Annu. Rev. Astron. Astrophys. 1991. 29: 543-79
Copyright © 1991 by . All rights reserved

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8. SUMMARY REMARKS

Several major themes have emerged from the preceding discussion:

1. In most or all galaxies, globular clusters are distinctly more metal-poor, by Delta[Fe/H] ~ -0.5, than the spheroid-population field stars.
2. Both the average and range of cluster metallicity increase with galaxy size. These correlations parallel the ones for the metallicities of the galaxies themselves, and support the view that similar enrichment processes generated both types of halo subsystems.
3. Globular clusters in all galaxies have similar, though not identical, luminosity distributions. For distance scale purposes, the calibrations of GCLFs are not yet adequate for use as high-precision (± 0.2-mag) standard candles. The present data are, however, sufficient to exert strong theoretical constraints favoring a universal cluster formation process insensitive to metallicity and with only modest later influences from dynamical evolution in most of the halo.
4. In giant ellipticals, GCSs are often (but not always) more spatially extended than the halo light. A few have been shown to have higher velocity dispersions as well, and thus to form a dynamically different halo population than the spheroid stars.
5. In most large galaxies, the inner ~ 1-2 kpc of their spheroids have probably been almost totally depopulated of globular clusters through many dynamical mechanisms. At larger Rgc, the effectiveness of these mechanisms falls off rapidly, leaving only gradual erosive processes. It is not yet clear if these processes act much differently in practice for disk galaxies as opposed to ellipticals.
6. In today's universe, few globular clusters are being formed (that is, the formation of dense clusters with a characteristic mass ~ 105-6 Msmsun is extremely rare). Though there is no reason to believe that the processes of star cluster formation 15 Gy ago were fundamentally different than today, the prevailing physical conditions of the protocluster gas then clearly favored more massive objects. The formation of globular clusters was an early, but secondary, process (that is, clearly associated with their surrounding protogalaxies).
7. Arguments based on GCS spatial distributions, metallicity distributions, and dynamics suggest that the high-SN giant ellipticals such as in Virgo and Fornax did not form by mergers of disk galaxies or dwarfs. The merger scenario is, however, more viable for E galaxies in sparse environments (smaller groups and the field) and perhaps for some cD galaxies.

How else may we use GCSs to understand the formation of galaxies? On the observational side, we need to fill in the many areas that are presently sketched out only with broad brush strokes:

1. The metallicity distribution of GCs has proven to be an effective touchstone of interpretive models. We need to accumulate spectra of clusters in a wider variety of galaxies, combined with multicolor photometry in metallicity-sensitive indices.
2. Luminosity function work has just begun to be exploited. For example, the luminosity distributions of clusters within Rgc approx 3 kpc in giant galaxies should carry the strongest imprint of dynamical evolution; direct observations could be straightforwardly made in many galaxies. Larger samples of clusters taken from the rich Virgo and Fornax systems will reveal fine structure in the GCLF and provide essential constraints on eventual theoretical modelling. And deep photometry of clusters in additional near-field galaxies should finally tell us how accurate GCLFs can be as distance indicators. In addition, the brightest globulars in giant E galaxies may prove to be useful long-range standard candles.
3. It is possible that the globular clusters in central giant ellipticals such as M87 are the oldest visible objects in the universe. High-S/N spectra of them, compared with integrated spectra of Milky Way globulars and fitted with population synthesis codes, may lead to useful age determinations relative to the Milky Way halo and to stronger limits on the Hubble time.
4. Modern spectroscopic and imaging techniques are finally putting a complete and accurate understanding of the important M31 cluster system within reach.
5. The advent of large-format CCD arrays will enable us to study the large-scale structures of globular cluster systems far more quantitatively and accurately.
6. Comprehensive radial velocity surveys of GCSs can place unique limits on the large-scale mass distribution of galaxies, and on the orbital characteristics of the halo clusters. For GCSs at or beyond Virgo-like distances, the velocity measurements do press the limits of current technology, but this field will be a rich mine for the new generation of 8-meter-class telescopes to explore.

On the theoretical side, recommendations for future work may be easy to prescribe but will be hard to execute. A formation model specific enough to predict an initial cluster mass spectrum as a function of density and metallicity would be a major achievement. The dynamical evolution of GCSs within galaxies of different types also needs to be modelled more comprehensively, with the eventual goal of predicting the full evolution of the GCLF as a function of parent galaxy type and galactocentric distance.

Because they are virtually the only remaining witnesses to the long-vanished first epoch of galaxy formation, globular clusters stand among the most powerful cosmological probes that we have. Although many intriguing new results and questions have emerged from the observational work of the past decade, we have also confirmed that globular cluster systems resemble each other rather closely. Thus by extending our study of these remarkable objects, we are uncovering a common theme in the earliest history of the galaxies.

ACKNOWLEDGMENTS

It is a pleasure to give credit for projects, conversations, and ideas generated together over the years to many colleagues and friends, including David Hanes, Gretchen Harris, Hugh Harris, Jim Hesser, Chris Pritchet, Sidney van den Bergh, Malcolm Smith, Michael Fall, and Richard Larson. The healthy state of our field today owes a great deal especially to the vision of René Racine, who in the 1970s first set in motion much of the work discussed above. I am pleased to acknowledge the hospitality of Kitt Peak National Observatory, and D. and M. Gehret of the Orinda IMAC, as well as financial support from the Natural Sciences and Engineering Research Council of Canada.

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