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Astronomers unmask missing black holes

An international team of astronomers including experts from Durham University have unmasked hundreds of black holes hiding deep inside dusty galaxies billions of light-years away.

The massive, growing black holes, discovered by NASA's Spitzer and Chandra space telescopes, represent a large fraction of a long-sought missing population of black holes.

Their discovery implies there are hundreds-of-millions of additional black holes growing in our young universe, more than doubling the total amount known at that distance.

The research was led by Emanuele Daddi of the Commissariat a l'Energie Atomique in France and two papers will appear in the November 10 issue of the Astrophysical Journal.

Co-author David Alexander, in the Department of Physics at Durham University, said: “The findings are the first direct evidence that most, if not all, massive galaxies in the distant universe spend their youths building monstrous black holes at their cores.”

For decades, large populations of active black holes have been considered missing.

These supermassive black holes produce highly energetic structures, called quasars, which consist of doughnut-shaped clouds of gas and dust that surround and feed the budding black holes.

As the gas and dust is devoured, it heats up and shoots out X-rays. Those X-rays can be detected as a general glow in space, but often the quasars themselves can't be seen directly because dust and gas blocks them from our view.

"We knew from other studies from about 30 years ago that there must be more quasars in the universe, but we didn't know where to find them until now," said Emanuele Daddi.

Daddi and his team initially set out to study 1,000 dusty, massive galaxies that are busy making stars, and were thought to lack quasars. The galaxies are about the same mass as our own spiral Milky Way galaxy, but irregular in shape.

At 9 to 11 billion light-years away, they exist at a time when the universe was in its adolescence, between 2.5 and 4.5 billion years old.

When the astronomers peered more closely at the galaxies with Spitzer's infrared eyes, they noticed that about 200 of the galaxies gave off an unusual amount of infrared light.

X-ray data from Chandra, and a technique called "stacking," revealed the galaxies were in fact hiding plump quasars inside. The scientists now think that the quasars heat the dust in their surrounding doughnut clouds, releasing the excess infrared light.

"We found most of the population of hidden quasars in the early universe," said Daddi. Previously, only the rarest and most energetic of these hidden black holes had been seen at this early epoch.

The newfound quasars are helping answer fundamental questions about how massive galaxies evolve. For instance, astronomers have learned that most massive galaxies steadily build up their stars and black holes simultaneously until they get too big and their black holes suppress star formation.

The observations also suggest that collisions between galaxies might not play as large a role in galaxy evolution as previously believed.

"Theorists thought that mergers between galaxies were required to initiate this quasar activity, but we now see that quasars can be active in unharrassed galaxies," said David Alexander.

The new observations were made as part of the Great Observatories Origins Deep Survey, the most sensitive survey to date of the distant universe at multiple wavelengths.

Consistent results were also recently obtained by Fabrizio Fiore of the Osservatorio Astronomico di Roma and his team. Their results will appear in the Jan 1, 2008, issue of Astrophysical Journal.

Alex Thomas | alfa
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