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Durham University News



Cosmic blobs point to key stage in galaxy evolution

(25 June 2009)

Illustration of Black Hole Outflows: (Credit: NASA/CXC/M.Weiss)

Illustration of Black Hole Outflows: (Credit: NASA/CXC/M.Weiss)

Cosmic "blobs" have helped pinpoint a crucial stage in galaxy and black hole evolution, suggests research led by Durham University.

The blobs – huge brightly glowing reservoirs of hydrogen gas in the early Universe – were discovered ten years ago but little was known about their power source.

Using data from NASA’s Chandra X-Ray Observatory and other telescopes, the Durham-led researchers found that a significant source of their power came from radiation associated with supermassive black holes within the blobs that are partially obscured by dense layers of dust and gas.

Bursts of star formation also provided power to the blobs which were seen at a time when the Universe was only 2 billion years old, the researchers found.

Galaxies are believed to form when gas flows inwards under the pull of gravity and cools by emitting radiation, potentially forming a blob. However, this process should stop when the gas is heated by radiation and material flowing out from galaxies and their black holes.

The implication of the Durham-led research is that the blobs represent a crucial stage in galaxy evolution called feedback - when adolescent galaxies and black holes are switching off their rapid growth. The glow from the blobs is a glimpse of this process in action.

The research, funded by the Science and Technology Facilities Council (STFC), is published in The Astrophysical Journal.

Lead author Dr Jim Geach, in the Department of Physics, at Durham University, said: “For ten years the secrets of the blobs had been buried from view, but now we've uncovered their power source.

"Now we can settle some important arguments about what role they played in the original construction of galaxies and black holes.”

Co-author Dr Bret Lehmer, also in the Department of Physics at Durham University, added: “We’re seeing signs that the galaxies and black holes inside these blobs are coming of age and are now pushing back on the infalling gas to prevent further growth.

“Massive galaxies must go through a stage like this or they would form too many stars and so end up ridiculously large by the present day.”

Chandra and a collection of other telescopes, including the Spitzer Space Telescope, have observed 29 blobs – several hundred thousand light years across - in one large field in the sky dubbed SSA22.

In five of these blobs, the Chandra data revealed the tell-tale signature of growing supermassive black holes - a point-like source with luminous X-ray emission.

Another three of the blobs in this field showed possible evidence for such black holes. Based on further observations, including Spitzer data, the research team was able to determine that several of these galaxies are also dominated by remarkable levels of star formation.

Giant black holes are thought to reside at the centres of most galaxies today, including our own Milky Way.

The radiation and powerful outflows from these black holes and bursts of star formation are, according to calculations, powerful enough to light up the hydrogen gas in the blobs they inhabit.

In the cases where the signatures of these black holes were not detected, the blobs are generally fainter.

Besides explaining the power source of the blobs, these results help explain their future.

Under the heating scenario, the gas in the blobs will not cool down to form stars but will add to the hot gas found between galaxies. SSA22 itself could evolve into a massive galaxy cluster.

"In the beginning the blobs would have fed their galaxies, but what we see now are more like leftovers," said Dr Geach.

"This means we'll have to look even further back in time to catch galaxies and black holes in the act of forming from blobs."

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