Suzaku Shows Clearest Picture Yet Of Perseus Galaxy Cluster

Galaxy clusters are millions of light-years across, and most of their
normal matter comes in the form of hot X-ray-emitting gas that fills
the space between the galaxies.

"Understanding the content of normal matter in galaxy clusters is a
key element for using these objects to study the evolution of the
universe," explained Adam Mantz, a co-author of the paper at NASA's
Goddard Space Flight Center in Greenbelt, Md.

Clusters provide independent checks on cosmological values established
by other means, such as galaxy surveys, exploding stars and the
cosmic microwave background, which is the remnant glow of the Big
Bang. The cluster data and the other values didn't agree.
NASA's Wilkinson Microwave Anisotropy Probe (WMAP) explored the cosmic
microwave background and established that baryons -- what physicists
call normal matter -- make up only about 4.6 percent of the universe.
Yet previous studies showed that galaxy clusters seemed to hold even
fewer baryons than this amount.

Suzaku images of faint gas at the fringes of a nearby galaxy cluster
have allowed astronomers to resolve this discrepancy for the first time.

The satellite's ideal target for this study was the Perseus Galaxy
Cluster, which is located about 250 million light-years away and
named for the constellation in which it resides. It is the brightest
extended X-ray source beyond our own galaxy, and also the brightest
and closest cluster in which Suzaku has attempted to map outlying gas.

"Before Suzaku, our knowledge of the properties of this gas was
limited to the innermost parts of clusters, where the X-ray emission
is brightest, but this left a huge volume essentially unexplored,"
said Aurora Simionescu, the study's lead researcher at the Kavli
Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University.

In late 2009, Suzaku's X-ray telescopes repeatedly observed the
cluster by progressively imaging areas farther east and northwest of
the center. Each set of images probed sky regions two degrees across
-- equivalent to four times the apparent width of the full moon or
about 9 million light-years at the cluster's distance. Staring at the
cluster for about three days, the satellite mapped X-rays with
energies hundreds of times greater than that of visible light.

From the data, researchers measured the density and temperature of the
faint X-ray gas, which let them infer many other important
quantities. One is the so-called virial radius, which essentially
marks the edge of the cluster. Based on this measurement, the cluster
is 11.6 million light-years across and contains more than 660
trillion times the mass of the sun. That's nearly a thousand times
the mass of our Milky Way galaxy.

The researchers also determined the ratio of the cluster's gas mass to
its total mass, including dark matter -- the mysterious substance
that makes up about 23 percent of the universe, according to WMAP. By
virtue of their enormous size, galaxy clusters should contain a
representative sample of cosmic matter, with normal-to-dark-matter
ratios similar to WMAP's. Yet the outer parts of the Perseus cluster
seemed to contain too many baryons, the opposite of earlier studies,
but still in conflict with WMAP.

To solve the problem, researchers had to understand the distribution
of hot gas in the cluster, the researchers say. In the central
regions, the gas is repeatedly whipped up and smoothed out by passing
galaxies. But computer simulations show that fresh infalling gas at
the cluster edge tends to form irregular clumps.

Not accounting for the clumping overestimates the density of the gas.
This is what led to the apparent disagreement with the fraction of
normal matter found in the cosmic microwave background.

"The distribution of these clumps and the fact that they are not
immediately destroyed as they enter the cluster are important clues
in understanding the physical processes that take place in these
previously unexplored regions," said Steve Allen at KIPAC, the
principal investigator of the Suzaku observations.

Goddard supplied Suzaku's X-ray telescopes and data-processing
software, and it continues to operate a facility that supports U.S.
astronomers who use the spacecraft.

Suzaku (Japanese for "red bird of the south") is the fifth Japanese
X-ray astronomy satellite. It was launched as Astro-E2 on July 10,
2005, and renamed in orbit. The observatory was developed at JAXA's
Institute of Space and Astronautical Science in collaboration with
NASA and other Japanese and U.S. institutions.

For more information and images about this finding, visit:

http://www.nasa.gov/astro-e2

Source: NASA

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