Research: Groups

The majority of galaxies in the Universe, including our own, appear to reside in groups. Most galaxy groups are "loose groups", in which the galaxies are separated by distances of several hundred kiloparsecs, however "compact groups" such as HCG97 (shown below) are much easier to identify optically. The study of groups and their galaxies is a special interest of the group at Birmingham. Groups are especially important since:

  • Groups may contain most of baryonic mass of the Universe.
  • Since most galaxies are found in groups, the effects of the group environment is essential to an understanding of galaxy evolution.
  • Groups are the best place in the Universe to look for galaxy collisions and mergers.
  • The effects of energetic phenomena within galaxies, such as starbursts and jets, can have a noticeable impact on the hot intergalactic medium within groups, providing a probe of galactic history.
  • Galaxy clusters are believed to be formed from the merger of galaxy groups.
  • Click below for information about some of our main projects involving galaxy groups.


    Contours of soft X-ray emission are shown superimposed on an optical plate of the galaxy group HCG97. This compact group contains only five galaxies (marked with crosses) in a very close configuration. The X-ray emission arises from hot (~10 million K) gas trapped in the gravitational potential well of the group. As can be seen, this gas extends well beyond the optical confines of the group, and it can be used to infer the existence of a large amount of dark matter, which is similarly extended.

Groups: Evolution

Galaxy groups are evolving structures, and this evolving environment can have  a profound effect on the evolution of the galaxies they contain. For example, the high densities and low relative velocities found within evolved groups, make them the most likely places to find galaxies merging. The evolution of galaxy groups is not well understood, and at Birmingham, we are attempting to improve this situation in a number of ways:-

  • Groups are observed to have very diverse properties, and it is very likely that these are related to their evolutionary status. For example, the Local Group in which our own Galaxy is located, is a low density group, dominated by spiral galaxies, which has yet to finish collapsing out of the expanding Hubble flow. It is very different from typical X-ray bright groups. To understand the relationship between the properties and evolution of groups, we are engaged in two major multiwavelength surveys of group properties:
    • The XI (XMM-IMACS) project - a collaboration between Birmingham and the Carnegie Institute in Pasadena to study the galaxy dynamics and hot intergalactic gas in a statistical sample of optically selected groups

Groups: Hot Gas

The hot gas in galaxy groups accounts for more mass than is found in the galaxies. This gas has a temperature of typically 1-10 million degrees, and must therefore be studied with X-ray telescopes. The X-ray properties of galaxy groups have only been accessible to study since the launch of the ROSAT Observatory in 1990. However with the launch of the Chandra (NASA) and XMM-Newton (ESA) Observatories in 1999, much more detailed studies of groups are now possible, and the Birmingham group has been taking full advantage of this. Some of the aims of our work on groups are:

  • Detailed modelling of the gas distribution in the brightest groups. This allows the dark matter profile, gas fraction etc. to be inferred, and compared with galaxy clusters.
  • Tracing the heavy elements (Fe, Si, S, O, Mg) ejected into the hot gas by galaxies. These elements produce diagnostic bright lines in X-ray spectra.
  • Exploring the nature of X-ray dim groups. Do these contain no gas, or is it simply too tenuous to have been detected by earlier telescopes? The answer is important, since if this gas is present, it is one of the main reservoirs of baryons in the Universe.
  • Study of the X-ray properties of galaxies within groups. Comparison with the properties of field galaxies allows a study of the effects of triggering of star formation and stripping of dark galaxy halos within groups.
  • Study of the evolution of galaxy groups, combining multi-wavelength observations and theory.

Researchers: Trevor Ponman, Alastair Sanderson, Abdulmonem Alshino, Ria Johnson, Nathan Slack, Somak Raychaudhury, Ali Dariush


An X-ray image of the central regions of the compact galaxy group HCG62, taken with the ACIS-S camera on-board the US Chandra Observatory, Chandra, launched in 1999, brings arcsecond imaging to X-ray astronomy for the first time. In HCG62, this reveals two remarkable "bubbles" in the gas surrounding the central galaxy (to top left and bottom right in the picture). These may be the result of high energy jets from a large black hole at the centre of this galaxy, although no such jets are visible today.

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

Clusters: Gravitational lensing

The light from distant galaxies and quasars is affected by the gravitational field of the intervening matter between us and the source, resulting in magnified or multiple images. To probe the distribution of matter in the Universe, which is overwhelmingly dark, one needs to directly map this gravitational field. The study of gravitational lensing therefore recently has become one of the most valued tools in surveying the universe and understanding its constituents and its evolutionary history. The most direct way of measuring the distribution of matter in a cluster of galaxies is to study gravitationally lensed images of galaxies lying behind the cluster, like in the image below.




Quadruple-image gravitational lens systems from the CASTLES database. The dominant lens is the central galaxy seen in these near-infrared images taken by the Hubble space telescope. The asymmetric position of the images about the galaxy, however, is due to the lensing effect of the cluster that the galaxy belongs to.

High-redshift quasars are often lensed into multiple images by galaxies, three of which are shown above. They can be used for accurate measurements of various global cosmological parameters, provided that the mass distribution of the gravitational lens system is well constrained. Since the light travel time along the path to the various images is different, observing delayed versions of the same source event in its various images provides a direct way to measure the Hubble constant, independent of local calibrators. The ``cleanest" systems to do these are in the four-image systems as above. However, in many of these cases, it is obvious that the lensing effect is not due to a single galaxy, but contributions from the group or cluster it belongs to are significant.We are carrying out a survey of x-ray detections of clusters and groups of galaxies that are required by gravitational lens models of multiply-imaged quasars but are not seen by optical means. Our analysis of the system B1422+231 (the image on the right above) was recently published, and mentioned in the September 2003 issue of Natural History magazine.

We have also developed an algorithm for the reconstruction of the distribution of matter in a cluster of galaxies from the observable distortion of background galaxies (``weak lensing"). In this method, called the Lens Mapping Algorithm, from the measured distortion (``shear") of the images, the mass distribution is directly derived. This is unlike other methods, where the convergence is first obtained. We have shown that the strength of this method is that for finite fields, the usual mass-sheet degeneracy and other boundary problems can be eliminated by an iterative scheme.


Analysis of systematic distortions of background galaxies (weak lensing) reveal significant structures in galaxy clusters that are not apparent from optical images. We are using XMM and Chandra observations to detect these dark subclusters (like in Abell 1722 above, from Dahle et al.), and investigate sites of suppressed star formation.

Researchers: Somak Raychaudhury, Rowan Temple, Habib Khosroshahi, Trevor Ponman