Article Posted on : - Oct 01, 2009

The spiral galaxy, NGC4402, is currently falling towards the centre of the Virgo cluster. Image credit: H. Crowl (Yale University) and WIYN/NOAO/AURA/NSF

Clusters

Galaxy groups and clusters are the largest known gravitationally bound objects to have arisen in the process of cosmic structure formation. They form the densest part of the large scale structure of the universe.

In models for the gravitational formation of structure with cold dark matter, the smallest structures collapse first and eventually build the largest structures - clusters of galaxies. Clusters are formed relatively recently between 10 billion years ago and now. Groups and clusters may contain from ten to thousands of galaxies.

Ram pressure stripping

Galaxy clusters are permeated by hot, X-ray emitting gas known as the intra-cluster medium. As individual galaxies move within such clusters, they experience this intra-cluster gas as a 'wind' - much like the wind experienced by a moving bicyclist, even on a still day.

'Ram pressure stripping' occurs if this wind is strong enough to overcome the gravitational potential of the galaxy to remove the gas contained within it.

The spiral galaxy NGC 4522 located some 60 million light-years away from Earth is a spectacular example of a spiral galaxy currently being stripped of its gas content.

The galaxy is part of the Virgo galaxy cluster and its rapid motion within the cluster results in strong winds across the galaxy as the gas within is left behind.

Evidences found in NGC 4402 that show Ram Pressure stripping is at work:

- The disk of dust and gas appears bowed. This indicates that the galaxy is having trouble holding onto the loosely bound dust and gas in the outer regions of the disk against the pressure of the 'wind'.

- The stellar disk appears to extend well beyond the star forming disk of dust and gas. The loosely bound dust and gas in the outer regions of the disk has been stripped from the galaxy after the formation of these stars.

- Streamers of dust and gas seen trailing behind the motion of the galaxy, obscuring and reddening the stars behind. At the same time, the 'wind' has pushed the dust and gas that would normally be found ahead of the motion of the galaxy up into the galaxy itself. This has revealed bright blue stars along the leading edge of the galaxy.

The result of ram pressure stripping is a galaxy which contains very little cold gas. This effectively halts star formation in the galaxy, supporting the belief that ram pressure stripping could be one of the processes responsible for the morphology density relation.

-Courtesy:-
                 astronomy.swin.edu.au