This Hubble Space Telescope image shows NGC 1275, the galaxy located in the centre of the Perseus Galaxy Cluster. The red threadlike filaments are composed of cool gas suspended by a magnetic field.
Credit: NASA/ESA/Hubble Heritage (STScI/AURA)
Bluer colors indicate less intense X-ray emission. The dashed circle is 11.6 million light-years across and marks the so-called virial radius, where cold gas is now entering the cluster. Red circles indicate X-ray sources not associated with the cluster. Inset: An image of the cluster's bright central region is shown to scale.
Credit: NASA/ISAS/DSS/A. Simionescu et al., Fabian et al.
WASHINGTON: X-ray observations made by the Suzaku observatory provide the clearest picture to date of the size, mass and chemical content of a nearby cluster of galaxies, providing the first direct evidence that million-degree gas clouds are tightly gathered in the cluster's outskirts.
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," said co-author Adam Mantz from NASA's Goddard Space Flight Centre in Maryland.
Resolving an important discrepancy
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% of the universe. Yet previous studies showed that galaxy clusters seemed to hold even fewer baryons than this amount.
Images from Suzaku, which is sponsored by the Japan Aerospace Exploration Agency (JAXA) with contributions from NASA, of faint gas at the fringes of a nearby galaxy cluster have allowed astronomers to resolve this discrepancy for the first time, having been published in the journal Science.
Huge volume of gas previously unexplored
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 California.
In late 2009, Suzaku's X-ray telescopes repeatedly observed the cluster by progressively imaging areas farther east and northwest of the centre. 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.
