On the Origin of Globular Clusters in Elliptical and cD Galaxies

Abstract

Perhaps the most noteworthy of recent findings in extragalactic globular cluster (GC) research are the multimodal GC metallicity distributions seen in massive early-type galaxies. We explore the origin of these distinct GC populations, the implications for galaxy formation and evolution, and identify several new properties of GC systems. First, when we separate the metal-rich and metal-poor subpopulations, in galaxies with bimodal GC metallicity distributions, we find that the mean metallicity of the metal-rich GCs correlates well with parent galaxy luminosity but the mean metallicity of the metal-poor ones does not. This indicates that the metal-rich GCs are closely coupled to the galaxy and share a common chemical enrichment history with the galaxy field stars. The mean metallicity of the metal-poor population is largely independent of the galaxy luminosity. Second, the slope of the GC system radial surface density varies considerably in early-type galaxies. However, the galaxies with relatively populous GC systems for their luminosity (called high specific frequency, SN) only have shallow, extended radial distributions. A characteristic of high SN galaxies is that their GCs are preferentially located in the outer galaxy regions relative to the underlying starlight. Third, we find that the ratio of metal-rich to metal-poor GCs correlates with SN. In other words, high SN galaxies have proportionately more metal-poor GCs, per unit galaxy light, than low SN galaxies. Fourth, we find steeper metallicity gradients in high SN galaxies. This is due to the greater number of metal-poor GCs at large galactocentric radii. We critically review current ideas for the origin of GCs in giant elliptical (gE) and cD galaxies and conclude that the gaseous merger model of Ashman & Zepf (1992, ApJ, 384, 50) is unlikely to account for the GC systems in these galaxies. Tidal stripping of the GCs from nearby galaxies appears to contribute to the GC population in the outer parts of cD galaxies, but we suggest that the vast majority of GCs in gE and cD galaxies have probably formed in situ. We speculate that these indigenous GCs formed in two distinct phases of star formation from gas of differing metallicity, giving rise to the bimodal GC metallicity distributions. The metal-poor GCs are formed at an early stage in the collapse of the protogalactic cloud. The metal-rich GCs formed out of more enriched gas, roughly contemporaneously with the galaxy stars. In this sense the metal-rich GCs in elliptical galaxies are the analog of the metal-poor halo GCs in spirals. The disk GCs in spirals may represent a third phase of this formation process.