By: Dr. MARCO RONCADELLI
INFN, Pavia, Italy (roncadelli@pv.infn.it)
Abstract
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.
http://arxiv.org/pdf/astro-ph/0301537.pdf
No comments:
Post a Comment