Most of the galaxies in the universe reside in clusters, where they experience close encounters with their neighbors and interact gravitationally with dark matter - the mysterious substance that seems to comprise nearly one-quarter of the mass of the universe, but has not yet been directly observed.

The encounters cause the smaller galaxies to spiral in slowly toward the center of the group, where they eventually merge to form a single giant galaxy, which over time will consume all of its neighbors.

After the process runs its course, the result is an object astronomers call a fossil cluster: Almost all of the stars fall into the central galaxy, which in turn sits in the center of a surrounding dark-matter halo. Astronomers can infer the presence of the halo from the corresponding hot gas that fills its gravitational well and emits X-rays.

The team of international astronomers studied in detail a puzzling physical feature of the hottest and most massive fossil group, which according to theoretical models, simply could not have formed in the time available.

The group investigated, called RX J1416.4+2315, is dominated by a single elliptical galaxy located about 1.5 billion light-years away and radiating about 500 billion times more luminously than the Sun.

The XMM-Newton and Chandra X-ray observations, combined with optical and infrared analyses, revealed the group sits within a hot gas halo extending across 3 million light-years and heated to a temperature of about 50 million degrees, mainly due to shock heating as a result of gravitational collapse.

Such a high temperature - about twice as high as previously estimated, is characteristic of galaxy clusters. Another interesting feature is the cluster's large bulk - over 300 trillion solar masses, with only about 2 percent in the form of stars in galaxies and 15 percent as X-ray-emitting hot gas. The major contributor to the mass of the system, however, is dark matter, which gravitationally binds the other components.

According to the team's initial calculations, a fossil cluster as massive as RX J1416.4+2315 could not have formed during the whole age of the universe. The key to the process is a factor called dynamical friction, which causes a large galaxy to lose its orbital energy to the surrounding dark matter. This process is less effective, however, when galaxies are moving more quickly, which they do in massive clusters.

"Simple models to describe the dynamical friction assume that the merging galaxies move along circular orbits around the center of the cluster mass," said team leader Habib Khosroshahi of the University of Birmingham. "Instead, if we assume that galaxies fall towards the center of the developing cluster in an asymmetric way, such as along a filament � the cluster formation process may occur in a shorter time scale."

The hypothesis is supported by a highly elongated X-ray emission pattern observed in RX J1416.4+2315, Khosroshahi said.

"The study of massive fossil groups such as RX J1416.4+2315 is important to test our understanding of the formation of structure in the universe," he added. "Cosmological simulations are underway, which attempt to reproduce the properties we observe, in order to understand how these extreme systems develop."

The XMM-Newton observations of the fossil galaxy cluster RX J1416.4+2315 were performed in July 2003. Chandra's observations of the same object where made in September 2001. The findings will appear in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.

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