Key Concepts:

1. What is dark matter?
2. How do we know dark matter exists?
3. What is "hierarchical large scale structure" and where does it come from?

Earth at Night

Evidence for Dark Matter

• rotation speed of the outer parts of galaxies is too great
• 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
• 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
• 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

Galaxy Clusters: 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 of images can be seen

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 clouds?
• could Newton's gravity be wrong?

Quiz 21a

---

Hierarchical Large Scale Structure

The Local Group

The Milky Way Galaxy M31 (The Andromeda) M33 LMC & SMC many dwarf satellites

The Local Cluster and Supercluster

Large Scale Structures: Great Walls and Fingers of God

Hierarchical Structure Formation

• Dense clumps of matter condense out of uniform environment


• High density regions (groups and clusters) grow denser by accumulating more matter(dark matter, stars)

Quiz 21b

---