Galaxy Clusters

Question 1

Describe the existence of hot gas in galaxy clusters.

Answer 1

Primordial gas falls into potential well of galaxy cluster, gains bulk KE, converted into internal energy by shocks, heats gas to high temps.

Question 2

What are the types of radiation are detected coming from galaxy clusters?

Answer 2

Thermal Bremss. (cutoff v~kT/h) => bright in X-rays. Synch and IC are weak/negligible.

Question 3

How can an estimate of the temperature of a galaxy cluster be estimated?

Answer 3

KE=3/2kT. PE = -GMm/R. m~m_p/2 (plasma). Virial equilibrium => 2KE = -PE.

Question 4

Why are cluster masses one of the best probes of cosmological parameters?

Answer 4

number and evolution of very massive clusters exponentially sensitive to initial conditions and expansion of Universe.

Question 5

If ICM were primoridial, we expect fully ionised H and HE, no emission lines. Why do we see strong emission lines in X-rays (heavier elements)?

Answer 5

ICM is enriched by SN in galaxies over life of cluster. fossil records of star formation in those member galaxies -> elements in ICM give info on relative numbers of type II and Ia SN.

Question 6

How is the cooling rate of Bremss. related to density?

Answer 6

rho squared.
denser core ICM cools more rapidly - > contracts, denser, cools faster.

Question 7

What is cooling flow predicted to lead to?

Answer 7

massive star formation in central galaxy.

Question 8

What is not observed, wrt to galaxy clusters?

Answer 8

Some energy input must balance Bremss. energy loss. Energy output from AGN in jets and radiation likely responsible (bubbles in ICM, push gas out).