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A fossil group is a galactic system in which some or all of the component galaxies have merged to form a single elliptical galaxy. This body has a mass and X-ray luminosity comparable to the sum of its components and is surrounded by a cloud of hot gas extending several galactic radii. Any members of the system not absorbed are typically distant and of far less magnitude than the dominant member. It has been suggested that these systems formed in the very distant past and represent the final stage of evolution for a galactic group, hence the term 'fossil'.
Most galaxies, including the Milky Way, are part of a local group. In the most compact groups, the galaxies are near enough to project that gravity will bring about their eventual merger into a single body, centered in a halo of dark matter. This is matter that does not emit or reflect radiation, but can be detected by its gravitational effects. The presence of dark matter is inferred from the vast clouds of gas emitting high levels of X-ray radiation that typically surrounds these systems. Such a process is thought to form a fossil group over the course of a few billion years.
Observations of galactic groups that appear to be in the process of forming a fossil group have been made. These reinforced the idea that it is the common outcome for systems of a certain density and the sole method of fossil group formation. Further research has shown evidence of fossil formation is less densely populated sectors, however. In addition, massive fossil groups have been examined that could not have resulted from common models of dynamical friction within the known life of the universe.
The most massive known fossil group was studied using the XMM-Newton and Chandra X-ray space observatories as well as ground based infrared telescopes. Its elliptical galaxy is 500 billion times more luminous than the sun, has a mass of over 300 trillion solar masses and sits in a halo of super-heated gas three million light years in diameter. According to the accepted view of fossil group formation, a structure this size should not have been possible in the time available.
An alternate formation theory has been proposed where the effects of dynamical friction acting on bodies moving in circular orbits about the center of mass is replaced by an asymmetric model. If mass is pulled toward the center along filaments, or threadlike galactic structures, known forces can account for formation at a much more rapid pace. Similarities between the elliptical galaxies of fossil groups and bright galaxies in large clusters suggest another formative process. The end process of fossil group formation could be the initial phase of formation for the cluster groups.