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 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).

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