In both M87 and M49, the efficiency of GC formation is found to have
the familiar "universal" value of
tot
0.25%
per unit baryon mass
(McLaughlin 1999).
In both galaxies, the
formation efficiencies of the separate MP and MR components is
roughly half that of the GC system as a whole. These findings
provide a challenge to models in which MR GCs form exclusively in
major mergers, and to models that require different
formation efficiencies for the MP and MR GCs.
Kinematic studies of the GC subpopulations in M87 and M49 reveal some interesting differences. In M87, both GC subpopulations show rapid rotation; in M49, the MP GCs show modest rotation, while the MR GCs form a non-rotating population. The existence of rapidly rotating GC subpopulations in these galaxies, and in the Milky Way and M31, suggests that mergers have probably played a role in the formation of the host galaxies, as the amount of angular momentum involved seems to exceed that generated by tidal torques alone (see Côté et al. 2001). With the exception of M49, the rapidly rotating MR GCs in these galaxies indicate that the formation of the MR GCs probably predated the mergers. Furthermore, the ages of the GC subpopulations in M87 and M49 suggest that if the mergers were predominantly dissipational in nature, then the mergers must have occurred at high redshift, as noted by Bekki et al. (2002) from an analysis of simulated and observed GC MDFs.
Many properties of the MP GCs in M87 and M49 (and in other galaxies) are consistent with the predictions of models in which these GCs form in low-mass, proto-galactic fragments that are later accreted into the main body of the galaxy. These proto-galactic fragments appear to be the analogs of the low-mass subhalos that form in cosmological simulations of structure formation. Indeed, the literature abounds with terminology for these objects: e.g., proto-galactic fragments, supergiant molecular clouds, proto-galactic disks, Searle-Zinn fragments, failed dwarfs, etc. In terms of their physical properties, however, these are the same objects.
While there may be an emerging consensus on the origin of the MP GCs, debate continues on the nature and origin of their MR counterparts. These clusters can be interpreted simply as those that formed in the most massive proto-galactic fragments, or as the endproducts of a star formation burst triggered by gas-rich mergers. The former interpretation has the æsthetic advantage of requiring a single mechanism to explain both GC populations, while the second scenario has strong support from observations of young GCs forming in local mergers. It is likely that both processes have played a part in shaping the GC systems that we observe today, and our task for the decade ahead is to determine which, if either, of these processes has played a dominant role.
I thank Judy Cohen, John Blakeslee, Andres Jordán, Ron Marzke, Dean McLaughlin and Mike West for allowing me to present preliminary results from our ongoing collaborations.