By: Dr. MARCO RONCADELLI
INFN, Pavia, Italy (email@example.com)
Primordial nucleosynthesis as well as anisotropies in the cosmic microwave background radiation imply that the total amount of baryons in the Universe largely exceeds the visible contribution, thereby making a strong case for baryonic dark matter. Moreover, certain recent developments lead to a consistent picture of the dark baryon budget in the present-day Universe.
Accordingly, dark baryons are mostly locked up in galactic halos – which are anyway dominated by nonbaryonic dark matter – and a sizable fraction of them consists of gas clouds. While a priori various forms of baryonic dark matter in galaxies can be conceived, observational constraints rule out most of the possibilities, leaving brown dwarfs and cold gas clouds mostly made of H2 as the only viable candidates (besides supermassive black holes).
So, it looks natural to suppose that baryonic dark matter in galaxies is accounted for by dark clusters made of brown dwarfs and cold H2 clouds. A few years ago, it was shown that indeed these dark clusters are predicted to populate the outer halos of normal spiral galaxies by the Fall-Rees theory for the formation of globular clusters, which was based on the standard cold dark matter paradigm described in Blumenthal et al. 1984 Nature 311, 517.
We review the dark cluster formation mechanism, and argue that its qualitative features are expected to remain true even in the contemporary picture of galaxy formation. We also discuss various ramifications of the dark cluster scenario in question, paying particular attention to its observational implications. One of them – the diffuse gamma-ray emission from the Milky Way halo – appears to have been confirmed by the discovery of Dixon et al. 1998 New Astronomy 3, 539. Whether this is actually fact or fiction only the future satellite missions AGILE and GLAST will tell.