5.5 Local Group of Galaxies

Question 1

Describe the local group of galaxies

Answer 1

Contains 80+ galaxies (mostly dwarfs)

1. 5Mpc radius with 10E12 solar masses
   - Edge of Virgo cluster

Question 2

Describe the distribution of galaxies in our universe

Answer 2

Not uniform

- Groups of 10-100 or clusters of 100-1000 or super clusters

Question 3

What are the 4 quantities that define the classification of galaxies

Answer 3

Virial mass, richness (how many galaxies), distance and radius

Question 4

What is the virial mass?

Answer 4

The mass including all objects subject to gravity

Stars, gas, dust, BHs, DM. Much greater than the mass of stars due to DM

Question 5

When can clusters of galaxies be included in the virial mass?

Answer 5

When the gravitational interactions have reached equilibrium and energy has been redistributed

Question 6

Describe the intra cluster medium

Answer 6

Hot ionised gas surrounding galaxies

• Driven away by SNe or AGN (winds/rad pressure)
• Peak emission at 1.5nm

Question 7

What is responsible for the x-ray emission from the ICM?

Answer 7

Thermal bremsstrahlung

- A free electron is deflected by an ion, losing KE

Question 8

What is the galaxy type for the densest regions of clusters, and the most massive galaxies?

Answer 8

Both elliptical

Question 9

What type of galaxy is most commonly observed at local group densities?

Answer 9

Spirals or irregulars

Question 10

Describe RAM pressure stripping

Answer 10

Gas inside the galaxy feels the pressure of the ICM gas and lags behind motion

• Eventually all gas is lost altering star formation
• End point of galaxy evolution - elliptical

Question 11

Describe what happens when two galaxies have a close encounter

Answer 11

Stars don’t collide

• Spiral arms stretch and distort -> irregular galaxies
• Harassment -> exchange of stars and mixing of popoulations
• Can get mergers (high velocity) or bound galaxies (low)
• Can get SFR from gas compression