The optical spectra of central galaxies in southern clusters: evidence for star formation

Abstract

We have observed 21 elliptical galaxies, of which nine are at the centre of clusters with cooling flows, and find that 12 have extended optical line emission. The spectra show line ratios indicating low ionization and are qualitatively similar to those observed in other cooling flows. A problem with the production of the large observed line luminosities is highlighted and explained by some ongoing star formation with the initial mass function of a disc galaxy. The rate of such star formation is as high as <f>$100{M}_{\\odot }\\text{yr}^{ \\overline{~}1}$</f> in the case of PKS 0745 – 191, and in all the galaxies there is quantitative agreement between the H<it>β</it> line flux and the 4000-Å break measured from our blue spectra. Most of the gas (more than 90 per cent) must cool from X-ray temperatures and form low-mass objects without producing any detectable optical line radiation or hot young stars.