Key Questions:

1. What is dark matter?
2. How is the presence of matter determined?
3. How do we know dark matter exists?
4. What is the nature of dark matter?

Earth at Night

Quiz 22A

Evidence for Presence of Dark Matter

• motions of stars in spiral and elliptical galaxies
• motions of galaxies and gas in galaxy clusters
• gravitational lensing

Mass of the Milky Way inside the Solar Circle

• use Modified Kepler's Law (also "Orbital Velocity Law"): M = aV²/G
• semi-major axis "a" is 8.5 kpc (about 2 billion AU)
• period:
• mass:


• mass-to-light ratio: if the luminosity of stars inside the Sun's orbit is 10¹⁰ solar luminosity, then the mass-to-light ratio is 10¹¹/10¹⁰ = 10 solar mass per solar luminosity.
• "missing mass problem" and need for dark matter: the circular rotation speed of stars and gas remains constant and does not fall off even at the visible edge of the MW (unlike in the Solar system on the left panel), suggesting a lot of invisible matter lies beyond the "edge" of the MW galaxy.

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• Cosmic Hide and Seek: the Search for the Missing Mass (by C. Miller)
• What's the Matter with the Universe? (by R. J. Nemiroff)

Mass of Elliptical Galaxies and Galaxy Clusters (here the movie)

Galaxy clusters represent a large concentration of:

1. galaxies (<1%)
2. hot gas (about 10%)
3. dark matter (about 90%?)

Optical Image (Coma Cluster)	X-ray Image (Coma Cluster)

Gravitational Lensing and Dark Matter

• gravity bends light - only depends on the mass
• different shapes are possible, depending on the source-lens geometry

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Mapping dark matter in clusters using gravitational lensing

What makes up the Dark Matter?

• MACHOs (Massive Compact Halo Objects): planets, brown dwarfs, white dwarfs, black holes?
• WIMPs (Weakly Interacting Massive Particles): axions, massive neutrinos, photinos?
• very cold gas/dust clouds?
• modified Newtonian dynamics (MOND): could Newton's gravity be wrong?

Quiz 22B

Reading assignment for next lecture: Unit 79-80