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Breathing life into the
Coma Cluster

Posted: 11 August 2010

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A long-exposure Hubble Space Telescope image of the Coma Cluster reveals a stunning look at spiral galaxy NGC 4911 as it dances through space with a companion galaxy.

The Coma Cluster is home to a menagerie of nearly one thousand galaxies, but the face-on galaxy NGC 4911 takes centre stage in this scene. Thick with gas and dust, its outermost skeletal arms sport joints of intense star-formation regions. "We think that the apparent 'kinks' are due to an interaction between the galaxy disc and the wider intracluster medium (hot X-ray gas) of the Coma Cluster," explains Michael Gregg of the University of California.

This Hubble image of the 320 million light year distant Coma Cluster combines data obtained in 2006, 2007, and 2009 from the Wide Field Planetary Camera 2 and the Advanced Camera for Surveys, and required 28 hours of exposure time. Image: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)/K. Cook (Lawrence Livermore National Laboratory).

NGC4911 is travelling at high speed through the cluster, and as the hotter gas of the intracluster medium rips away the cooler material of the galaxy's disc, regions of star formation are revealed. Similarly, where star formation has yet to happen, the gas is smoothed out, heated, and further star formation suppressed. "So the 'kinks' are where there is already star formation preceding the interaction," continues Gregg. "As the interaction progresses, large star cluster complexes will be revealed at the sites of intense star formation. The spacing of the kinks is probably telling us something about the physical scales of star formation and galaxy cluster interactions – exactly what has yet to be worked out!"

A nearby lenticular (S0 class) galaxy, which is seen roughly edge on in this image, is nudging up to NGC4911, teasing out material from the spiral galaxy. "There is a light bridge and dust lanes between the two galaxies which run along the major axis of NGC4911A, on the side towards NGC4911, and which are absent on the opposite side," says Gregg, who comments that in a wider field of view than shown in the release image it is possible to make out additional tidal debris. In time, the stripped material will be dispersed through the cluster to fuel new populations of stars.

Other spiral galaxies near the densely populated centre of the cluster are also subject to the gravitational tug-of-war between neighbours, colliding and merging to spawn the next generation of the cluster's elliptical galaxy population.

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