Question:
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?
Replies:
1. The halo is present inside as well.
2. The existences 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
objects.
4. Halo outside a galaxy does not pull a galaxy apart, this follows from a well
known theorem.
Jasjeet.
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 particle's
position.
NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.
