The Net Advance of Physics: The Nature of Dark Matter, by Kim Griest -- Section 7G.

Interpretation of Bulge Events

The large number of events we (and the OGLE group [34]) have

found towards the galactic center came as a great surprise to

everyone. The line-of-sight toward the bulge goes though the

stellar disk, so bulge microlensing is sensitive to halo dark matter,

disk stars, and any disk dark matter which might be present. The

early predictions [55, 56, 57] included all these sources, but still

predicted many fewer events than have now been observed. It seems

the microlensing experiments have discovered a new component of

the Milky Way. A standard way of quoting the microlensing

probability is the optical depth , which is the probability that any

given source star is lensed by a magnification of 1.34 or greater.

Optical depth has larger statistical errors than the event rate, but

has the great advantage of being independent of the masses of the

lenses. Early predictions of bulge microlensing were in the

range [55, 56, 57], while using the sample of events above we find

[41]. We have not finished the complete

efficiency calculation for our bulge events, so this estimate uses a

sub-sample of 15 giant star events, for which our preliminary

efficiencies should be acceptable [41].

Several models have now been proposed to explain the high

microlensing rate. They include [58, 59, 60]

1. A ``heavy" disk. Perhaps the disk of the Milky Way is

   substantially more massive than normally considered.

2. A ``bar" at small inclination. Perhaps the Milky way is not a

   grand design spiral as usually assumed, but is a barred spiral,

   with a very large bar, previously overlooked since it points

   nearly toward us.

3. A highly flattened, or disk-like halo.

4. Some combination of the above, and/or extra material in the

   bulge.

The suggestion of a Galactic bar has been around for a few years,

and seems to be corroborated by other data [61], though it is still not

clear whether this alone is sufficient to explain the microlensing

data. Extensive work is being undertaken in trying to resolve these

questions. One method is to map out the bulge area with

microlensing. A bar-like structure will give a different pattern of

microlensing than a disk-like structure. Use of a satellite, or of the

fine-structure of the microlensing lightcurve has also been

suggested [62, 63].

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