Introduction

• X-ray images of the cores of many clusters of galaxies show the emission to be strongly peaked.

• The radiative cooling time of the X-ray emitting gas within the inner 100-200 kpc is less than a Hubble time. It decreases inward so that it is less than a billion years within the inner few tens of kpc.

• This region where the gas loses much of its thermal energy to radiation forms a cooling flow.

• X-ray spectra from the Einstein Observatory SSS and ROSAT PSPC measure characteristic temperatures of 1-2 keV in the cores of some clusters, significantly lower than the mean temperature of the cluster.

• The Einstein Observatory FPCS found evidence in the core of the Perseus cluster for gas at a characteristic temperature of ~0.5 keV through observations of OVIII and Fe XVII lines.

• These spectroscopic observations show that gas does cool as predicted. The mass accretion rate from this cooling may be several hundred Solar masses per year in a large flow.

• The detection in cooling flows of X-ray absorption in excess of that from the Galaxy suggests that some of the cooled gas remains as cold clouds before forming any stars, which from optical constraints must be of low mass.