Fossil Groups of Galaxies
Fossil Groups of Galaxies are a special kind of galaxy group. While the general definition of a "galaxy group" is rather loosely simply a conglomeration of galaxies, often showing signs of interactions (but that is not a requirement), a fossil group is a rather well defined object in astronomy:
A group of galaxies, dominated by a single, very luminous (and thus massive) galaxy in the center, for which the second brightest galaxy is at least 2 magnitudes less luminous (and therefore much smaller). These fossil groups usually have very bright X-ray halos, indicating a very massive dark matter halo hosting this group. From a theoreticians point of view, these fossil groups have at least a mass of 1013 solar masses.
Some of the most massive galaxies in the Universe (all of them elliptical galaxies) are the central galaxies of such fossil groups.
The current picture of how such groups form is that they are rather old (therefore the naming). They have assembled as a local over-density at very high redshifts, and in time most of the surrounding galaxies have merged to build up the massive central galaxy and heat up the gas in this process, causing the observed X-ray bright halos of hot gas.
Small galaxies are still living inside the massive and relaxed dark halo, orbiting around the central galaxy.
Simulations seem to support this picture of fossil group formation, however, many aspects of the growth of such structures still remain to be explained in the future.