Miércoles 1 de Agosto de 2007, Ip nº 201

Our local black hole may have a baby brother
Por Maggie McKee

IS A second black hole lurking at the heart of the Milky Way? A relatively simple test could answer the question: look for a pair of stars fleeing the galaxy at breakneck speed.

A colossal black hole - about 3.6 million times the mass of the sun - is thought to lie at the centre of the Milky Way. Yet there may also be a second black hole there that weighs as much as 1000 to 10,000 suns. The evidence comes from observations of a cluster of young stars located just a fraction of a light year from the monstrous black hole - where gravitational forces should prevent any stars from forming. The cluster could have formed further away and migrated there under the influence of a second, middleweight black hole at the galactic centre, but so far no one has managed to verify this idea.

Now Youjun Lu, an astrophysicist at the University of California, Santa Cruz, and colleagues say the case could be clinched if a pair of "hypervelocity" stars can be found speeding out from the centre of the galaxy. Ten of these cosmic speed demons have turned up since December 2004. It is thought that the stars were shot out by an interaction between three or more objects. The question is: which objects?

In one scenario, a pair of stars wanders too close to a single supermassive black hole, and one star gets captured while the other gets flung outwards at up to 4000 kilometres per second. In the second scenario, a single star approaches a pair of black holes and is ejected at similarly high speed.

Now Lu has come up with a variation on the second theme (www.arxiv.org/abs/0707.1872). A pair of black holes orbiting around each other would have a much larger sphere of influence than a single black hole. This would mean that a pair of stars approaching them would behave as one and be shot outwards at hypervelocities just like a single star approaching a binary black hole.

"For a single black hole, the probability of ejecting a hypervelocity binary star is negligible," says Lu. He argues, therefore, that finding a pair of hypervelocity stars hurtling towards the edge of the Milky Way at 1000 kilometres per second or more would therefore be "definitive evidence" of the existence of two large black holes at the centre of the galaxy.

About 10 per cent of the stars in the sun's neighbourhood have a close partner. If that proportion is the same near the galactic centre, one of the 10 hypervelocity stars already observed could actually be a binary.

Finding a second black hole would support standard theories of how galaxies grow.

  28/07/2007. New Scientist Magazine.