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

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1. INTRODUCTION

When the first review paper on this topic was written (108), it was just beginning to emerge as a distinct observational discipline. A dozen years later, the study of globular cluster systems in other galaxies has proliferated in several directions, given tremendous impetus in the mid-1980s by the widespread deployment of CCD cameras, and by the achievement of routine subarcsecond seeing quality at observatories such as the CFHT.

A globular cluster system (GCS) will be defined here as the ensemble of all the old star clusters (tau gtapprox 10 Gy) found in the spheroid and halo regions of a given galaxy. Globular clusters are likely to be the oldest directly visible stellar entities, and virtually all large galaxies appear to contain them. This paper will emphasize the GCSs in galaxies beyond the Local Group, their global properties, and their use as tracers for galactic structure and galaxy formation. Previous reviews of GCS properties have usually concentrated on the Local Group members, with brief additions for the Virgo galaxies and a few other systems. Today, the wealth of new data makes such an approach unmanageable (this review may well be the last one to survey GCSs in all types of galaxies). Excellent recent papers on the systematics of globular clusters in the various Local Group members include Zinn (225) and Armandroff (5) for the Milky Way; Elson & Walterbos (54), Fusi Pecci (69), and Battistini et al. (8) for M31; Christian & Schommer (37) for M33; Fusi Pecci (68) and Da Costa & Mould (45) for the dwarf ellipticals; and Westerlund (220) and van den Bergh (213) for a guide to the vast literature on the Magellanic Cloud clusters.

This review deliberately emphasize the results of the past decade. For a more comprehensive discussion of earlier material, the reader is urged to see Harris & Racine (108), Hanes (80), and additional comments in Harris (98, 99). A review by Hesser (115) places the study of GCSs within the broader context of globular cluster research; another overview by Racine (171) should also be seen for its identification of many key issues that remain surprisingly valid today.

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