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Start of darkness 30 Jun 01

The vast black hole at the core of our Galaxy could have come together bit by bit

WHICH came first, the galaxy or the black hole? That question has been puzzling astronomers ever since they discovered that most galaxies have a monster black hole at their core.

Now observations of one such black-hearted galaxy suggest that the galaxy came first. Black holes formed later from individual stars in tight-knit clusters, which merged to make a "middleweight" black hole that migrated to the galactic centre.

Astronomers have long wanted to explain the size gap between small black holes-up to 100 times the mass of the Sun-and supermassive ones that are millions of times as massive. The large ones could have formed in one go, as an enormous cloud of gas collapsed under gravity early in a galaxy's life. But computer models do not support this, suggesting that the cloud would fragment as it collapsed.

The alternative-that stars collapse to form black holes, merge with others and eventually make a monster-doesn't fit the models either. Simulations predict that small black holes in a cluster would tend to be flung out by the gravity of nearby stars.

Now a team led by Toshikazu Ebisuzaki at the Japanese Institute of Physical and Chemical Research in Wako says star clusters do seem to be the key-as long as they're very densely packed with stars.

The astronomers' work was prompted by images taken last year by the orbiting Chandra X-ray Observatory, which revealed several X-ray spots in the core of nearby galaxy M82. Astronomers believe the brightest spot is the first middleweight black hole ever found, weighing in at around 700 solar masses (New Scientist, 23 September 2000, p 19). "This was very exciting-we'd found a stepping stone," says team member Piet Hut of the Institute for Advanced Study in Princeton, New Jersey.

Follow-up observations of M82 by the Subaru Telescope in Hawaii have shown that at least four of the spots coincide with compact young star clusters. Ebisuzaki says large stars inside a very dense cluster could merge to form a colossal star a hundred times as massive as the Sun. This would burn its fuel furiously and collapse into a black hole. The black hole would continue to grow into a middleweight by gobbling nearby stars.

The scientists have shown that these dense star clusters could slowly fall into the galaxy's centre, building up a supermassive black hole. Stuart Shapiro of the University of Illinois at Urbana-Champaign says the idea is intriguing. "It joins a growing list of plausible but unproven scenarios which try to account for these objects."

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Hazel Muir

From New Scientist magazine, vol 170 issue 2297, 30/06/2001, page 20