X-RAY LUMINOSITY AND SPIRAL FRACTION OF NEARBY CLUSTERS OF GALAXIES -ASTROPHYSICAL CONSEQUENCES OF AN OBSERVATIONAL BIAS

The spiral fraction in nearby clusters (z < 0.05) has been underestima ted in the past, and increasingly so with redshift. The tentative valu e of spiral fraction with zero bias is about 50-60%. Distant clusters (z is similar to 0.4) such as 3C295, cl 0024+1645 and cl 0939+4713, wh ich were claimed to have an ''excess'' of spiral galaxies f(sp) is sim ilar to 50%, have in fact a normal spiral fraction. Conversely, distan t clusters such as cl 0016+16 and II Zw 1305.4+2941 which were claimed to have a ''normal'' spiral content f(sp) is similar to 0-10% have in reality a low spiral fraction. The tight correlation between spiral f raction and X-ray luminosity previously found is the result of two obs ervational biases, namely the apparent increase of X-ray luminosity wi th redshift, due to the Malmquist bias, and the simultaneous decrease of the observed spiral fraction. The interpretation that smaller (i.e. low X-ray luminosity) clusters have a high spiral fraction and that l arger cluster have a low spiral fraction is thus no longer true. Conse quently, galaxies do not have to change morphological type when cluste rs merge. This resolves the conflict between the observations and the necessity, if the correlation were real, of such a morphological evolu tion.