XIII. Galaxies and the Structure of the Universe

1. Proof of other galaxies

Shapley - Curtis debate: nebulae in Milky Way or separate galaxies?

Hubble: Cepheids in galaxies (1924)

Period - Luminosity relation --> distance --> "island universes"

• like the milky way: disc with spiral arms

• no structure
  • no gas or dust--> no young stars (no star formation)
  • no rotation --> no flat discs, but spheroidal shapes
• Globular clusters are like very small elliptical galaxies
  • But: ellipticals have very energetic centers (supermassive black holes?) while globulars have no evidence for central black holes

• no regular structure--> distorted by tidal forces?

• The 'missing mass' in the universe?

e) Why ellipticals and spirals? Still a riddle

1. Not age - old stars in both
2. Not rotation - won't cause lack of gas & dust in ellipticals
3. Galaxies collide --> ellipticals
   • Stimulates rapid early star formation (Starburst galaxies?)
   • Consumes gas & dust which are not available for making young stars
   • Cancels angular momentum
   • Ejects hot gas --> another way to explain lack of gas
   • But: no star collisions!(distances still too large)

3. Masses of galaxies

• Doppler effect --> velocities

• galactic rotation curves --> velocities--> Mass

• broadening of lines from stars --> velocities--> Mass
• Large central velocities --> supermassive black holes

• Individual velocities (Kepler's 3rd law) --> m1 + m2andm1/ m2

• velocities of member galaxies
• All cases: deduce amount of gravity needed to keep things from flying apart

Much more matter than observed in stars is needed to keep the structures together ! --> Dark matter problem everywhere

which our Milky Way galaxy and the Andromeda galaxy are in

b) Types of Clusters

• Regular clusters (spherical shape): contain only ellipticals
• Irregular clusters: contain ellipticals and spirals

Evidence in favor of scenario with collisions:

• Regulars --> interactions (collisions?) --> ellipticals and spherical group
• Regulars: hot X-ray emitting gas between galaxies (from collisions)
• Regulars: giant ellipticals at center --> "galactic cannibalism"

• expanding from close to distant
• Standard candles - "know" luminosity: Brightness -> Distance
• Standard rulers - "know" size: Angular size -> Distance

Observed: Large redshift --> object far away

v_away = H x Distance (H: Hubble constant)

Hubble's Law can be used the other way around:

Redshift --> distance (galaxy map of universe)

c) Superclusters:the "Great Wall", sheets and voids (bubbles)

• the largest structures in the universe - a riddle
• are the voids really empty or full of dark matter? - a riddle

• Energetic jets emerging from their centers

• Spirals with very energetic "star-like" centers
• Like center of Milky Way, but more energetic

• Ellipticals with very energetic centers

d) Quasars "quasi-stellar radio sources"

1. Look star-like, but give radio (synchrotron), IR, UV, X, gamma
• Some can be radio-quiet but they are still called quasars.
2. Huge redshifts (Hubble)--> large distance
3. Distance: If we see them --> huge luminosity
4. Far away --> light needs long time --> objects in early universe
5. Time changes --> energy generated in small regions
   • Can't get big object to change quickly
   • Even if it did, we'd see photons arrive at different times
   • --> Energy of 1000's of galaxies generated in the size of the solar system
6. Quasars = cores of forming galaxies?
   • Seyferts, BL Lacs = cores of formed galaxies?
     • Evidence for galaxy evolution?
   • All these objects have bright point-like centers
     • Evidence for supermassive black holes

Importance: clues to how & when galaxies form
Energy generation: a riddle
Supermassive black holes?
Is black hole formation part of galaxy formation?

Chapter 14