Annu. Rev. Astron. Astrophys. 1996. 34: 511-550 Copyright © 1996 by Annual Reviews. All rights reserved

2.2. The Old Cluster Population

The Magellanic Clouds have a number of red clusters long suspected of being analogues to the Galactic globular clusters. A list of these old LMC clusters is given in Suntzeff et al (1992) and is repeated in Table 1 with a few revisions. Reticulum, NGC 1841, and NGC 1466 have at times been considered to be Milky Way globulars (e.g. Webbink 1985). All three have now been shown from color-magnitude diagrams (Gratton & Ortolani 1987, Walker 1992a for Reticulum; Walker 1990 for NGC 1841Walker 1992b for NGC 1466) and velocities (Storm et al 1991, Suntzeff et al 1992, Olszewski et al 1991) to be LMC members. The few old clusters in the SMC are also listed in Table 1. The lower age cutoff in Table 1 is about 8 Gyr.

While the early papers of Arp (1958a, 1958b), Tifft (1962), Hodge (1960) were among those that first showed that some of the redder Magellanic Cloud clusters were similar to Galactic globulars, direct evidence for the old ages of the red clusters comes only from deep color-magnitude diagrams (CMD), which can be measured from ground-based data for the clusters in uncrowded regions of the MCs. The expected main-sequence turnoff of the old population in the LMC will be at V ~ 22.5. The CMDs for the LMC clusters NGC 1466, NGC 1841, and Reticulum listed above, and for NGC 2257 (Testa et al 1995), all show main-sequence turnoffs roughly consistent with the ages of Galactic globular clusters, but the quality of the published color-magnitude diagrams is inadequate to say with certainty whether the LMC clusters are truly ancient or are a few Gyr younger. Two major programs have been undertaken in Cycle 5 of HST to observe the oldest LMC clusters listed in Table 1 to improve the age determinations.

Because all the LMC clusters known to be old based on CMD dating are also very metal poor, we have assumed all metal-poor LMC clusters to be old. The list of LMC clusters in Table 1 is therefore a compliation of all clusters with known old ages or low metallicities. In some cases, the old age is verified by the existence of RR Lyrae variables. A number of LMC clusters that could be considered old based on their red colors are excluded from Table 1 because their metallicity was found to be similar to the intermediate-age clusters (Olszewski et al 1991). No new metal-poor clusters have been found in recent spectroscopic surveys of MC clusters (Olszewski et al 1991).

In the SMC, NGC 121 (Stryker et al 1985) is the oldest cluster, but all three clusters listed in Table 1 are younger than the ancient Galactic globular clusters (Olszewski et al 1987, Rich et al 1984) based on ages from CMDs (see also Sarajedini et al 1995).

Some of the oldest MC clusters may no longer be in the Magellanic Clouds. Lin & Richer (1992) have argued, on the basis of cluster positions and velocities, that the Galactic globulars Pal 12 and Rup 106 have been captured from the LMC. This idea is actually not radical, for we know that the Sagittarius dwarf spheroidal is contributing four globular clusters to our Galaxy (see Da Costa & Armandroff 1995). van den Bergh (1994) has pointed out a possible problem with the Lin & Richer scenario. Pal 12 has a rather high metallicity to be an LMC cluster of its age, unless it is somehow an analogue to the unique LMC cluster ESO121-SC03. The possibility that Pal 12 came from the SMC should be examined.

It is probably true that no luminous old clusters (M_V -7) remain to be found in the Clouds. The large clusters have had well-determined integrated UBV photometry for many years (van den Bergh 1981). Most have been or will be searched for RR Lyraes and observed with HST in the next few years. There may be old clusters among the generally less-luminous clusters cataloged by Olszewski et al (1988), Kontizas et al (1990), Bica et al (1991, 1992). To find them efficiently, we will have to resort to indirect indicators of age such as low metallicity, integrated colors, and the presence of RR Lyrae variables.

2.2.1. RR LYRAES    It is generally accepted that the existence of RR Lyrae variables or a blue horizontal branch (BHB) is prima facie evidence of the existence of an old population. Exactly how old is not well determined observationally or theoretically. RR Lyraes are low-mass helium core-burning stars. The thickness of the envelope determines whether such a star is redward of, blueward of, or in the instability strip. The envelope mass is a complicated function of age, metallicity, helium abundance, rotation, and mass loss; the latter is poorly understood (Lee et al 1994).

