Hello Andrzej!
The origin of the gas is not completely clear. The standard idea is that the bulk of the gas (in fraction of mass) is primordial and collapsed with the dark matter when the system components (the groups that later merged into present-day clusters) collapsed.
However, the abundance of heavy elements is roughly 1/3 solar in clusters, instead of near-zero predicted if the gas wewre primordial. So, in the standard picture, the gas is polluted by galaxy winds caused by supernovae, and the bulk of the metals (heavy elements) comes from the galaxies, even though the galactic winds contribute little to the mass of the hot intracluster gas.
Gary, and how about the more "natural" explanation of that pollution with heavy elements. What I mean is that the intracluster gas (partly) comes from... the galaxy collisions. Such collisions are inelastic w.r.t. the gas and so the scenario seems to be the following:
- The stars "pass through" although, of course, the structure/shape of the galaxies is - as we perfectly know - significantly disturbed.
- The galactic gas stays "in place"
- The galactic gas heats up.
The net result is the following:
- Galaxies emerging from collisions are deprived of some gas and since the
gas mainly sits in galactic disks they are deprived of disks i.e. the become more and more elliptical. And *this* is what we actually observe!
- Intracluster gas gets enriched by the galactic gas which in turn - as we
perfectly know again - is reprocessed ("polluted") in the ("original") galaxies mainly by SNe.
- The gas is hot because the kinetic energy of colliding galaxies is
converted into the heat.
Well, the standard picture that I had in mind is very close to yours (Spitzer & Baade 1951 revisited?), except that the collisions do not involve the galaxies as we see them, but the galaxies with their dark matter halos, as well as gas reservoirs originally attached to these halos. The other difference is that when two of these halos merge (and they do so more often than the visible parts of galaxies, because the halo cross-section is naturally much larger than that of the visible galaxies), the hot gas (and dark matter) becomes common to both galaxies. With subsequent halo merging, you build a group ... then a cluster.
The gas reservoirs are shock heated during halo collisions, and the temperatures found with X-ray spectra are consistent with the velocity dispersions of the groups and clusters (i.e. the ratio called beta_spec between orbital kinetic and thermal energies is close to unity).
This idea rests upon the assumption (that I have not verified) that most of the gas connected to spiral galaxies lies outside of the visible (optical) disk, and perhaps outside of the HI disk (since the gas reservoir is expected to have a spheroidal shape similar to that of the dark matter). The mass of gas in this spheroidal reservoir should be equal to the universal baryonic fraction (set by, e.g., WMAP) times the mass within the virial radius minus the mass in stars and in disk gas, and one should compare this to the mass in disk gas.
Another cute idea (Larson, Tinsley & Caldwell 1980; Whitmore, Gilmore & Jones 92?) is that, in clusters, the gas reservoirs are tidally stripped before they can begin to cool and settle into a cold disk to form stars, and Whitmore et al. have shown that the amount of X-ray gas corresponds to the mass of these tidally stripped reservoirs. My criticism of that idea is that clusters are built from the mergers of smaller groups of galaxies, themselves built from the mergers of single galaxies, for which the tidal effects should be small, while the shock heating of the colliding gas reservoirs is inevitable.
best regards
Gary