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RE: Galaxy clusters
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New evidence from NASA's Spitzer Space Telescope reveals that most galaxies undergo a huge stellar baby boom when they first enter a "cosmic city", or galaxy cluster. And the more distant the galaxy cluster, the greater the star formation rate.

"The infrared Spitzer observations let us peek at otherwise hidden, powerful star formation harboured in some of these cluster galaxies. By looking at both nearby and distant galaxy clusters, we can look back in time and observe an increase in the fraction of galaxies undergoing these intense star-forming events" - Dr. Amelie Saintonge, of the Institute for Theoretical Physics, University of Zurich, in Switzerland.

Sanitonge and Dr. Kim-Vy Tran, also of the University of Zurich, studied a total of 1,300 galaxies in eight clusters spread across 7 billion light-years. The galaxies were observed by Spitzer's Multiband Imaging Photometer (MIPS), and archived for the astronomical community to use.

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The universe's first "galactic cities" did not sprout up randomly across space. On the contrary, a new statistical analysis of observations from NASA's Spitzer Space Telescope confirms that these ancient galactic metropolises may have developed much like sprawling cities joining together into a larger urban whole.
Across the cosmos, galaxies rarely stand alone. Instead, they are grouped into large, densely populated communities containing thousands of galactic residents, called galaxy clusters.

"Previously, we only knew of a handful of galaxy clusters that existed when our universe was in its first few billion years. Now, thanks to Spitzer's superb sensitivity, we've identified over a hundred" - Dr. Mark Brodwin, of the National Optical Astronomy Observatory, in Tucson, Arizona.

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Abell catalogue
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The Abell catalogue of rich clusters of galaxies is an all-sky catalogue of 4,073 rich galaxy clusters of nominal redshift z = 0.2. This catalogue supplements a revision of George Ogden Abells original Northern Survey of 1958, which had only 2,712 clusters, with a further 1,361 clusters the Southern Survey of 1989 from those parts of the south celestial hemisphere that had been omitted from the earlier survey.

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RE: Galaxy clusters
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Formation of cold filaments in cooling flow clusters
How hot gas in galaxy clusters (with temperature higher than 100 millions degrees) may cool and flow towards the cluster centre in order to feed the central galaxy ?
A team of astronomers from Paris Observatory proposes a new scenario, taking advantage of the jets emitted by the central active galactic nuclei (AGN). Very high resolution numerical simulations allowed to understand the formation of the puzzling filaments of cool gas, observed in the atmosphere of galaxy clusters, like Perseus. Those filaments result from the cooling of hot gas, trapped in the wake of plasma bubbles formed by the central AGN. This gas, dragged at higher radius, has time to cool down to relatively low temperature (below 10 000 degrees) and to fall back, forming filamentary structures. The mass and the kinematics of the predicted filaments are in excellent agreement with the observations.

filaments56

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Title: Formation of cold filaments in cooling flow clusters
Authors: Y. Revaz, F. Combes, P. Salome

Emission-lines in the form of filamentary structures is common in bright clusters characterised by short cooling times. In the Perseus cluster, cold molecular gas, tightly linked to the H\alpha filaments, has been recently revealed by CO observations. In order to understand the origin of these filamentary structures, we have investigated the evolution of the hot ICM gas perturbed by the AGN central activity in a Perseus like cluster. Using very-high resolution TreeSPH simulations combined with a multiphase model and a model of plasma bubbles, we have been able to follow the density and temperature evolution of the disturbed ICM gas around the bubbles. Our simulations show that a fraction of the 1-2
m{keV} gas present at the centre of clusters is trapped and entrained by the rising buoyant bubble to higher radius where the AGN heating is less efficient. The radiative cooling makes it cool in a few tens of Myr below 10^4
m{K}, forming cold filamentary structures in the wake and in the rim of the bubbles.

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Title: H.E.S.S. observations of galaxy clusters
Authors: W. Domainko, W. Benbow, J. A. Hinton, O. Martineau-Huynh, M. de Naurois, D. Nedbal, G. Pedaletti, G. Rowell, for the H.E.S.S. Collaboration

Clusters of galaxies, the largest gravitationally bound objects in the universe, are expected to contain a significant population of hadronic and leptonic cosmic rays. Potential sources for these particles are merger and accretion shocks, starburst driven galactic winds and radio galaxies. Furthermore, since galaxy clusters confine cosmic ray protons up to energies of at least 1 PeV for a time longer than the Hubble time they act as storehouses and accumulate all the hadronic particles which are accelerated within them. Consequently clusters of galaxies are potential sources of VHE (> 100 GeV) gamma rays. Motivated by these considerations, promising galaxy clusters are observed with the H.E.S.S. experiment as part of an ongoing campaign. Here, upper limits for the VHE gamma ray emission for the Abell 496 and Coma cluster systems are reported.

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