There is evidence that RR Lyraes are not all extremely old. Taam et al (1976), Kraft (1977) have argued that the metal-rich RR Lyraes with thick-disk kinematics in the Milky Way may be younger than the ancient clusters, but the age difference is not known. How young can an RR Lyrae be? Clusters of known ages containing RR Lyraes provide a most direct way of estimating the youngest age for an RR Lyrae. We note, however, that it is difficult to age-date the metal-rich field RR Lyraes in either the Milky Way or the Magellanic Clouds. Although one can envision mechanisms that would make these stars very young, new data from HST (J Liebert, 1996, private communication) show that some metal-rich globular clusters do contain substantial blue horizontal-branch populations, thus allowing the possibility that metal-rich field RR Lyraes are old. Da Costa & Armandroff (1995) give the most recent list of Milky Way "young halo" globulars; many of these 21 objects have long been known (Sawyer Hogg 1973) to contain RR Lyraes. Ruprecht 106 is thought to be more than 3 Gyr younger than the typical globular (Buonanno et al 1993), yet it contains 12 RR Lyraes (Kaluzny et al 1995).

From Table 1, we see that the 12-Gyr old SMC cluster NGC 121 has four RR Lyraes whereas the 9-Gyr old cluster Lindsay 1 has none. This comparison is often cited as evidence that the youngest RR Lyraes are older than 10 Gyr (Olszewski et al 1987). However, L1 is 7 times fainter than NGC 121 (van den Bergh 1981). Scaling NGC 121's RR Lyrae population to L1, we expect less than one RR Lyrae. While the number of known RR Lyraes in the concentrated cluster NGC 121 is likely an underestimate, the expected number ot RR Lyraes in L1 will still be subject to small number statistics, and will not provide a good constraint on the age of the youngest-possible RR Lyraes. None of the other relatively luminous, older SMC clusters have RR Lyraes (Walker 1989b). For LMC clusters, it is unlikely that any of the luminous clusters in Table 1 are young enough to constrain the minimum age of the RR Lyraes. There are no known luminous LMC clusters in the age range 4-12 Gyr (see Section 3.4 below). It may be possible to constrain the age of the youngest RR Lyraes by seaching Milky Way old open clusters, the globulars in the Fornax dwarf spheroidal, and the luminous intermediate-color star clusters in M33.

In summary, the existence of RR Lyraes probably means that the population is old, but there is no strong observational evidence that can rule out their presence in a substantially younger population. The gravitational lens experiments such as MACHO should be very effective in searching for RR Lyraes in the less-luminous clusters in the MCs, which may overcome the low a priori chance of finding RR Lyraes in any individual low-luminosity cluster.

2.2.2. LOW METAL ABUNDANCES    The metallicity of a cluster is often used to indicate old age (cf Andersen et al 1984) in analogy to our Galaxy where the metal-poor globular clusters are all old. For the Galaxy and the LMC, low metallicity is apparently a sufficient condition to indicate that a population is old. In the SMC, however, low abundance does not imply old age. The field giants near NGC 121 have ages of about 8 Gyr, with a mean metallicity [Fe/H] = -1.6 (Suntzeff et al 1986), and the 8-Gyr SMC cluster Kron 3 has a metallicity of [Fe/H] = -1.3 (Rich et al 1984).

The mean metal abundance of the LMC old clusters is [Fe/H] = -1.84, whereas that of the Galactic globulars outside of the solar circle is -1.70 (Suntzeff et al 1992). These similarities imply that fragments of long-destroyed galaxies not very different from the LMC could have assembled the halo (e.g. Searle & Zinn 1978). Van den Bergh (1993, 1994) has argued that objects like the current LMC could not have donated many clusters, but the differences between LMC clusters and Galactic halo clusters are subtle. The entire Galactic halo cannot simply be the debris from LMC-like protogalaxies, since inside the solar circle the Galaxy shows a metallicity gradient (Zinn 1985, Suntzeff et al 1991), implying (in part) a dissipational evolution involving chemical feedback from earlier generations of stars. Of course, a mechanism in which the largest density fluctuations in a protogalaxy were themselves sites of star and cluster formation would also be an explanation (Sandage 1990).

2.2.3. INTEGRATED COLORS    A third indication of an old population can be found in the integrated colors or spectra. The broad-band integrated color of a star cluster changes continuously with age, although the color separation between very disparate ages is sometimes small (Searle et al 1980;, Elson & Fall 1985, 1988;, Bica et al 1991, 1992;, Girardi et al 1995). Nevertheless, broad-band colors remain a useful indicator of clusters with potentially interesting properties. Bica and collaborators have obtained integrated UBV photometry for 624 LMC clusters and a number of SMC clusters (Bica et al 1986); this sample can be searched for possible old clusters. The LMC clusters Hodge 7 (Bica et al 1992) and NGC 2155 (Searle et al 1980) were both claimed to be ancient clusters. Our CMDs show that these clusters are not old, illustrating the potential problems of age classifications of lower luminosity clusters in crowded fields using broad-band photometry (see also Girardi & Bica 1993, Girardi et al 1995).