Unequal-mass galaxy mergers and the creation of cluster S0 galaxies

It is a longstanding and remarkable problem when and how red S0 galaxies were formed in clusters of galaxies. We here propose that the major mechanism for the S0 creation is galaxy merging between two spirals with unequal mass. Our numerical simulations demonstrate that galaxy merging exhausts a large amount of interstellar medium of two gas-rich spirals owing to the moderately enhanced star formation, and subsequently transforms the two into one gas-poor S0 galaxy with structure and kinematics strikingly similar to the observed ones. This secondary S0 formation with enhanced star formation explains a smaller fraction of S0 population recently observed in some distant clusters of galaxies. Unequal-mass galaxy mergers thus provide an evolutionary link between a larger number of blue spirals observed in intermediate redshift clusters and red S0s prevalent in the present-day ones.