![]() | Annu. Rev. Astron. Astrophys. 1994. 32:
531-590 Copyright © 1994 by Annual Reviews. All rights reserved |
9.2. Pregalactic and Protogalactic Cooling Flows
Although cooling flows provide a natural way of turning gas into low mass stars with high efficiency, those observed in the centers of clusters could not themselves be responsible for either the cluster dark matter (since this is distributed throughout the cluster) or the halo dark matter in galaxies outside clusters. In order to account for the usual dark matter problems, one therefore needs cooling flows on the scale of galaxies or below. Only the most massive cluster galaxies exhibit cooling flows at the present epoch - but it would not be surprising if smaller scale cooling flows occurred at earlier cosmological epochs since X-ray data already suggest that cooling flows evolve hierarchically to larger scales (Evrard 1990, Katz & White 1993).
These considerations prompted
Ashman & Carr (1988) and
Thomas & Fabian (1990)
to consider the circumstances in which one could expect high-pressure
quasi-static flows to occur at pregalactic and protogalactic epochs. The
situation is best illustrated for the hierarchical clustering scenario,
in which, as
time proceeds, increasingly large gas clouds bind and virialize. The
mass fraction
of a cloud cooling quasi-statically is maximized when the cooling time
tc is comparable to the free-fall time
tf: Collapse does not proceed at all for
tc >> tf, whereas it is not
quasi-static for tc << tf. In
any particular variant
of the hierarchical clustering scenario, one can specify the mass
binding as a function of redshift. For a cloud of mass M, the
dynamical time will just
be of order the Hubble time at that redshift, whereas the cooling time will
depend upon the density and virial temperature of the cloud (which are
themselves determined by M and z). Thus, one can specify a
region in the (M, z)
plane of Figure 8 in which bound clouds will
cool within a dynamical time.
This applies above a lower mass limit associated with molecular hydrogen or
Lyman- cooling and below
an upper mass limit associated with atomic hydrogen cooling
(Rees & Ostriker 1977)
or the Compton cooling of the microwave background.
The condition tc ~ tf will be
satisfied at the boundary of the region (shown
shaded) and the intersection of this boundary with the binding curve
M(z)
singles out two characteristic mass-scales and redshifts. These
correspond to
what Ashman & Carr term "Pervasive Pregalactic Cooling Flows" (PPCFs) and
what Thomas & Fabian term "Maximal Cooling Flows" since the amount of gas
cooling quasi-statically is maximized. The associated mass-scales are always
of order 104-108
M and
1011
M
, but
the redshifts depend on the particular
scenario. Figure 8 shows the binding curves
corresponding to the Cold Dark
Matter scenario, the broken curve corresponding to the biased version,
and the baryon-dominated isocurvature scenario
(with
specifying the exponent in the
mass dependence of the density fluctuations at decoupling).
One might anticipate most of the dark matter being made on the smaller scale because much of the gas will have been consumed by the time atomic cooling becomes important. However, this does not happen in the Cold Dark Matter picture because the spectrum of fluctuations is very flat on subgalactic scales and, in the isocurvature models, M(z) may never be small enough for low mass PPCFs to occur after decoupling. Both these features are indicated in Figure 8. Ashman & Carr (1991) therefore argue that most of the dark matter would need to be made by high mass protogalactic PPCFs. Another argument in favor of the protogalactic PPCFs is that the pressure is probably too low to make LMOs on the smaller scale.
One problem with invoking protogalactic cooling flows to make the dark matter is that one might expect most of the gas to have gone into clouds with tc < tf and such clouds should make ordinary stars. This "cooling catastrophe" raises the question of whether there could be enough gas left over to make ordinary galaxies (White & Frenk 1991, Blanchard et al 1992). One way around this is to invoke supernovae to reheat the gas so that most of it can avoid cooling until the protogalactic epoch (Thomas & Fabian 1990). Another way is to argue that even clouds with tc < tf can make a lot of dark matter (Ashman 1990). The idea here is that gas always drops out at such a rate as to preserve the PPCF condition tc ~ tf for the surviving gas. One thus gets a two-phase medium, with cool dense clouds embedded in hot high-pressure gas. This was originally proposed as a mechanism to make globular clusters at a protogalactic epoch (Fall & Rees 1985), but Ashman (1990) argues that sufficiently small clouds would fragment into dark clusters rather than visible clusters in the presence of molecular hydrogen. By applying the same idea to other galaxies, he predicts that the fraction of dark mass in spirals should increase with decreasing disk mass and this may be observed (Persic & Salucci 1990).