A wide-field survey of the globular cluster systems of elliptical and spiral galaxies.

摘要

We have undertaken a survey of the globular cluster (GC) systems of a large sample of elliptical and spiral galaxies, with the aim of using their observed properties to test models of galaxy formation. The survey combines wide-field, ground-based CCD imaging with archival Hubble Space Telescope data to determine the total numbers, spatial distributions and color distributions of GCs around the galaxies. We use photometry in three filters and image analysis to reduce contamination in the GC candidate samples. As a result, they are up to an order of magnitude less contaminated than those of past studies. Furthermore, we typically have more than twice the radial coverage of previous CCD surveys.; In this thesis, I present results for the GC system properties of six galaxies spanning a range of morphological types, from ellipticals to late-type spirals. The derived spatial distributions suggest that we have observed the full radial extent of the galaxies' GC systems. Using these distributions, we derive global values for the total number (N_GC) and specific frequency (S_N) of GCs in each galaxy. The S_N values for three of the five galaxies studied previously are reduced by 20–75%, and in all cases the errors are 2–3 times smaller. The GC systems of the early-type galaxies have multimodal color distributions and modest negative color gradients. We compare the relative numbers of blue (metal-poor) GCs in the galaxies and find that merging the GC populations of spirals can account for the blue GCs in a moderate-luminosity field elliptical, but is insufficient to account for the blue GC populations of luminous cluster ellipticals. Furthermore, we find a rough trend in the sense that more luminous galaxies have larger relative numbers of blue GCs. Finally, we find that N_GC in spirals seems to depend primarily on the luminosity of the galaxy and find little or no correlation with morphological type or bulge properties. Our results seem to point toward a picture of galaxy and GC system formation in which metal-poor GCs form at high redshift in protogalactic building blocks and metal-rich GC populations are built up over time during subsequent gaseous mergers.