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 Luminosity Functions of Old and Intermediate-Age Globular Clusters in NGC 3610

The WFPC2 Camera on board HST has been used to obtain high-resolution images of NGC 3610, a dynamically young elliptical galaxy. These observations supersede shorter, undithered HST observations where an intermediate-age population of globular clusters was first discovered. The new observations show the bimodal color distribution of globular clusters more clearly, with peaks at (V-I)o = 0.95 and 1.17. The luminosity function (LF) of the blue, metal-poor population of clusters in NGC 3610 turns over, consistent with a Gaussian distribution with a peak Mv ~= -7.0, similar to old globular-cluster populations in ellipticals. The red, metal-rich population of clusters has a LF that is more extended toward both the bright and faint ends, as expected for a cluster population of inter-mediate age. It is well fit by a power law with an exponent of alpha = -1.78 +-0.05, or -1.90+-0.07 when corrected for observational scatter. A Kolmogorov-Smirnov test confirms the significant difference between the LFs of the red and blue clusters, with a probability of less than 0.1% that they come from the same population. A comparison with the Fall & Zhang cluster disruption models shows marginal agreement with the observed LF, although there are differences in detail. In particular, there is no clear evidence of the predicted turnover at the faint end. A by-product of the analysis is the demonstration that, at any given metallicity, the peak of the LF should remain nearly constant from 1.5 Gyr to 12 Gyr, since the effect of the disruption of faint clusters is almost perfectly balanced by the fading of the clusters. This may help explain the apparent universality of the peak of the globular cluster luminosity function. (Abridged)Comment: 32 pages, LaTeX, 13 PS figures, 1 table; to appear in AJ (July 2002

Some Constraints On the Effects of Age and Metallicity on the Low Mass X-ray Binary Formation Rate

We have studied the low mass X-ray binary (LMXB) populations within and outside globular clusters (GC) in NGC 4365 and NGC 3115. Using published age and metallicity constraints from optical and IR observations of their GCs, we do not find any evidence for an increase in the LMXB formation rate in the intermediate age cluster population of NGC 4365, as has been proposed in some scenarios of dynamical LMXB formation in GCs. The old, metal-rich, red population of GCs in NGC 3115 on the other hand is {\it at least} three times as efficient at creating LMXBs as the old, metal-poor, blue clusters. These data suggest that the higher formation efficiency of LMXBs in the red GC subsystems of many galaxies is largely a consequence of their higher metallicity. A comparison of the densities of field LMXBs in different galaxies does not reveal an obvious correlation with the age of the field stars as predicted by models in which the LMXB formation rate in the field drops monotonically with time after an initial burst. This suggests that either a significant fraction of the field LMXBs are created in GCs and subsequently injected into the field, or the LMXB formation rate has a more complex time evolution pattern.Comment: Accepted for publication in ApJ Letters. 7 pages including 5 figure

Hubble Space Telescope FUV observations of M31's globular clusters suggest a spatially homogeneous helium-enriched sub population

We present high spatial resolution, far ultraviolet (FUV) F140LP observations of 12 massive globular clusters in M 31 obtained using the ACS/SBC on the Hubble Space Telescope (HST). These observations resolve the cluster profiles to scales similar to their core radii and enable the study of the spatial distribution of blue and extreme horizontal branch (HB) stars, which dominate the emission in the F140LP images. We confirm that some of these clusters have excess FUV emission, suggesting additional hot populations beyond those expected by canonical single stellar populations models. We find no evidence that the hot populations are spatially distinct from the majority populations in these clusters, as would be expected if the excess FUV emission is a result of a dynamically enhanced population of extreme-HB stars. We conclude that a second population of stars with significantly enhanced helium abundance is a viable explanation for the observed FUV emission which is both bright and distributed similarly to the rest of the cluster light. Our results support the use of FUV observations as a path to characterising helium enhanced sub populations in extragalactic clusters. These M31 clusters also show a correlation such that more massive and denser clusters are relatively FUV bright. Similar to extant Milky Way results, this may indicate the degree of helium enrichment, or second population fraction increases with cluster mass.Comment: 12 pages, 7 figures, 1 table, accepted for publication in MNRA

Global Properties of the Globular Cluster Systems of Four Spiral Galaxies

We present results from a wide-field imaging study of the globular cluster (GC) systems of a sample of edge-on, Sb-Sc spiral galaxies ~7-20 Mpc away. This study is part of a larger survey of the ensemble properties of the GC populations of giant galaxies. We imaged the galaxies in BVR filters with large-format CCD detectors on the WIYN 3.5-m telescope, to projected radii of ~20-40 kpc. For four galaxies (NGC 2683, NGC 3556, NGC 4157, and NGC 7331), we quantify the radial distributions of the GC systems and estimate the total number, luminosity- and mass-normalized specific frequencies (S_N and T), and blue (metal-poor) fraction of GCs. A fifth galaxy (NGC 3044) was apparently too distant for us to have detected its GC system. Our S_N for NGC 2683 is 2.5 times smaller than the previously-published value, likely due in part to reduced contamination from non-GCs. For the spiral galaxies analyzed for the survey to date, the average number of GCs is 170+/-40 and the weighted mean values of S_N and T are 0.8+/-0.2 and 1.4+/-0.3. We use the survey data to derive a relationship between radial exent of the GC system and host galaxy mass over a factor of 20 in mass. Finally, we confirm the trend, identified in previous survey papers, of increasing specific frequency of metal-poor GCs with increasing galaxy mass. We compare the data with predictions from a simple model and show that carefully quantifying the numbers of metal-poor GCs in galaxies can constrain the formation redshifts of the GCs and their host galaxies.Comment: 30 pages, including 14 figures and 13 tables; accepted for publication in The Astronomical Journal, Oct 2007 issu