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 underestimated in the past, and increasingly so with redshift. The tentative value 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, which were claimed to have an ''excess'' of spiral galaxies f(sp) is similar to 50%, have in fact a normal spiral fraction. Conversely, distant 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 fraction and X-ray luminosity previously found is the result of two observational biases, namely the apparent increase of X-ray luminosity with 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 larger cluster have a low spiral fraction is thus no longer true. Consequently, galaxies do not have to change morphological type when clusters merge. This resolves the conflict between the observations and the necessity, if the correlation were real, of such a morphological evolution.