Star Formation in Cooling Flows (Invited Paper)

Abstract

AbstractX-ray data show that substantial quantities of hot gas are cooling near the centres of many clusters and groups of galaxies. The existence of such cooling flows has been challenged because of the lack of evidence for star formation from the cooled gas. Spectra of cooling flow galaxies show filling in of the continuum shortward of the break at 4000 Å relative to normal elliptical galaxies. This is consistent with some continuing star formation. Extended regions of line emission are commonly associated with cooling flows. If the initial-mass-function of the newly formed stars which affect the 4000 Å break is like that which applies in the solar neighbourhood, then these stars can also power the line emission. The strength of the 4000 Å break is shown to correlate with the Hβ flux in the manner expected when this is the case. This allows us to esimate the star formation rate from the line luminosity.The rate of star formation required to account for the line emission still falls well short of the rate at which gas is inferred to be cooling. It is argued that, nevertheless, the cooling gas is probably forming into stars. The overall initial-mass-function must be different from that which applies in the solar neighbourhood, but this should not be surprising given the different ambient conditions in a cooling flow.