Dark matter distribution
Name: Stuart Briber
Assuming the force that holds spinning galaxies together comes largely from
the gravity of dark matter, why is it believed this dark matter forms haloes
around the OUTSIDE of a galaxy where its gravity would contribute to pulling
a galaxy apart?
1. The halo is present inside as well.
2. The existence of dark matter beyond the visible extent of galaxies is
inferred by looking at velocities of HII regions seen beyond the main galaxy
and are believed to be bound to the galaxy (gravitationally bound).
3. Another evidence for this comes from gravitational lensing of distant
4. Halo outside a galaxy does not pull a galaxy apart, this follows from a
well known theorem.
Jasjeet S Bagla
The previous answer is basically correct: the "Dark Matter" is believed to
be present throughout the entire galaxy. It extends beyond the visible
galaxy's edge, however. Point 4 in the previous response deserves a little
more explanation. It is only true if the mass distribution is spherical.
If the halo is spherical, as is believed to be the case, the force felt by
any particle only depends on how much material lies interior to that
Richard A Gerber
Nature magazine had 2 nice articles on the "dark matter" problem. One was
a review paper. The other was a measurement in dwarf galaxies that made an
impressive case against "cold" dark matter. I think that the upshot is that
almost all dark matter could be hadronic (and the universe can only be
closed by Einstein's fiddle-factor).
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Update: June 2012