XI: The Birmingham-Carnegie XMM-IMACS project

An unbiased study of galaxy groups

In the low redshift Universe, most of the normal (baryonic) matter which cosmologists believe to be present has yet to be detected. Cosmological simulations suggest that this missing matter is hot, and that most of it lies within galaxy groups and large filamentary structures. A census of the gas contents of galaxy groups is therefore essential to our understanding of the Universe.

Existing X-ray studies of galaxy groups are mainly based on systems that have already been detected in ROSAT X-ray data. However, such groups need not be representative of the overall group population. To address this problem, we have teamed up with John Mulchaey (Carnegie Observatories, USA), for a detailed study of a sample of 25 groups selected from the 2dF galaxy redshift survey, using a well-defined overdensity criterion.

We are studying this sample by means of optical spectroscopy (using the multi-aperture IMACS instrument on one of the 6.5m Magellan Telescopes in Chile), combined with X-ray observations, for which we have been awarded substantial XMM-Newton time. We plan to study all galaxies down to small dwarfs in these groups, and look at the velocity structure and star-formation activity of the galaxies from their spectra, and how these relate to the hot baryonic intergalactic medium.

The figure below shows X-ray images for the first four XI groups for which we received XMM-Newton data. Diffuse X-ray emission from hot gas in these systems is either faint and irregular or remains undetected. This suggests that the amount of hot gas in these groups is very low, and that the nature of the groups is very different from "classic" X-ray selected groups, which tend to show bright, regular X-ray emission. XMM data for a further 5 groups show a similar pattern, apart from one group which is X-ray bright, containing a large mass of hot gas.

XI groups

XMM-Newton soft X-ray images of four of the XI groups.

On the optical side of the project, three-band photometry has now been completed for 16 of the 25 groups, and spectroscopic coverage extends to nearly 4000 galaxies in the fields of these 16 groups. These spectra reveal a large fraction of emission-line galaxies, indicating significant star formation and nuclear activity in many of the group galaxies.


Composite BVR optical images of two of the XI groups, MZ4577 (also shown in X-rays above) and MZ3698, taken with the 2.5m Irenee du Pont Telescope at the Carnegie Observatories, Las Campanas, Chile.

To help investigate the state of the baryons in these groups, we are also looking at the amount and distribution of cold gas in selected systems. Together with Chandreyee Sengupta (RRI, India), we have observed one of the groups at 21cm with the Giant Metrewave Radio Telescope (GMRT) in India, and plan to extend these observations to further groups in the sample.

Researchers: Somak Raychaudhury, Trevor Ponman, Ali Dariush, Smriti Mahajan

External collaborators: John S. Mulchaey & Jesper Rasmussen (Carnegie Observatories, Pasadena, USA) and Chandreyee Sengupta (Raman Research Institute, Bangalore, India).