The majority of galaxies in the Universe, including our own, appear to reside in groups. Most galaxy groups are "loose groups", in which the galaxies are separated by distances of several hundred kiloparsecs, however "compact groups" such as HCG97 (shown below) are much easier to identify optically. The study of groups and their galaxies is a special interest of the group at Birmingham. Groups are especially important since:

• Groups may contain most of baryonic mass of the Universe.
• Since most galaxies are found in groups, the effects of the group environment is essential to an understanding of galaxy evolution.
• Groups are the best place in the Universe to look for galaxy collisions and mergers.
• The effects of energetic phenomena within galaxies, such as starbursts and jets, can have a noticeable impact on the hot intergalactic medium within groups, providing a probe of galactic history.
• Galaxy clusters are believed to be formed from the merger of galaxy groups.

Click below for information about some of our main projects involving galaxy groups.

• Group evolution
• Hot gas in groups
• Fossil groups
• XI
• Scaling properties
• Millennium simulations



Contours of soft X-ray emission are shown superimposed on an optical plate of the galaxy group HCG97. This compact group contains only five galaxies (marked with crosses) in a very close configuration. The X-ray emission arises from hot (~10 million K) gas trapped in the gravitational potential well of the group. As can be seen, this gas extends well beyond the optical confines of the group, and it can be used to infer the existence of a large amount of dark matter, which is similarly extended.