Throughout this chapter, an attempt has been made to relate observable features of globular cluster systems (kinematics, metallicity distributions, luminosity and mass distributions, specific frequencies) to the competing hypotheses for galaxy formation: in situ, mergers, or accretions. Where do we stand, at the current stage of development of the subject?
Each of these models in its nominal form represents an extreme view of the formation process, and each has merits. The in situ model comes in two basic varieties: the Searle/Zinn (1978) picture whereby a large galaxy is assembled over a relatively long period of time (perhaps several Gyr) from many small, dwarf-sized gas clouds; and the monolithic collapse picture of Eggen, Lynden-Bell, & Sandage (1962). The arguments that are presented to contrast, and choose between, these two extremes now involve a far greater body of evidence than was available when these key papers were written. For example, the full complexity of behavior of the globular cluster systems in these galaxies - the large ranges in metallicity, the existence of two or more distinct subpopulations in most large galaxies, the kinematical subgroups, and the evidence for late infall and accretion - clearly favors the SZ scheme, especially for their outer halos. The ELS picture may still have relevance for describing simpler, smaller systems such as nucleated dwarf ellipticals or the central bulges of larger galaxies, in which the principal formation epoch may have been dominated by rapid dissipation and collapse. Nevertheless, for most large galaxies, elliptical or spiral, it seems necessary to invoke at least two major formation stages. Surprisingly, we may need only two stages for at least some gE galaxies if we interpret the two-component metallicity distributions in the simplest possbile way.
The basic accretion picture would start with a large initial galaxy which had already formed through an in situ process. As described above, we then add a sequence of smaller galaxies to it and thus build up the metal-poor halo component. Here also, we can invoke two basic varieties: If the accreted objects are gas-free, then we would build up a larger galaxy with a normal specific frequency and a broad MDF, but with a halo that should have an average metallicity that is intermediate or moderately low. If, however, the accreted smaller galaxies are very gas-rich, then new clusters and halo stars can form in the process and (possibly) change the GCS specific frequency and weight the MDF increasingly toward the metal-rich end. However, in a rather direct sense, this latter alternative could be regarded as a version of the generic SZ picture.
Lastly, the merger approach involves the amalgamation of pre-existing galaxies of roughly equal size. The result, almost invariably, should be an elliptical of low specific frequency if the merging objects are pre-existing disk galaxies. Once again, the presence or absence of gas will play an important role in the outcome. If the mergers are taking place at high redshift (that is, at very early times), then we can expect the progenitors to be largely gaseous, in which case the majority of stars in the merged product would actually form during the merger of gas clouds. This, too, can be viewed as an extension of the basic SZ galaxy formation picture. On the other hand, if the merging is happening at low redshift nearer to the present day, then the galaxies are much more likely to have smaller amounts of gas, much less star formation can happen during the merger, and the result will be a low-SN elliptical such as we see in small groups.
No one of these three approaches can be taken as the answer
to all cases. Instead, we need all of them to tell the complete story.
The evidence is plain that mergers and accretions are happening
today and must be part of the ongoing construction of large
galaxies. Similarly, in situ formation at very early times
is clearly an essential part of the history of giant ellipticals
and BCGs. In their full range of possibilities, these
formation pictures make up a continuum of processes which can be
seen to operate in the real world, and thus the competition
among them which often appears in the literature is more than
a little artificial. The challenge we face is, for any one
galaxy, to identify the particular set of processes which has
ended up dominating its present-day structure.
For practical help and many conversations on these topics, I am grateful to Dean McLaughlin, J. J. Kavelaars, and Gretchen Harris.