Clusters of galaxies contain a hot gas with a temperature of millions of degrees that is so hot that it produces X-rays. This gas has a total mass that is usually much larger than the total mass in stars and is commonly referred to as `intra-cluster medium' or `intra-cluster gas'. X-ray observations suggest that this gas also contains a significant amount of heavy elements (like iron, oxigen etc.; note that all the elements heavier than helium are usually called `metals' in astronomy). Since the only sources of such heavy elements are fusion reactions in the centres of stars, their detection suggests that a substantial fraction of this gas must have been ejected from the stars in galaxies into the intra-cluster medium. A few unresolved questions are: how were these metals transported from the galaxies into the intra-cluster gas? When did this chemical pollution occur? And which galaxies are mainly responsible for the large amount of metals in the intra-cluster gas? Astronomers believe that an effective way to transport metals from the intergalactic medium into the intracluster gas is by `galactic winds'. These are generated by supernovae heating of the intergalactic gas to such high temperatures that the thermal energy of the gas becomes larger than the binding energy of the galaxy so that the heated gas is allowed to `escape'. Figure 1 shows a beautiful example of such a wind.

Researchers at Max-Planck-Institute for Astrophysics have investigated these questions combining high resolution numerical simulations and `semi-analytic' techniques. The basic assumption of these models is that galaxies form at the centre of dark matter haloes, whose evolution is followed using the results of high resolution numerical simulations. The evolution of the baryons inside these haloes is then modelled using simplified prescriptions that are physically motivated and supported by observational results. This kind of modelling is quite complex, since one has to follow many mixing and exchange processes between different phases. Figure 2 shows a schematic representation of the physical processes that are included in our model.

Model results can be compared to specific observational data in order to fix the free parameters of the models. The models can then be used to make predictions that can be tested by completely different observational data. With this kind of tooll it is for example possible to predict the amount of metals in different phases as a function of redshift. This is shown, for a particular model in Figure 3. Using this kind of approach, researchers at Max-Planck-Institute for Astrophysics have shown that metals have been ejected into the intra-cluster medium at high redshifts and that `massive' galaxies represent important contributors to the chemical enrichment of the intra-cluster medium. The results obtained suggest that gas and its associated metals must be ejected very efficiently from galaxies and their associated dark matter haloes. Once the material leaves the halo, it must remain in the diffuse intergalactic medium for a time that is comparable to the age of the Universe.

Gabriella De Lucia

For further information:

Gabriella De Lucia, Guinevere Kauffmann and Simon D. M. White: Chemical enrichment of the intra-cluster and intergalactic medium in a hierarchical galaxy formation model, 2004, MNRAS, vol. 349, p. 1101