Cooling flows in clusters of galaxies

Cooling gas in the centre of many clusters of galaxies is compressed and pushed inward by the thermal pressure of the hot, uncooled outer gas. Gravity acts as a focusing agent until the gas has cooled to galactic temperatures. The cooling process is thermally unstable, giving rise to optical filamentation. Once the gas cools below 10⁴ K it presumably forms stars which add to the mass of a central galaxy. Accretion rates of up to 400 M_solar yr^-1 are inferred from X-ray measurements so that the mass of such central galaxies may be due to cooling flows. Their optical and UV appearance emphasize that the gas predominantly turns into low-mass stars with an initial mass function unlike that of the solar neighbourhood. Optical and X-ray observations of cooling flows can be used to investigate the continuing formation of the largest known galaxies.