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.