THE CAMBRIDGE-CAMBRIDGE ROSAT SERENDIPITY SURVEY .1. X-RAY-LUMINOUS GALAXIES

We report on the first results obtained from a new optical identificat ion programme of 123 faint X-ray sources with S(0.5-2 keV) > 2 x 10(-1 4) erg s(-1) cm(-2) serendipitously detected in ROSAT PSPC pointed obs ervations. We have spectroscopically identified the optical counterpar ts to more than 100 sources in this survey. Although the majority of t he sample (68 objects) are QSOs, we have also identified 12 narrow emi ssion line galaxies which have extreme X-ray luminosities (10(42) < L( X) < 10(43.5) erg s(-1)). Subsequent spectroscopy reveals them to be a mixture of starburst galaxies and Seyfert 2 galaxies in approximately equal numbers, Combined with potentially similar objects identified i n the Einstein Extended Medium Sensitivity Survey, these X-ray-luminou s galaxies exhibit a rate of cosmological evolution, L(X) proportional to (1 + z)(2.5+/-1.0), consistent with that derived for X-ray QSOs, T his evolution, coupled with the steep slope determined for the faint e nd of the X-ray luminosity function (Phi(L(X)) proportional to L(X)(-1 .9)), implies that such objects could comprise 15-35 per cent of the s oft (1-2 keV) X-ray background.