Distant clusters of galaxies detected by X-rays

The dynamical masses of groups and clusters of galaxies decrease on average with increasing redshift, after the hierarchical cosmogonies dominated by direct collapses of dark matter overdensities. We show that the masses of intracluster plasma emitting in the X-ray band are to decrease more rapidly. We also show that consideration of the intrinsic spread in the dynamical formation times leads us to predict more numerous faint sources at given dynamical mass. The model we compute yields steeper luminosity functions in the X-ray band with a specific change in lookback time: the bright end shifts back. Such negative evolution is fast even at modest redshifts z <~ 0.5 if the external gas infalling into groups of clusters was preheated and has cooled down after z ~ 1.5-2. If so, the evolution is considerably faster in the X-ray than in the optical band and compares interestingly with the data from the existing surveys.