* Astronomy

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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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.



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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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