Outcast black holes surround the Milky Way

WANDERING naked or in tattered cloaks of dark matter, as many as 2000 black holes booted from their homes now live on the outskirts of the Milky Way. That's the prediction of new computer simulations, which looked at how our galaxy grew through mergers with smaller galaxies.

Theory says that every galaxy may have a black hole at its centre. As galaxies merge, their central black holes merge too, building a supermassive black hole millions of times the mass of the sun.

But collisions between black holes create gravitational waves, which can kick a newly merged black hole out of its host galaxy.

Valery Rashkov and Piero Madau of the University of California, Santa Cruz, ran simulations that show 70 to 2000 of these outcasts may now linger in the halo of the Milky Way, depending on the properties of the objects that collided (arxiv.org/abs/1303.3929).

Some might have been stripped bare, while others may carry a few clusters of stars and dark matter, says Avi Loeb of Harvard University, who has proposed a similar idea. Though faint, these star clusters should be observable with current or future telescopes. Finding them may tell us more about early black hole growth.

This article appeared in print under the headline "Outcast black holes ring the Milky Way"

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