3.1 Baryonic dark matter

According to the standard model, baryonic dark matter is required to make up the difference between the visible matter density Ωv and the baryon density Ωb required by standard BBN models. Exactly where these baryons might be hiding depends on the nature of the objects being studied. For high redshifts > 3, most of the baryons might still be in the form of an intergalactic medium still in the process of collapse [144], while recent data from a large sample of nearby x-ray emitting clusters of galaxies have shown that for these clusters most of the baryon fraction f is in the surrounding hot gas of the intracluster medium (ICM) [87]. Indeed, if it is assumed that the cluster matter has the same fraction of baryonic matter as the universe as a whole, i.e. f ≃ Ωb∕ Ωm, then it seems that the ICM accounts for baryons up to the required BBN levels in these clusters. On the scale of individual galaxies the situation is much less clear [31Jump To The Next Citation Point]. Rotation curves of galaxies imply the existence of dark-matter centrally clustered halos. Λ-CDM N-body simulations (see Figure 4View Image) suggest the halo composition should follow the underlying matter distribution of the universe but with some enhancement of the baryonic proportion through more efficient dissipative collapse.

This naturally leads to the conclusion that there is probably unseen baryonic matter in galaxies, but that it is unlikely to be sufficient to entirely explain the rotation curves. The brown dwarf (BD) candidate entry in Figure 3View Image includes any compact object with masses below 0.08M ⊙. Many searches have been carried out looking for these MAssive Compact Halo Objects (MACHOs) using microlensing data. These are reviewed in [31]. Although a number of candidate microlensing events have been seen, the apparent mass determinations for the lenses and their locations cast doubt on whether the lenses are indeed MACHOs in the halo of the Milky Way. The most recent estimates put the most likely MACHO contribution to the halo at 20% [8], and the masses of these objects appear to be ∼ 0.5 M ⊙. This suggests a population of white dwarfs and might indicate an early epoch of star formation in the Galactic halo. To explain all the dark matter with compact objects larger than brown dwarfs would have produced too many heavy elements during their evolution as stars prior to collapse and so these are still excluded as halo baryons in Figure 3View Image, at least as far as providing the bulk of the Galactic dark matter. However, above ∼ 105M ⊙, super massive objects (SMOs) might collapse immediately to black holes. SMOs would still produce microlensing effects and would also give rise to dynamical effects, such as the heating of disk stars and the disruption of globular clusters [32]. Finally, it remains possible in principle that cold clouds with masses ∼ 10−3 M ⊙ might provide some of the halo dark matter [103140141].

View Image

Figure 4: High-resolution N-body simulation of a galactic dark matter halo [89Jump To The Next Citation Point].

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