Building upon a suggestion by Schweizer (1987), Ashman & Zepf (1992) examined the possibility that a factor-of-two difference in specific frequency, SN, between normal elliptical and spiral galaxies (e.g., Harris 1991) might be explained if: (1) normal elliptical galaxies formed as a result of major mergers of disk galaxies, and (2) the merger process led to the formation of new, presumably MR, GCs. There seems to be little doubt that a process of this sort has played in role in the formation of some elliptical galaxies, in light of the detection of young clusters forming in interacting galaxies (e.g., Whitmore et al. 2001). But is it plausible that major mergers of disk-galaxy pairs is the generic mechanism for the formation of normal elliptical galaxies, more than half of which show bimodal GC MDFs (e.g., Larsen et al. 2001)?
The most comprehensive study of GC formation in disk mergers is that of Bekki et al. (2002), who used an N-body/TREESPH code to investigate mergers of pairs of gas-rich disk galaxies. These disks were posited to contain a pre-existing population of exclusively MP GCs. Using a simple pressure criterion for the onset of GC formation, Bekki et al. showed that the end-products of such mergers do indeed resemble elliptical galaxies, with distinct MP and MR GC subpopulations. The newly-formed MR GCs show a greater degree of concentration toward the center of the merger remnant, as is observed. Bekki et al. also examined the kinematic properties of the MP and MR GC subpopulations, finding the MP component to show a large velocity dispersion and a net rotation; by contrast, the MR component was found to show a lower dispersion and modest rotation. By examining the GC MDFs of the merged galaxy, Bekki et al. also noted that the mean metallicity of the MR GCs far exceeded those measured for MR GCs in real ellipticals. As no plausible change in the assumed range of gas metallicities, abundance gradients in the progenitor disks, or GC formation criteria was able to alter this basic result, they concluded that "if most elliptical galaxies are formed by major mergers, then it must have occurred at high redshift".