and Cosmology

2.5 The Galactic Microlensing Effect: The Quest for Compact Dark Matter
servations and analysis (Fig. 2.28), and more groups
have begun the search for microlensing events, choosing
various lines-of-sight. The most important results
from these experiments can be summarized as follows:
About 20 events have been found in the direction
of the Magellanic Clouds, and of the order a thousand
in the direction of the bulge. The statistical analysis of
the data revealed the lensing probability towards the
bulge to be higher than originally expected. This can
be explained by the fact that our Galaxy features a bar
(see Chap. 3). This bar was also observed in IR maps
such as those made by the COBE satellite. The events
in the direction of the bulge are dominated by lenses
that are part of the bulge themselves, and their column
density is increased by the bar-like shape of the bulge.
On the other hand, the lens probability in the direction of
the Magellanic Clouds is smaller than expected for the
case where the dark halo consists solely of MACHOs.
Based on the analysis of the MACHO collaboration,
the observed statistics of lensing events towards the
Magellanic Clouds is best explained if about 20% of the
halo mass consists of MACHOs, with a characteristic
mass of about M ∼ 0.5M ⊙ (see Fig. 2.29).
Interpretation and Discussion. This latter result is not
easy to interpret and came as a real surprise. If a result
compatible with ∼ 100% had been found, it would have
been obvious to conclude that the dark matter in our
Milky Way consists of compact objects. Otherwise, if
very few lensing events had been found, it would have
been clear that MACHOs do not contribute significantly
to the dark matter. But a value of 20% does not allow
any unambiguous interpretation. Taken at face value,
the result from the MACHO group would imply that the
total mass of MACHOs in the Milky Way halo is about
the same as that in the stellar disk.
Furthermore, the estimated mass scale is hard to understand:
what could be the nature of MACHOs with
M = 0.5M ⊙ ? Normal stars can be excluded, because
they would be far too luminous not to be observed.
White dwarfs are also unsuitable candidates, because to
73
Fig. 2.28. In this 8 ◦ × 8 ◦ image of the LMC, 30 fields are
marked in red which the MACHO group has searched for microlensing
events during the ∼ 5.5 years of their experiment;
images were taken in two filters to test for achromaticity. The
positions of 17 microlens events are marked by yellow circles;
these have been subject to statistical analysis
Fig. 2.29. Probability contours for a specific halo model as
a function of the characteristic MACHO mass M (here denoted
by m) and the mass fraction f of MACHOs in the halo. The
halo of the LMC was either taken into account as an additional
source for microlenses (lmc halo) or not (no lmc halo), and two
different selection criteria (A,B) for the statistically complete
microlensing sample have been used. In all cases, M ∼ 0.5M ⊙
and f ∼ 0.2 are the best-fit values