Unequal-Mass Galaxy Mergers and the Creation of Cluster S0 Galaxies

It is a longstanding and remarkable problem when and how red S0 galaxies wereformed in clusters of galaxies. We here propose that the major mechanism for the S0 creation is galaxymerging between two spirals with unequal mass. Our numerical simulations demonstrate that galaxy merging exhausts a largeamount of interstellar medium of two gas-rich spirals owing to the moderatelyenhanced star formation, and subsequently transforms the two into one gas-poorS0 galaxy with structure and kinematics strikingly similar to the observedones. This secondary S0 formation with enhanced star formation explains a smallerfraction of S0 population recently observed in some distant clusters ofgalaxies. Unequal-mass galaxy mergers thus provide an evolutionary link between alarger number of blue spirals observed in intermediate redshift clusters andred S0s prevalent in the present-day ones.Comment: 14 pages 3 figures (ps file), ApJL in pres