The ROSAT deep cluster survey: The X-ray luminosity function out to z = 0.8

We present the X-ray luminosity function (XLF) of the ROSAT Deep Cluster Survey sample over the redshift range 0.05-0.8. Our results are derived from a complete flux-limited subsample of 70 galaxy clusters, representing the brightest half of the total sample, which have been spectroscopically identified down to the flux limit of 4 x 10(-14) ergs m(-2) s(-1) (0.5-2.0 keV) and have been selected via a serendipitous search in ROSAT PSPC pointed observations. The redshift baseline is large enough that evolutionary effects can be studied within the sample. The local XLF (z less than or equal to 0.25) is found to be in excellent agreement with previous determinations using ROSAT All-Sky Survey data. The XLF at higher redshifts, when combined with the deepest number counts constructed to date (f > 2 x 10(-14) ergs cm(-2) s(-1)), reveals no significant evolution at least out to z = 0.8, over a luminosity range of 2 x 10(42) to 3 x 10(44) ergs s(-1) in the 0.5-2 keV band. These findings extend the study of cluster evolution to the highest redshifts and the faintest fluxes probed so far in X-ray surveys. They complement and do not necessarily conflict with those of the Einstein Extended Medium-Sensitivity Survey, leaving open the possibility of negative evolution of the brightest end of the XLF at high redshifts.