The Net Advance of Physics: The Nature of Dark Matter, by Kim Griest -- Section 2E.
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While we don't know what the dark matter (DM) is, we have a
fairly reasonable idea as to how much of it there is in the Galaxy,
how it is distributed, and how fast it is moving. This information
comes from the rotation curve of the Milky Way, and is crucial to
all the direct searches for dark matter. If we say that the rotation
curve of the Milky Way is constant at about
km/sec out to
as far as it is measured, then we know that
the density must drop as
at large distances.
This velocity also sets the scale for the depth
of the potential well and says that the dark matter must also move
with velocities in this range. Assuming a spherical and isotropic
velocity distribution is common, and a usual parameterization is
where kpc is the distance of the Sun from the galactic
center, a is the core radius of the halo, and is the
density of dark matter near the Sun. Also, a typical velocity
distribution is
It should be noted that the specifics of the above models are not
very secure. For example, it is quite possible that the halo of our
Galaxy is flattened into an ellipsoid, and there may be a component
of the halo velocity which is rotational and not isotropic. Also,
some (or even most) of the rotation curve of the Milky Way at the
solar radius could be due to the stellar disk. Canonical models of the
disk have the disk contributing about half the rotation velocity, but
larger disks have been envisioned. Recent microlensing results may
be indicative of a larger disk as well (see Section 7.).
Finally, other important points about our Galaxy's geography
include the fact that the nearest two galaxies are the LMC and
SMC, located at a distance of 50 kpc and 60 kpc respectively; that
the halo of the Milky Way is thought to extend out at least this far;
and that the bulge of the Milky Way is a concentration of stars in
the center of our Galaxy (8.5 kpc away) with a size of about 1 kpc.
Distribution of Dark Matter in the Milky Way
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