GALAXIES

1
GALAXIES

• Visible contents of the cosmos

2
Galaxies Local or Distant?

• Angular diameters span ratio of about 100
• Angular diameter gt actual diameter to distance
  ratio large ratio gt large distances
• Clincher Cepheid variables in Andromeda galaxy
  (1922) 2 million ly!

3
Galaxies-Distances

• Distance indicators Objects whose luminosities
  estimated (cepheids/P-L main method)
• Measure flux from whole galaxy, apply
  inverse-square law for light, get a distance
  (with an error, sometimes large!)

4
Galaxies Shapes

• Elliptical Football little starbirth now
• Disk (spiral) Starbirth in disks
• Irregular No special shape lots of starbirth
• Peculiar shapes Usually involve collisions

5
Galaxies-Properties

• Actual diameter from angular diameter and
  distance
• Luminosity from flux and distance, apply
  inverse-square law for light
• Mass Rotation curve (disks) from Doppler shifts
  (Newtons version Keplers 3rd)!

6
Hubble Law

• Measure red shifts (radial velocities) and
  distances
• Find direct proportion Greater distances, larger
  red shifts (radial velocities) implies the
  universe is expanding!
• Expansion rate now about 20 km/s/Mly Hubble
  constant

7
Age of Cosmos

• Time distance/rate (rate H)
• If H 30 km/s/Mly, age about 10 billion years
  (whoops!)
• If H 20 km/s/Mly, age about 15 billion years
  (OK)
• If H 15 km/s/Mly, age about 20 billion years
  (better yet!)

8
Mass/Luminosity Ratio

• Divide mass (M in solar masses) by luminosity (L
  in solar luminosities) for sun, M/L 1 near
  sun, M/L a few
• Add mass but not luminosity (dark matter)
  increases M/L
• M/L up to 100 in galaxies what/where is the dark
  matter?

9
Clusters Superclusters

• Red shifts as proxy for distances
• Contain few hundred to few thousand galaxies
• Size up to billion light years
• M/L a few hundred (lots of dark matter)
• Superclusters largest entities in cosmos!