Chapter 2: Bremsstrahlung And Galaxy Clusters Flashcards
What is Bremsstrahlung?
The acceleration of a charged particle due to the Coulomb field of another charged particle
Why do galaxy clusters satisfy the Virial Theorem?
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.
What is the Virial Theorem?
2K+U=0
Why is the mass of a galaxy cluster greater than expected?
The presence of intracluster gas
How is intracluster gas heated when the cluster is first formed?
Through gravitational collapse
What are the two brightest objects in the X-ray sky?
AGN and Galaxy Clusters
What is the percentage breakdown of the cluster mass?
5% galaxies, 10-15% intracluster mass, 80-85% dark matter
How are clusters an accurate representation of the ratio between baryonic and dark matter?
Deep gravitational potential wells of a cluster means essentially nothing ‘escapes’
Describe the differences between rich and poorer clusters in terms of temperatures and line emissions
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
What is the equation for total energy input by SN?
Total Energy in SN/Total mass of gas
Where is the effect of SN (or AGN) feedback greatest and why?
Lower mass clusters. Energy input scales linearly with mass but the thermal energy scales with grav pot of cluster (M^2)
How does Bremsstrahlung emissivity scale with number density and temp?
Epsilon is proportional to (T^(1/2))*n^2
How does cooling time scale with T and n_e?
t_cool = E_TH/epsilon which is proportional to (T^(1/2))/n_e
What is cooling flow?
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.
How does cooling flow lead to star formation in the simplest models?
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.
Why are emission lines of some ions in the cluster eg FeXVII much weaker than expected?
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.
Why do we treat the ion as fixed in Brem?
It has much larger inertia relative to an electron.
Why do we expect the dominant emission mechanism for gas in clusters of galaxies to radiate at X-Ray energies?
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
