XII. The Milky Way

a) See band of stars in the sky we are in disc full of stars

• use distance of stars to get spatial distribution
• (spectroscopic parallax (HR diagram))

Kapteyn concluded: We are in the center!

• Shapley used Globular clusters
• Distance from Cepheids in globulars 3D map (1917)

Galactic Halo consists of:

1. Globular clusters (old Population II stars) Left over from pre-galactic-collapse
2. Individual Population II stars, high velocities (ejected from globulars?)
3. 21 cm hydrogen gas

• The Milky Way has features in common with other spiral galaxies:

1. Flat disc (rotation)
2. Emission nebulas (red) and 21 cm --> H gas
3. Radio maps of Milky Way show spiral structure:
• spirals would wind up after a few rotations
• but: galaxy > 20 rotations --> not the same material in arms
4. Young hot (blue) stars light up arms:
   • density waves --> compression --> more self-gravity --> star formation
5. Dust

• Doppler shifts of globulars etc. --> "galactic rotation curve"

• Velocity does not fall off with distance from center
• i.e. non-Keplerian orbits --> most mass outside disc(not seen in stars)
• missing mass problem.

• Observed:
  • UV absorption lines --> hot (ions) and turbulent (Doppler) gas
  • Most mass of galaxy? --> missing mass ?
• What kind?Brown Dwarfs? Black Holes? Strange form of Matter?

Exciting observations:

1. Expanding & rotating rings (Doppler effect) --> explosions in center?
2. Synchrotron radiation "blob" near center --> explosion?
3. SgrA* - small intense radio source
4. Need 3 x 106 Msun in SgrA* to keep things from flying away
• SgrA* = supermassive black hole (~H100 Msun)?
5. Rotating (Doppler) molecular disc around SgrA*
6. Matter falling in --> energy to power all the bizarre features?
7. Gammas from electron-positron annihilation
8. Other galaxies: rapid central rotation --> supermassive black holes?

Chapter 13