gas question

[Gary Mamon]

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:
>
> 1. The stars "pass through" although, of course, the structure/shape of the
>    galaxies is - as we perfectly know - significantly disturbed.
> 2. The galactic gas stays "in place"
> 3. The galactic gas heats up.
>
> The net result is the following:
>
> 1. 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!
>
> 2. 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.
>
> 3. 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