On the Origin of Globular Cluster Systems in Elliptical and cD Galaxies

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, S_N) only have shallow, extended radial distributions. A characteristic of high S_N 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 S_N. In other words, high S_N galaxies have proportionately more metal-poor GCs, per unit galaxy light, than low S_N galaxies. Fourth, we find steeper metallicity gradients in high S_N 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) i