Galaxy-Cluster interactions

Most galaxies in the nearby Universe reside in groups and clusters of galaxies. The dominant component of visible matter in these systems is hot, X-ray emitting gas. It is believed that interactions between group/cluster galaxies and their gaseous environment can strongly modify the properties and evolution of the galaxies themselves and the group/cluster as a whole. Various pieces of evidence exist today in support of the idea that galaxy-cluster interactions take place. For example:

  • In almost all cases, the jets of cluster radio galaxies show severe distortions compared to the well-collimated shapes of similar radio galaxies outside clusters.
  • The intracluster gas contains a vast amount of heavy elements (e.g., Fe and O). It is well-known that heavy elements are produced by stars inside galaxies. Galaxy-cluster interactions should play a major role in the removal of metal-rich gas from galaxies and distributing it throughout the cluster.
  • Many group and cluster galaxies show distorted gas morphologies, probably due to the pressure felt by the galaxies as they move through the cluster gas.

The study of galaxy-cluster interactions and their impact on the galaxies and clusters are a special interest of the group at Birmingham. There is information about some of our projects below.

Galaxy - Cluster interactions: Tailed radio galaxies

We are currently studying the interaction of radio jets with the wider cluster environment, specifically:

  • how the cluster medium affects radio galaxy morphology, e.g. the formation of wide angle tailed radio galaxies (Jetha et al 2005, Hardcastle & Sakelliou 2003).
  • how the cluster and the motion of the radio galaxy through the cluster causes jet bending.

To facilitate our program we have radio observations (VLA and ATCA) of the most powerful radio galaxies in clusters, XMM observations of merging clusters with tailed radio galaxies, and Chandra observations of tailed radio galaxies to investigate the regions where the jets decelerate.


A Wide-Angle Tailed Radio Galaxy (WAT) (e.g. 3C465 above) is a radio galaxy that contains very well collimated inner jets, which resemble an FRII radio galaxy (Fanaroff & Riley 1974), that flare suddenly into diffuse plumes which resemble those found in FRI galaxies. In addition, the jets and plumes can also be bent into wide C-shapes (e.g. 2151+085 below).


Using Chandra X-ray data for 0110+152 and 2230-176 we have found that the base of the radio lobes co-incides with a sharp increase in the temperature gradient (Jetha et al 2005). However, it is unclear as to whether the location of the increase in the temperature gradient is due to the presence of the radio lobes, or if the location of the lobe base is determined by the presence of the cool core. Further work is required to determine which of these scenarios is correct.

More radio images can be found here.

Galaxy-Cluster interactions: Gas stripping in galaxy groups

We are studying the physical processes through which galaxies are affected by their environment. It is well known that the hot, dense X-ray gas in clusters of galaxies can have an impact on the evolution of cluster galaxies. For example, galaxies travelling through this hot cluster gas will feel a "wind" which can strip out gas from within the galaxy. However, it is less clear whether this can also occur in smaller systems, groups of galaxies, where most galaxies live. We are investigating the importance of such processes in groups, by combining X-ray, optical, and radio observations of group galaxies.

The example shown below is a Chandra X-ray image of the hot gas in the spiral galaxy NGC 2276, a member of a small galaxy group. The gas in this galaxy appears compressed to the right of the galaxy, and stretches out in a tail in the opposite direction. Because of its motion through the hot gas in the group, this galaxy is being stripped of its gas at a rate of several solar masses per year.


We intend to follow up on such observations with detailed hydrodynamical simulations, to better constrain the nature and importance of the various physical processes associated with gas stripping from this galaxy, and in galaxy groups in general.

Researchers: Trevor Ponman, Ian Stevens, Jesper Rasmussen, Nazirah Jetha