Annu. Rev. Astron. Astrophys. 1989. 27: 139-59 Copyright © 1989 by Annual Reviews. All rights reserved

4.3 Globular Clusters

The presence of true globular clusters can be used as an indicator that a galaxy has at least some 15-Gyr-old stars. The problem, however, is to identify the clusters as true globular clusters, as a glance at the checkered history of our knowledge of the globular cluster population of the Magellanic Clouds will quickly show. For unresolved clusters, we must rely on colors, structure, and spectra. Use of UBV colors alone is not sufficient, as the age relations for this color system are double valued at certain colors. Because of the effect of the horizontal branch, which almost mimics the effect of a hotter main sequence turnoff, it is difficult to tell a 15-Gyr- old cluster from a 0.8-Gyr-old one in the Magellanic Clouds. The addition of another color, such as R, can help.

Table 1 gives the results from the application of these criteria to Local Group galaxies. Only 16 have been demonstrated so far to have a population of 15-Gyr-old stars. It is likely, on the basis of less stringent criteria such as the presence of an envelope of faint red stars, that the other members also contain at least a few very old members. A more detailed discussion of this problem can be found in Hodge (68).

Table 1. The presence of 15-Gyr-old stars ^a

	Main sequence	Sample	RR Lyrae	Sample	Globular	Sample
Galaxy	fitting	reference	stars	reference	clusters	reference
M31			x	101	x	17
MWG	x		x		x
M33					(x)	23
LMC	x	118	x	93	x	118
SMC	x	119	x	51	x	119
NGC 205					x	62
M32
NGC 6822					x	68
NGC 185					x	57
IC 1613					(x)	68
NGC 147			x	106	x	60
Fornax			(x)	86	x	20
GR8
Sculptor	x	29	x	124
Leo I			(x)	73
Leo II			x	121
Ursa Minor			x	123
Draco			x	15
Carina			x	100
a x, 15-Gyr-old stars present, based on listed criterion. Parentheses indicate uncertain identification. Adapted from Hodge (68).