Components of the Milky Way The light from galaxies is centrally concentrated. But is the mass also centrally concentrated? Does Mass follow Light in Galaxies?

Examples of Rotation Curves

Expected Versus Observed Rotation Curves for Our Galaxy

The Maybe Correct Way to Think about Our Galaxy The “Mass Size of the Galaxy is 10 times larger (1000 times more volume) than the “Light Size” of the galaxy  this seems weird? Shouldn’t there be “DarkGalaxies”  appearance of light (baryons) in the DM halo is random?

Dark Matter in M31 – Flat Rotation Curve

Dark Matter in Other Spiral Galaxies – Flat Rotation Curves NGC 3198

Dark Matter in Elliptical Galaxies – Evidence from X-ray Gas NGC 4697 optical NGC 4697 Chandra

X-ray Evidence for Dark Matter in Clusters of Galaxies Need enough gravity to keep the X-ray gas from “boiling off” into intergalactic space.

Possible Dark-Matter Candidates Dark Matter is material that emits low light per unit mass

Galaxy Motions in a Cluster of Galaxies The Coma Cluster

What about ordinary low mass stars: Clearly ruled out by Space Telescope observations. Left picture shows simulation if these stars dominate stellar clusters at the level required for being DM candidates  observations strongly fail to confirm their existence at the required number density.

Gravitational Lensing in Abell 2218

Gravitational Microlensing

Microlensing Light Curves Length of the lensing event is determined by the mass of the lensing object.

Wimps: Weakly Interacting Massive Particles left over from early big bang now filling the Universe Dark matter elastically scatters off nuclei Nuclear recoils detected by phonons, scintillation, ionization, …, … e,  DM Attisha

Three Real Choices DM is baryonic  most likely ruled out DM is a particle  hasn’t been detected despite 30 years of experiments and expectation that detection is just around th corner. Maybe its gravity that is broken (covered in the third homework assignment). Until DM is actually detected, it remains just like an epicycle.