Cooling Flows

• Occur in both clusters of galaxies and individual galaxies.

• When the cooling time of gas (with T=temp. and n=density) is shorter than the Hubble time, or the time since the last major merger of the system, a cooling flow will be set up.

• In cooling flow clusters, large amounts of gas (~100s Msun/yr) are cooling radiatively – this happens first in the center where the gas is most dense, then outer gas flows in to maintain hydrostatic equilibrium.