7.2-4

Question 1

Local Group Galaxies

how many, properties, examples

Answer 1

Over 50 galaxies, mostly low luminosity and low metallicity.

Dominant galaxies:

• MW
• Andromeda (M31)
• Triangulum (M33)

Question 2

Galaxy Interactions

Answer 2

• Common in galaxy clusters
• Affect evolution
• Types:
  – small-big
  – big-big
  – one-many

Question 3

Possible Galactic Collision

Answer 3

Andromeda and MW

• May collide in ≈ 4 Gyr
• Current separation 0.74 Mpc
• Relative velocity: ≈ -110 km/s

Question 4

Brightest Local Group Galaxies

Answer 4

• Milky Way (SBb): -20.8 mag ≈ -21 mag
• Andromeda (M31, Sb): -21.6 mag ≈ -22 mag
• Triangulum (M33, Sb): -18.9 mag ≈ -19 mag

Question 5

Galaxy Interaction Effects

Answer 5

Includes:

• Tidal distortion
• Enhanced spiral structure
• Triggered star formation

Question 6

AGN Luminosity

for both types

Answer 6

• Seyfert galaxies: ~ 10^37 W ≈ 10¹¹ L_⊙
• Quasars: ~ 10^39 W ≈ 10¹³ L_⊙

Question 7

Eddington Luminosity Limit

formula + two examples

Answer 7

L_E = 10^4.5 M/M_⊙ · L_⊙

Mass for stability:

• > 10^6.5 M_⊙ for Seyfert galaxy
• > 10^8.5 M_⊙ for quasar

Question 8

Black Hole Accretion

what is the efficiency of the energy production?

Answer 8

Energy production in accretion disk. Efficiency parameter:

η < 10%

Question 9

Eddington Accretion Rate

example numbers for 10⁸ M_⊙

Answer 9

For 10⁸ M_⊙ black hole:

• ~ 2 M_⊙/yr
• Luminosity ~ 10^12.5 L_⊙