Fossil groups are galactic systems where component galaxies merge to form a single elliptical galaxy. They have a mass and X-ray luminosity comparable to the sum of their components and are surrounded by hot gas. The most massive known fossil group challenges the accepted view of their formation. An alternative theory proposes an asymmetric model. Fossil groups could be the initial stage of formation of cluster groups.
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 that extends several galactic radii. All unabsorbed members of the system are typically far apart and much smaller in magnitude than the dominant member. It has been suggested that these systems formed in the very distant past and represent the final stage in the evolution of 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 close enough to project that gravity will determine 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 surround these systems. Such a process is thought to form a fossil group over the course of a few billion years.
Observations have been made of galactic groups that appear to be in the process of forming a fossil group. These reinforce the idea that it is the common outcome for systems of a certain density and the only method of formation of fossil groups. However, further research has shown that fossil formation is in less densely populated areas. In addition, huge fossil groups were examined that could not have arisen from common models of dynamical friction within known life in the universe.
The most massive known fossil group was studied using the XMM-Newton and Chandra X-ray space observatories and 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 superheated gas three million light-years in diameter. According to the accepted view of the formation of fossil groups, a structure of this size should not have been possible in the time available.
An alternative formation theory has been proposed in which the effects of dynamic friction acting on bodies moving in circular orbits around the center of mass are replaced by an asymmetric model. If the mass is pulled toward the center along thread-like filaments or galactic structures, known forces can unfold the formation at a much faster rate. The similarities between elliptical galaxies in fossil groups and bright galaxies in large clusters suggest another formative process. The final process of formation of fossil groups could be the initial stage of formation of cluster groups.
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