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 3 includes any compact object with masses below . Many searches have been carried out looking for these MAssive Compact Halo Objects (MACHOs) using microlensing data. These are reviewed in . 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% , and the masses of these objects appear to be . 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 3, at least as far as providing the bulk of the Galactic dark matter. However, above , 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 . Finally, it remains possible in principle that cold clouds with masses might provide some of the halo dark matter [103, 140, 141].
© Max Planck Society and the author(s)