OPTICAL SPECTROSCOPY OF THE ROSAT X-RAY BRIGHTEST CLUSTERS

We present the results from the first stage of an optical follow-up study of the X-ray brightest clusters of galaxies detected in the ROSAT All-Sky Survey (RASS). The redshifts of the central galaxies in 29 of the X-ray brightest Abell and Zwicky clusters in the RASS have been measured using the Faint Object Spectrograph on the Isaac Newton Telescope. Approximately 40 per cent of the central galaxy spectra obtained show strong optical line emission. In several cases this emission is quite spectacular. The central cluster galaxy in Zwicky 3146 is the most luminous such galaxy in optical lines yet discovered, having an Halpha luminosity approximately twice that of NGC 1275. We re-examine the link between optical line emission and the presence of an excess of blue continuum flux in the spectra and find that the two are correlated. The spectral shape of the excess flux is well matched by B stars.