Galaxy mergers and the creation of cluster S0 galaxies

When and how red S0 galaxies were formed is a longstanding and noteworthy p roblem. Recent morphological and photometric studies of S0s in distant clus ters of galaxies have revealed a smaller S0 population fraction and the exi stence of S0s with bluer colours, which suggests that some physical process es drive continuous creation of S0s with younger stellar populations in hig her-redshift clusters. We propose here that the major mechanism for S0 crea tion is galaxy merging between two spirals of unequal masses. Our numerical simulations demonstrate that galaxy merging exhausts a large amount of the interstellar medium of two gas-rich spirals owing to the moderately enhanc ed star formation, and subsequently transforms the two into a single gas-po or S0 galaxy with structure and kinematics strikingly similar to those obse rved. This secondary S0 formation via unequal-mass merging thus provides an evolutionary link between a larger number of blue spirals observed in inte rmediate-redshift clusters and the red S0s prevalent in present-day ones.