So Vrot ~ constant over the disk of the
Galaxy. This should confuse the hell out of you.
Things to ponder:
- What does the rotation curve of the
Solar System look like?
- What should the rotation curve
of the Milky Way look like?
- What does it mean that the rotation curve is
Note that our galaxy isn't the only one that does this. Here is a
compendium of rotation curves from Sofue (1999):
Look at the dynamical mass:
So as we go further and further out in the
galaxy, there is more and more mass. And we don't see it. This
is the Milky Way's dark matter halo.
How far out does it go? 100s of kpc? And there's
a lot of stuff out there.
So 90% of the Milky Way's mass is invisible dark matter.
- Dynamical mass estimates: ~ 1012
- Star + Gas mass: < 1011 Msun
So what could the
dark matter be?