Metallicity of the Intra-cluster medium.

The X-ray spectrum of cluster gas consists of the usual featureless thermal bremsstrahlung continuum, with additional emission lines mostly from inner-shell transitions of the Fe ion, see Figure 9.1. By modelling the spectrum, one can estimate the amount of Fe present in the gas. Comparison with the total amount of gas gives a mean metallicity $M_{\rm Fe}/M_{\rm gas}\approx {1/3}$ of the solar value^9.3.

Figure 9.1: X-ray spectrum of the central region of the Centaurus cluster, taken by the ASCA X-ray satellite. It shows some emission lines and an absorption edge (which are labelled) due to different components of the hot intra cluster medium.

This is highly surprising: it means that the gas in clusters has a higher metallicity than for example globular cluster stars, or even the stars and gas in most dwarf galaxies. The only way we know how to produce Fe is in stars - in fact we think that most of the Fe is produced during the thermo-nuclear deflagration of a Super Nova. Somehow Fe produced in stars, themselves presumably inside a galaxy, was able to escape from that galaxy and end-up in the intra cluster medium. If the metals were able to escape, then may be also some of the explosion energy was able to escape the galaxy, and heat the surrounding gas, preventing gas cooling and galaxy formation. So the presence of hot metal enriched gas in clusters, suggests that star formation in proto-clusters provided a negative feedback mechanism, which kept most of the gas at temperatures too high for efficient galaxy formation.