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The Net Advance of Physics: The Nature of Dark Matter, by Kim Griest -- Section 2E.

Next: Dark Matter Candidates Up: Physical Evidence Previous: The Baryonic Content

Distribution of Dark Matter in the Milky Way

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 tex2html_wrap_inline221 km/sec out to

as far as it is measured, then we know that the density must drop as

tex2html_wrap_inline119 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 tex2html_wrap_inline227 kpc is the distance of the Sun from the galactic

center, a is the core radius of the halo, and tex2html_wrap_inline231 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.

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Next: Dark Matter Candidates Up: Physical Evidence Previous: The Baryonic Content