Chapter 2: Bremsstrahlung And Galaxy Clusters

Question 1

What is Bremsstrahlung?

Answer 1

The acceleration of a charged particle due to the Coulomb field of another charged particle

Question 2

Why do galaxy clusters satisfy the Virial Theorem?

Answer 2

The time for a sound wave to cross the cluster (~10^8 years) is less than the age of the system (~10^10 years), therefore the cluster should have settle into hydrostatic state with pressure varying with radius.

Question 3

What is the Virial Theorem?

Answer 3

2K+U=0

Question 4

Why is the mass of a galaxy cluster greater than expected?

Answer 4

The presence of intracluster gas

Question 5

How is intracluster gas heated when the cluster is first formed?

Answer 5

Through gravitational collapse

Question 6

What are the two brightest objects in the X-ray sky?

Answer 6

AGN and Galaxy Clusters

Question 7

What is the percentage breakdown of the cluster mass?

Answer 7

5% galaxies, 10-15% intracluster mass, 80-85% dark matter

Question 8

How are clusters an accurate representation of the ratio between baryonic and dark matter?

Answer 8

Deep gravitational potential wells of a cluster means essentially nothing ‘escapes’

Question 9

Describe the differences between rich and poorer clusters in terms of temperatures and line emissions

Answer 9

Rich clusters have temperatures of a few 10^7-10^8K and are dominated by thermal Brem. Poorer clusters are of a temperature ~10^7K and are dominated by line cooling

Question 10

What is the equation for total energy input by SN?

Answer 10

Total Energy in SN/Total mass of gas

Question 11

Where is the effect of SN (or AGN) feedback greatest and why?

Answer 11

Lower mass clusters. Energy input scales linearly with mass but the thermal energy scales with grav pot of cluster (M^2)

Question 12

How does Bremsstrahlung emissivity scale with number density and temp?

Answer 12

Epsilon is proportional to (T^(1/2))*n^2

Question 13

How does cooling time scale with T and n_e?

Answer 13

t_cool = E_TH/epsilon which is proportional to (T^(1/2))/n_e

Question 14

What is cooling flow?

Answer 14

Gas in central regions cools, there is a drop in pressure and so matter flows inwards from outside the cooling region to maintain hydrostatic equilibrium.

Question 15

How does cooling flow lead to star formation in the simplest models?

Answer 15

Gas in a cooling flow can be thought of as an emulsion of gas clouds with different densities and temperature. These clouds slip slowly down grav pot of cluster with densest clouds cooling fastest. Eventually lose all E_TH and form small, cold molecular clouds in which stars can form.

Question 16

Why are emission lines of some ions in the cluster eg FeXVII much weaker than expected?

Answer 16

Current leading explanation is heat input from a central AGN. Balance between heating and cooling. Whilst it cools to 30% of its Virial temp, at most 10% cools further.

Question 17

Why do we treat the ion as fixed in Brem?

Answer 17

It has much larger inertia relative to an electron.

Question 18

Why do we expect the dominant emission mechanism for gas in clusters of galaxies to radiate at X-Ray energies?

Answer 18

Use equipartition and find that the gas has temp kT~mv^2. Assuming intracluster gas is moving at similar velocity to the galaxies then temperature is 10^7-10^8K. At these temps the gas will be ionised, and the free electrons will produce Bremsstrahlung emission. Cutoff at hbarw~kT which corresponds to X-ray freqs