Galaxies found with a hole in the middle

Most galaxies in the universe are like a Polo mint: they have a hole in the middle, according to new research using the Hubble Space Telescope.

Unlike terrestrial sweets, though, the centre of galaxies is not empty space but a supermassive black hole, with a mass millions of times that of our Sun.

The results emerge from a study of 27 "nearby" galaxies by the orbiting observatory, and gives astronomers an important clue to how galaxies develop and age. Notably, the findings help to explain the phenomenon of quasars - incredibly powerful sources of energy which are observed in very young galaxies.

The three key findings are that nearly every large galaxy has a supermassive black hole at its centre; that its mass is proportional to the galaxy's size; and that the number and mass of the black holes discovered are consistent with the requirements to power the quasars that have been observed.

According to Doug Richstone of the University of Michigan, who led the international team of astronomers, "most galaxies at one time burned as brightly as a quasar". Black holes are created as the galaxy forms and gas collects at its centre, eventually forming an enormously heavy star, whose gravity becomes so great as it collapses in on itself that not even light can escape.

A quasar is the result of gas falling into the black hole: as it approaches the vortex it accelerates almost to the speed of light, emitting vast quantities of radiation. "Gas falling into a black hole is the most efficient power source that we know of," said Sir Martyn Rees, of the Institute of Astronomy, yesterday. "Occasionally a star might get swallowed ... but that's only a tiny fraction of the total mass of the galaxy."

Eventually though, the quasar will burn itself out, leaving only the black hole that powered it. "We believe we are looking at fossil quasars," said Professor Richstone.

However the results, announced at this week's annual meeting of the American Astronomical Society, do not provide any answer for the "missing mass" of the universe, said Sir Martyn. "These black holes have a mass only about one-thousandth that of their parent galaxy," he said. But astronomers are looking for much bigger amounts of unseen mass; the observed galaxies account for only a few per cent at most of the mass of the universe.

Equally, the presence of a black hole at the centre of the galaxy does not mean that our eventual fate is to be swallowed up by it. "I don't think so," said Sir Martyn. "You have to remember that these are comparatively small compared to the mass of the whole galaxy." The black holes could not therefore drag the whole galaxy's mass into it, he argues.