1. Chapter 23
Our Galaxy

2. Our Galaxy

3. Looking Towards the Galactic Center

4. Galaxy Size & Structure

5. Neutral Hydrogen Emission at 21cm

6. Spiral Arms at 21 cm

7. Velocity Field

8. Interpretation of the Rotation Curve
If all the mass were concentrated in the center of the Galaxy
(as it is for the solar system) then one would expect Keplerian
orbits,
Balancing gravity and centripetal forces
GMm = mv2
r r
Then m (the mass of a test particle, like a star for example) cancels
v = √ GM/r where M is the mass of the galaxy interior to radius r
But what actually is observed is v ~ constant

9. The Flat Rotation Curve is Evidence for Dark Matter

10. Furthermore, the rotation curve remains flat beyond the
visible extent of the galaxy, so there is apparently mass
where there is no light!
To get a flat rotation curve requires a mass density   1/r2
which causes the mass, M(r), to grow as r, where there is no
1ight!
This is strange, because stars are the only form of mass
that is “visible”, so perhaps there is some “invisible form”
of mass, that astronomers call “dark matter”, which can
explain these strange results.
The problem gets worse for clusters of galaxies, for which
astronomers measure very large mass to light ratios.

11. Plausible Dark Matter Candidates
MACHOS – Massive Compact Halo Objects (e.g. dead stars)
WIMPS - Weakly Interacting Massive Particles (e.g. Neutrinos)
MOND – Modified Newtonian Dynamics.
(e.g. changing Newton’s law of gravity).

12. The Winding Dilemma
A consequence of v = const is that the angular velocity, w = v/r,
decreases with radius, which leads to the dilemma that the spiral arms should wind up very quickly;

13. Hence the Density Wave Theory
That the spiral arms are a pattern that the stars move through, much
like traffic on a congested highway;
There is a traffic jam that may last all day, but individual cars
only spend a short time in the jam itself by passing the slow
moving vehicle.

14. Density enhancement in Spiral Arms
Crowding of interstellar gas clouds in the arms is important
for star formation.

16. Evidence for a BH in the Galactic Center