Space

Wanted: Fugitive Black Holes Escaped from Home Galaxies

Astronomers are hunting the elusive rogue black holes

By Lucian Dorneanu, Science Editor

24th of May 2007, 15:15 GMT

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Black holes are space objects which have an immense gravitational field that cuts off a region of space from the rest of the universe, trapping all matter and radiation that enters that region. Black holes are thought to form in two ways, as a direct result of the gravitational collapse of a star, or by collisions between neutron stars.

Now scientists are trying to track down a weird kind of black holes, that have been ejected from the centers of their home galaxies and speed across space. Some of them are not sure that such a phenomenon can even be spotted, since it must be gathering matter from the accretion disk to make it shine.

Moreover, if these black holes are ejected from the core of a galaxy and sent racing into space, its accretion disk should be left behind.

New calculations of space theorist Avi Loeb, from

the Harvard-Smithsonian Center for Astrophysics, could bring new hope for actually tracking the fugitives. He proved that, in theory, a black hole ejected from the center of a galaxy could bring its accretion disk along for the ride and remain visible for millions of years.

"Matter in the disk is swirling around the black hole much faster than the typical black-hole ejection speed. That matter is so tightly bound that it follows the black hole like a herd of sheep around a shepherd," said Loeb.

The most easily understandable example would be that of two galaxies that collided and merged, creating a supermassive black hole that emits powerful gravitational radiation in a preferred direction.

Using computer simulations, he showed that the net momentum carried by the radiation gives the remnant black hole a large kick in the opposite direction, and that would make the black hole recoil at speed up to ten million miles per hour, which is actually fast enough to traverse an entire galaxy in a cosmically short time of only ten million years.

The problem is that even if such a fugitive black hole were to travel more than 30,000 light-years from the center of its galaxy, in cosmic terms it wouldn't mean much. So, the astronomers face a heavy task, since tracking the black hole would be like detecting the movement of a light spot that shifted its position by the size of a 10 cents coin.

TAGS:

black-hole | galaxy | light

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