|Annu. Rev. Astron. Astrophys. 1994. 32:
Copyright © 1994 by . All rights reserved
In this section, we focus specifically on the Low Mass Object (LMO) scenario. There are several reasons why LMOs currently seem to be the most plausible option. Firstly, there may be direct evidence from cluster cooling flows that baryons can turn into low mass stars with high efficiency even at the present epoch. [This topic is reviewed by Fabian (1994) in this volume, so I merely summarize the key points below and omit references.] Secondly, recent data on the stellar IMF in our own Galaxy suggests there may be a higher fraction of LMOs when the metallicity is low. Thirdly, as we saw in Section 7, microlensing data may already indicate that there is dark matter in the form of LMOs.
9.1 Cooling Flows in Clusters
X-ray observations suggest that the cores of many clusters contain hot gas which is flowing inwards because the cooling time is less than the Hubble time. This condition is satisfied in 70-80% of EXOSAT clusters and in some poor clusters and groups as well. Direct evidence for cooling comes from Fe XVII line emission, since this shows that the temperature decreases as one goes inwards. The mass flow rates are typically in the range 50-100 M y-1, extending up to 103 M y-1 in some cases, and they seem to have persisted for at least several billion years. There is consistency between the flow rates derived from spectral measurements and those derived from surface brightness analysis.
The mass appears to be deposited over a wide range of radii with a roughly M R distribution (which requires that the gas be very inhomogeneous), but it cannot be going into stars with the same mass spectrum as in the solar neighborhood, or else the central regions would be bluer and brighter than observed. Some cooling flows do exhibit a blue optical continuum over the central few kpc, but the associated massive star formation rate must be less than a few M y-1, which is only a fraction of the total inflow rate. This suggests that the cooling flows produce very low mass stars, possibly because the high pressure (of order 106 cm-3 K) reduces the Jeans mass. An important feature of a cooling flow is that it is quasi-static, in the sense that the cooling time exceeds the local dynamical time, and it is this condition which is supposed to preserve the high pressure. The Jeans mass could be as low as 0.1 M if the cloud gets as cool as the microwave background radiation (T = 3 K); this is not inconceivable because observations suggest that the gas is mainly molecular, which could allow grains to form abundantly.
A recent twist in this scenario has been the detection of large amounts of cold X-ray absorbing material in many clusters (White et al 1991). The cold gas extends out to 100 kpc and the mass involved is usually around 1012 M (comparable to that expected from a cooling flow that has persisted for a cosmological time). This raises the question of whether we still need low mass stars, especially in view of the Pfenniger et al (1994) proposal that the dark matter in galactic halos could be cold gas. Of course, if cooling flows do make low mass stars, one might expect some cold gas as an intermediate state. This issue has yet to be resolved.