The formation of giant elliptical galaxies and their globular cluster systems

Abstract

We examine the formation of giant elliptical galaxies using their globular cluster (GC) systems as probes of their evolutionary history. The bimodal distributions of GC metallicities in such galaxies are often cited as evidence for the formation of giant elliptical galaxies through mergers involving gas-rich spirals, with the metal-rich GCs forming during the merger process. We explore an alternative possibility: that these metal-rich clusters represent the galaxy's intrinsic GC population and the metal-poor component of the observed GC metallicity distribution arises from the capture of GCs from other galaxies, either through mergers or through tidal stripping. Starting with plausible assumptions for the luminosity function of galaxies in the host cluster and for the dependence of GC metallicity on parent galaxy luminosity, we show using Monte Carlo simulations that the growth of a preexisting seed galaxy through mergers and tidal stripping is accompanied naturally by the capture of metal-poor GCs whose chemical abundances are similar to those that are observed to surround giant ellipticals. We also investigate the spatial distribution of GCs in isolated galaxies of low and intermediate luminosity and conclude that, at the epoch of formation, the GC systems of such galaxies are likely to have been more spatially extended than their constituent stars. Thus, the capture of GCs through tidal stripping, unlike mergers, does not necessarily conserve GC specific frequency. Comparisons of model GC metallicity distributions and specific frequencies to those observed for the well-studied galaxies NGC 4472 (=M49) and NGC 4486 (=M87), the two brightest cluster members of the nearby Virgo cluster, show that it is possible to explain their bimodal GC metallicity distributions and discordant specific frequencies without resorting to the formation of new GCs in mergers or by invoking multiple bursts of GC formation. Finally, we discuss the possibility of using the ratio of the numbers of metal-poor to metal-rich GCs in giant elliptical galaxies as a diagnostic of their merger histories. We use this method to derive upper limits on the number of galaxies and total luminosity accreted to date by NGC 4472.