The evolution of the galactic morphological types in clusters

The morphological types of galaxies in nine clusters in the redshift range 0.1 less than or similar to z less than or similar to 0.25 are derived from very good seeing images taken at the NOT and the La Silla-Danish telescope s, with all galaxies at M-V < -20 and within the central <similar to> 1 Mpc (2) area being classified. With the purpose of investigating the evolution of the fraction of different morphological types with redshift, we compare our results with the morphological content of nine distant clusters studied by the MORPHS group, five clusters observed with HST/WFPC2 at redshift z = 0.2-0.3, and Dressier's large sample of nearby clusters. After having chec ked the reliability of our morphological classification both in an absolute sense and relative to the MORPHS scheme, we analyze the relative occurrenc e of elliptical, S0, and spiral galaxies as a function of the cluster prope rties and redshift. We find a large intrinsic scatter in the S0/E ratio, mo stly related to the cluster morphology. In particular, in our cluster sampl e, clusters with a high concentration of ellipticals display a low S0/E rat io and, vice versa, low concentration clusters have a high S0/E. At the sam e time, the trend of the morphological fractions (%E's, %S0's, %Sp's) and o f the S0/E and S0/Sp ratios with redshift clearly points to a morphological evolution: as the redshift decreases, the S0 population tends to grow at t he expense of the spiral population, whereas the frequency of E's remains a lmost constant. We also analyze the morphology-density (MD) relation in our dusters and find that-similarly to higher redshift clusters-a good MD rela tion exists in the high-concentration clusters, while it is absent in the l ess concentrated clusters. Finally, the comparison of the MD relation in ou r clusters with that of the MORPHS sample suggests that the transformation of spirals into S0 galaxies becomes more efficient with decreasing local de nsity.