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Homeless black holes could 'play back' galaxy mergers
Posted: JULY 10, 2009

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It has long been speculated that rogue black holes may be wandering intergalactic space, ejected from galaxies after they have merged.

In April, a report from scientists at the Harvard–Smithsonian Center for Astrophysics suggested that these black holes could possess a cluster of stars around them, ripped forcibly out of the galaxy they were ejected from. They suggested looking in the Milky Way’s halo for black holes ejected during mergers between dwarf galaxies when our Milky Way Galaxy was being assembled (see our news story here).

Now a team of American and German astronomers have taken the idea one step further, suggesting that supermassive black holes could be wandering the Coma and Virgo galaxy clusters, among others, and their orbiting stars mistaken for a globular cluster. If one could be found, they say, it would then be possible to calculate the violence of the black hole’s ejection from its original galaxy by measuring the velocities of its retinue of orbiting stars.

An artist's impression of a black hole and its ‘hypercompact system’ of stars being ejected from a galaxy merger. Image: STScI.

Galaxies merge all the time; such cosmic collisions are considered to play an important part in galaxy evolution and, in the crowded centres of galaxy clusters, collisions are still a frequent occurrence. When galaxies collide and combine, the supermassive black holes within them engage in a dangerous dance around one another, until they inevitably spiral inwards and coalesce. The recoil energy of this merger (depending upon the angle at which the galaxies and black holes merge, and their individual motions) is enough to kick the combined supermassive black hole right out of the galaxy. But it doesn’t go easily, spitefully tearing stars out from the galaxy on its way out.

“Even if the black hole gets kicked out of one galaxy, it is still going to be gravitationally bound to the whole cluster of galaxies,” says lead researcher Professor David Merritt from the Rochester Institute of Technology. Consequently, these black holes should still be floating around galaxy clusters today with their herd of stars and could have already been seen but mistaken for globular clusters (the giant elliptical galaxy M87, at the heart of the Virgo Cluster, has 16,000 orbiting globular clusters, and although we know that M87 still possesses a supermassive black hole, the point is that globular clusters can be resolved even at distances of over 50 million light years).

So how could we tell a faraway globular cluster and the ‘hypercompact system’ of an ejected black hole apart? It’s all down to the motions of the stars, says Merritt. In a hypercompact system, the stars would be moving around extraordinarily fast, driven by the gravity of the black hole. The only problem is that it would require long exposures on a large telescope to make such a difficult and precise observation, and with tens if not hundreds of thousands of globulars in each galaxy cluster, it could be a painstaking job, like looking for a needle in a haystack.

There could be an easier way, if we are prepared to be patient. The stereotypical representation of a black hole in the media is an object that sucks things in. Sooner or later a star will wander too close to the evicted black hole and be ripped apart, causing a powerful flash of X-rays; the telltale sign of material being torn asunder and drawn into the maw of a mighty black hole.

Either way, it could be worth the effort. The velocities of the stars orbiting the black hole in a hypercompact system will have been determined by how big a kick the black hole received to push it out of a galaxy. “You can measure how big the kick was by measuring how fast the stars are moving around the black hole. Only stars orbiting faster than the kick velocity remain attached to the black hole after the kick,” says Merritt. “These stars carry with them a kind of fossil record of the kick, even after the black hole has slowed down.”

Consequently, this could allow scientists to play back the ejection of the black hole, and possibly work out which galaxy it originated from and when the merger took place. It would be an unprecendented look into the history of a galaxy merger.

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