Bimodal Infrared Colors of the M87 Globular Cluster System: Peaks in the Metallicity Distribution

The globular cluster (GC) systems of many galaxies reveal bimodal optical color distributions. Based on stellar evolutionary models and the bimodal colors and metallicities of Galactic GCs this is thought to reflect an underlying bimodal metallicity distribution. However, stars at many different phases of stellar evolution contribute to optical light. The I-H color is a much cleaner tracer of metallicity because it primarily samples the metallicity sensitive giant branch. Therefore, we use deep HST-NICMOS H, and WFPC2 optical observations, of M87 GCs to study their metallicity distribution. The M87 clusters are bimodal in I-H, for which there is no known physical explanation other than a bimodal metallicity distribution. Moreover, the two modes defined by the B-I and I-H colors are comprised of roughly the same two sets of objects, confirming that optical colors also primarily trace the metallicity. This is inconsistent with a recent suggestion based on one model of metallicity effects on the horizontal branch that bimodality arises from an underlying unimodal metallicity distribution due to a specific color-metallicity relation. We also find no discernable variation in the peak colors of the M87 GCs out to roughly 75 kpc due to the declining ratio of red-to-blue GCs, as implied by this model. Similarly, there is no evidence that the bimodal peaks are bluer for systems with large blue-to-red GC ratio. Our observations confirm that the primary cause of bimodality in cluster systems is an underlying bimodal metallicity distribution, and not the specific color-metallicity relationship defined by this horizontal branch model.Comment: Accepted for publication in ApJ Letters. 5 pages, 4 figs. Version 2 is identical to version

The Globular Cluster System of the Spiral Galaxy NGC7814

We present the results of a wide-field photometric study of the globular cluster (GC) system of the edge-on Sab spiral NGC7814. This is the first spiral to be fully analyzed from our survey of the GC systems of a large sample of galaxies beyond the Local Group. NGC7814 is of particular interest because a previous study estimated that it has 500-1000 GCs, giving it the largest specific frequency (S_N) known for a spiral. Understanding this galaxy's GC system is important in terms of our understanding of the GC populations of spirals in general and has implications for the formation of massive galaxies. We observed the galaxy in BVR filters with the WIYN 3.5-m telescope, and used image classification and three-color photometry to select GC candidates. We also analyzed archival HST WFPC2 images of NGC7814, both to help quantify the contamination level of the WIYN GC candidate list and to detect GCs in the inner part of the galaxy halo. Combining HST data with high-quality ground-based images allows us to trace the entire radial extent of this galaxy's GC system and determine the total number of GCs directly through observation. We find that rather than being an especially high-S_N spiral, NGC7814 has <200 GCs and S_N ~ 1, making it comparable to the two most well-studied spirals, the Milky Way and M31. We explore the implications of these results for models of the formation of galaxies and their GC systems. The initial results from our survey suggest that the GC systems of typical ellipticals can be accounted for by the merger of two or more spirals, but that for highly-luminous ellipticals, additional physical processes may be needed.Comment: 28 pages, incl. 4 figures; accepted for publication in The Astronomical Journal, November 2003 issu

Environmental influences on galaxy evolution

We investigate the role of mergers and interactions in the evolution of galaxies by studying galaxies in compact groups. Compact groups of galaxies have high spatial densities and low velocity dispersions making these regions ideal laboratories in which to study the effect of interactions and mergers. Based on a detailed spectroscopic and multi-color imaging study, we find that both the isophotal shapes and the stellar kinematics indicate that many of the elliptical galaxies in compact groups have been affected by tidal interactions. At the same time, however, we find that only a few elliptical galaxies in compact groups have evidence for the young stellar populations that would be expected if they are the result of recent merger of two spiral galaxies. Therefore, we conclude that tidal interactions affect galaxy properties at the current epoch, but the bulk of basic galaxy formation and transformation must have occurred at much higher redshift

Globular cluster systems as clues to galaxy evolution

We investigate the properties of systems of globular clusters in light of the hypothesis that galaxy mergers play a major role in galaxy evolution. In a previous paper, we presented a model in which the formation of globular clusters occurs during galaxy interactions and mergers. We discussed several predictions of the model, including the existence of young globular clusters in currently merging galaxies and the presence of two or more metallicity peaks in the globular clusters systems of normal elliptical galaxies. Here, we present recent observational evidence which supports both of these predictions and suggests that mergers may have a significant influence on the formation and evolution of galaxies and their globular clusters