Groups: Fossils

Discovered in 1994, "fossil" groups of galaxies show the end result of galaxy merging over many billions of years. Mergers of galaxies within the bound system of a group are predicted to be common, because the galaxy velocities in the group are low, similar to their internal velocities. However, groups where most galaxies have merged were not discovered until X-ray observations revealed a halo of hot gas, as seen in normal groups of galaxies, surrounding a giant galaxy, with no other luminous galaxies in the group. The other luminous galaxies have been swallowed by the giant galaxy, leaving only the X-ray halo and some dwarf galaxies surrounding the central giant.


The original fossil (RX J1340.5+4017): An optical R band false-colour image of the original "fossil group" which we discovered in 1993, with X-ray contours overlaid. The system, at a redshift of 0.17, has X-ray properties (luminosity and temperature) similar to other groups of galaxies, but this group is completely dominated by a single giant elliptical galaxy, probably the result of merging of the original group galaxies. The solid contours are from the ROSAT PSPC X-ray instrument, showing the overall structure, and the dashed contours are from the higher resolution HRI instrument, showing detail in the core.

The giant elliptical galaxy in the fossil group shown above appears to be a completely normal elliptical, supporting the idea that most elliptical galaxies were formed by mergers. However recent studies which we have carried out suggest some structural differences which could be related to their environments.

We have assembled a sample of fossil groups using data from the WARPS X-ray/optical survey, and have been studying their properties in detail using X-ray data from the Chandra and XMM-Newton observatories, in combination with optical observations. An example is shown below.


The nearest fossil group (NGC 6482): Chandra observations of the giant elliptical galaxy NGC 6482, at the centre of this image, show that it is surrounded by a cloud of hot gas (shown in blue), with a temperature of about 10 million degrees, over 700,000 light years across. This giant galaxy is believed to have grown to its present size by cannibalising its neighbours, leaving only the X-ray halo to tell the tale ... more

We are also collaborating with theorists to investigate the formation and evolution of fossil groups, using the Millennium cosmological hydrodynamical simulations. The results support the idea that most fossil groups are old systems which formed earlier in the history of the Universe than "normal" galaxy groups.

We have also discovered that some large galaxy clusters satisfy the requirement for a fossil group, that their bright central galaxy should be almost ten times brighter than any other galaxy in the system. This is surprising, since the mergers which cause the central galaxy to grow are expected to be less common in large clusters, where galaxies move at very high velocities. We are exploring this further, using clusters from the LoCuSS (REF????) cluster survey.

Researchers: Trevor Ponman, Ali Dariush, Somak Raychaudhury, Graham Smith