
Galaxies coming of age in cosmic blobs
CHANDRA X-RAY CENTER NEWS RELEASE Posted: June 24, 2009

The "coming of age" of galaxies and black holes has been pinpointed, thanks to new data from NASA's Chandra X-ray Observatory and other telescopes. This discovery helps resolve the true nature of gigantic blobs of gas observed around very young galaxies. About a decade ago, astronomers discovered immense reservoirs of hydrogen gas -- which they named "blobs" -- while conducting surveys of young distant galaxies. The blobs are glowing brightly in optical light, but the source of immense energy required to power this glow and the nature of these objects were unclear.

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A long observation from Chandra has identified the source of this
energy for the first time. The X-ray data show that a significant
source of power within these colossal structures is from growing
supermassive black holes partially obscured by dense layers of dust
and gas. The fireworks of star formation in galaxies are also seen to
play an important role, thanks to Spitzer Space Telescope and ground-
based observations.
"For ten years the secrets of the blobs had been buried from view, but
now we've uncovered their power source," said James Geach of Durham
University in the United Kingdom, who led the study. "Now we can
settle some important arguments about what role they played in the
original construction of galaxies and black holes."
Galaxies are believed to form when gas flows inwards under the pull of
gravity and cools by emitting radiation. This process should stop
when the gas is heated by radiation and outflows from galaxies and
their black holes. Blobs could be a sign of this first stage, or of
the second.
Based on the new data and theoretical arguments, Geach and his
colleagues show that heating of gas by growing supermassive black
holes and bursts of star formation, rather than cooling of gas, most
likely powers the blobs. The implication is that blobs represent a
stage when the galaxies and black holes are just starting to switch
off their rapid growth because of these heating processes. This is a
crucial stage of the evolution of galaxies and black holes - known as
"feedback" - and one that astronomers have long been trying to
understand.
"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," said coauthor Bret Lehmer, also of
Durham. "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 Spitzer have
observed 29 blobs in one large field in the sky dubbed "SSA22." These
blobs, which are several hundred thousand light years across, are
seen when the Universe is only about two billion years old, or
roughly 15% of its current age.
In five of these blobs, the Chandra data revealed the telltale
signature of growing supermassive black holes - a point-like source
with luminous X-ray emission. These giant black holes are thought to
reside at the centers of most galaxies today, including our own.
Another three of the blobs in this field show 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.
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. The authors show that black holes bright
enough to power these blobs would be too dim to be detected given the
length of the Chandra observations.
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 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."
These results will appear in the July 10 issue of The Astrophysical
Journal. NASA's Marshall Space Flight Center in Huntsville, Ala.,
manages the Chandra program for NASA's Science Mission Directorate in
Washington. The Smithsonian Astrophysical Observatory controls
Chandra's science and flight operations from Cambridge, Mass.
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