hole clear across a galaxy. He published his findings in 2007 with RIT co-authors Manuela Campanelli, Carlos Lousto and Yosef Zlochower.

Building on his earlier discovery, Merritt points to a relationship between the size of the kick and the number of stars a black hole takes with it. “The bigger the kick, the fewer the stars will remain bound to the black hole. If it goes fast, it will leave more behind. If it goes slow, it will take more with it.”

The existence of a supermassive black hole is something astrophysicists must infer from the radiation emitted as light and heat by gas and stars surrounding the mass. A belt or doughnut-shaped object known as an accretion disk fuels the black hole.

If the black hole is kicked out of a galaxy with some of its surrounding gas—its fuel supply—it will remain bright until the store of energy is consumed. The limited supply of gas means the black holes’ luminosity is finite, and it will shine for, perhaps, a million years—a blip of time in astrophysical terms, Merritt notes.

The case is different when a black hole thrust from a galaxy rips away a million stars on its way out. The black holes’ gravity will disrupt the captured stars, which will shoot out flares of light as they are consumed. Merritt and Komossa suggest this event might take place 10 to 20 times as a black hole moves from the center to the edge of a galaxy over the course of a million years.

An ejected black hole bound by stars will appear similar to compact and luminous masses, such as globular clusters (tight knots of stars). Key to differentiating the new objects will be careful observation of the radiation produced when a black hole crushes a star.

Although the signature flares are detectable in ultraviolet and visible light, scientists can best detect the sudden increase in radiation from the stars by looking at X-ray wavelengths. Upcoming X-ray satellite missions making wide surveys of space will likely collect examples of these black holes and their retinue of stars.

Susan Gawlowicz

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