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 [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
. 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 [32]. Finally, it remains possible in principle that cold clouds
with masses
might provide some of the halo dark matter [104,
140,
141].
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Experimental Searches for Dark Matter
Timothy J. Sumner http://www.livingreviews.org/lrr-2002-4 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |