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Do solo black holes roam universe?

Even gravitational monsters can get the heave-ho. Two mysterious bright spots in a disheveled, distant galaxy suggest that astronomers have found the best evidence yet for a supermassive black hole being shoved out of its home.

If confirmed, the finding would verify Einstein's theory of general relativity in a region of intense gravity not previously tested. The results would also suggest that some giant black holes roam the universe as invisible free floaters, flung from the galaxies in which they coalesced.

Although loner black holes may be an entity that has to be reckoned with, they would still be rare, notes theorist Laura Blecha of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Blecha, along with observational astronomer Francesca Civano of Harvard-Smithsonian and their colleagues, took detailed X-ray observations of the distant galaxy CID-42, nearly 4 billion light-years from Earth. The team focused on the region after a Hubble Space Telescope survey revealed two compact visible-light sources within the starlit body: one of them at or near the galaxy's center, the other offset from the core.

Because CID-42 appears to be the remnant of two giant galaxies that collided relatively recently, it seemed likely that one or both of the compact sources were supermassive black holes.

Observations with NASA's Chandra X-ray Observatory revealed that only one of the compact visible-light sources, a blob that lies about 8,000 light-years from the galaxy's estimated center, emits X-rays. The high-energy radiation is believed to be a sign that this blob is a supermassive black hole munching on surrounding gas.

Blecha, who simulated collisions between galaxies that contain black holes, says the findings suggest two key possibilities. The sources could be two supermassive black holes brought together by the collision of the galaxies that formed CID-42. In that case, the central compact source, like its off-center black hole sibling, would emit X-rays. The absence of X-rays detected by Chandra would then have to be explained by a high concentration of dust that hid the high-energy radiation from the flying observatory.

Alternatively, says Blecha, the central source could simply be a new hot spot of star formation, truly devoid of any X-ray emission. The off-center X-ray-emitting source would then be a single black hole several million times as heavy as the sun, being expelled from the galaxy at about 2,000 kilometers per second.

Two papers by the team will appear Sunday in The Astrophysical Journal.

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