7.6

Question 1

Clusters of Galaxies (3)

not the types but general properties

Answer 1

• 100 - 10’000 members
• gravitationally bound
• dominated by giant elliptical galaxies in the center

Question 2

Regular Clusters

Answer 2

• Well-defined structure
• Central big ellipticals
• > 1000 galaxies
• Example: Coma cluster
  – d = 100 Mpc
  – R ≈ 3 Mpc

Question 3

Irregular Clusters

Answer 3

• Center often not well defined
• Galaxies irregularly distributed
• Example, Virgo cluster:
  – d = 16.5 Mpc
  – R ≈ 2.2 Mpc
  – 1300 galaxies

Question 4

M/L Ratio for Galaxy Clusters

Answer 4

M/L ≈ 100 M_☉/L_☉, cluster of galaxies contain a lot of dark matter

Question 5

Hot X-ray Emitting Gas (4)

process, temperature, volume, luminosity

Answer 5

Xrays telescopes detect hot gas in all clusters of galaxies
- Emission process is bremsstrahlung
- acceleration of charged particles -> EM radiation
- Xray emission requires T ≈ hv/kT
-> X-ray emission requires T ≈ 10⁸ K
- Typical volume ≈ 1 Mpc³
- Luminosity ≈ 10³⁷ W (= 2.5 · 10^10 L_⊙)
- very low luminosity and very long cooling time

Question 6

Origin of the Gas

in galaxies and its qualitative properties

Answer 6

• M_gas > M_stars
• Metallicity half solar
• Processed gas comes from infalling spirals

not sure what is meant with metalicity half solar

Question 7

Dark Matter and Gravitational Lensing

i think a more appropriate title would be: dark matter in galaxies

Answer 7

Visible mass only ≈ 10 - 30% of the mass derived from the virial theorem

Question 8

Gravitational Lensing

Answer 8

Independent method to derive the mass of an object, formula for deflection angle:

Δϕ = 4GM/(c²R)

Question 9

Einstein Ring and Angle

Answer 9

• r_E = sqrt((4GM/c²)(D_LSD_L/D_S))
• θ_E = sqrt((4GM/c²)(D_LS/(D_LD_S)))

i think i will learn at most that θ ∝ √M