Chapter 21

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```					Chapter 23

Our Galaxy
Our Galaxy
Looking Towards the Galactic Center
Galaxy Size & Structure
Neutral Hydrogen Emission at 21cm
Spiral Arms at 21 cm
Velocity Field
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
r2   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
The Flat Rotation Curve is
Evidence for Dark Matter
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.
Plausible Dark Matter Candidates
• MACHOS – Massive Compact Halo
• WIMPS - Weakly Interacting Massive
Particles (e.g. Neutrinos)
• MOND – Modified Newtonian Dynamics.
• (e.g. changing Newton’s law of gravity).
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;
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.
Density enhancement in Spiral Arms
Crowding of interstellar gas clouds in the arms is important
for star formation.
Evidence for a BH in the Galactic
Center

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 views: 1 posted: 5/17/2013 language: English pages: 16