Formation of shells in major mergers

Numerical simulations are used to study the fate of disks in mergers between equal-mass galaxies. Contrary to popular belief, mergers between equal-mass galaxies can form shells, loops, and ripples. Material that was originally in the outer disk of the pre-merger spirals falls into the remnant late in the merger event, long after the inner region of the remnant has relaxed. Thus, its evolution is similar to that of an accreted dwarf satellite that forms shells through "phase wrapping." However, the mechanism described in this letter avoids a number of difficulties with the accretion model; specifically, it can explain the observed correlation between properties of the host galaxies and shell alignments and luminosities. In view of this, we argue that many shell systems may have originated through "major" mergers of comparable-mass galaxies rather than via "minor" mergers or accretions. In particular, the shell elliptical NGC 3923 appears to be a good candidate for shell formation by a major merger.