Spectroscopy of a Globular Cluster in the Local Group dIrr NGC 6822

We present low-resolution Keck spectroscopy for the globular cluster H VIII in the Local Group dIrr galaxy NGC 6822. We find the metallicity of the cluster to be [Fe/H]= -1.58 +/- 0.28 and the age of the cluster to be 3-4 Gyr, slightly older than but consistent with previous age estimates. H VIII seems to be more metal-poor than most intermediate-age globular clusters in the Local Group, and appears most similar to the anomalous Small Magellanic Cloud clusters Lindsay 113 and NGC 339.Comment: 5 pages, including 3 figures. Accepted by MNRAS, uses mn2e.cl

Detailed abundances from integrated-light spectroscopy: Milky Way globular clusters

We test the performance of our analysis technique for integrated-light spectra by applying it to seven well-studied Galactic GCs that span a wide range of metallicities. Integrated-light spectra were obtained by scanning the slit of the UVES spectrograph on the ESO Very Large Telescope across the half-light diameters of the clusters. We modelled the spectra using resolved HST colour-magnitude diagrams (CMDs), as well as theoretical isochrones, in combination with standard stellar atmosphere and spectral synthesis codes. The abundances of Fe, Na, Mg, Ca, Ti, Cr, and Ba were compared with literature data for individual stars in the clusters. The typical differences between iron abundances derived from our integrated-light spectra and those compiled from the literature are less than 0.1 dex. A larger difference is found for one cluster (NGC 6752), and is most likely caused primarily by stochastic fluctuations in the numbers of bright red giants within the scanned area. As expected, the alpha-elements (Ca, Ti) are enhanced by about 0.3 dex compared to the Solar-scaled composition, while the [Cr/Fe] ratios are close to Solar. When using up-to-date line lists, our [Mg/Fe] ratios also agree well with literature data. Our [Na/Fe] ratios are, on average, 0.08-0.14 dex lower than average values quoted in the literature, and our [Ba/Fe] ratios may be overestimated by 0.20-0.35 dex at the lowest metallicities. We find that analyses based on theoretical isochrones give very similar results to those based on resolved CMDs. Overall, the agreement between our integrated-light abundance measurements and the literature data is satisfactory. Refinements of the modelling procedure, such as corrections for stellar evolutionary and non-LTE effects, might further reduce some of the remaining offsets.Comment: 35 pages, 16 figures, accepted for A&

Globular Clusters in NGC 4365: New K-band Imaging and a Reassessment of the Case for Intermediate-age Clusters

We study the globular cluster (GC) system of the Virgo giant elliptical galaxy NGC 4365, using new wide-field VIK imaging. The GC colour distribution has (at least) two peaks, but the colours of the red GCs appear more strongly weighted towards intermediate colours compared to most other large ellipticals and the integrated galaxy light. The intermediate-color/red peak may itself be composed of two sub-populations, with clusters of intermediate colours more concentrated towards the centre of the galaxy than both the blue and red GCs. Nearly all intermediate-colour and red GCs in our sample show an offset towards red V-K and/or blue V-I colours compared to SSP models for old ages in a (V-K,V-I) diagram. This has in the past been interpreted as evidence for intermediate ages. We also combine our VIK data with previously published spectroscopy. The differences between observed and model colour-metallicity relations are consistent with the offsets observed in the two-colour diagram, with the metal-rich GCs being too red (by about 0.2 mag) in V-K and too blue (by about 0.05 mag) in V-I compared to the models at a given metallicity. These offsets cannot easily be explained as an effect of younger ages. We conclude that, while intermediate GC ages cannot be definitively ruled out, an alternative scenario is more likely whereby all the GCs are old but the relative number of intermediate-metallicity GCs is greater than typical for giant ellipticals. The main obstacle to reaching a definitive conclusion is the lack of robust calibrations of integrated spectral and photometric properties for stellar populations with near-solar metallicity. In any case, it is puzzling that the intermediate-colour GCs in NGC 4365 are not accompanied by a corresponding shift of the integrated galaxy light towards bluer colours.Comment: 23 pages, including 20 figures and 5 tables. Accepted for publication in A&

Star Clusters in M31: V. Internal Dynamical Trends: Some Troublesome, Some Reassuring

We present internal velocity dispersions and precise radial velocities for 200 globular clusters (GCs) in M31 that are derived using new high-resolution spectra from MMT/Hectochelle. Of these, 163 also have King model structural parameters that allow us to estimate their mass-to-light ratios. This is, by far, the largest such dataset available for any galaxy, including the Milky Way. These data strongly confirm earlier suggestions that the optical and near-infrared mass-to-light ratios of M31 GCs decline with increasing metallicity. This behavior is the opposite of that predicted by stellar population models for a standard initial mass function. We show that this phenomenon does not appear to be caused by standard dynamical evolution. A shallower mass function for metal-rich GCs (with dN/dM ~ M^-0.8 to M^-1.3 below one solar mass) can explain the bulk of extant observations. We also observe a consistent, monotonic correlation between mass-to-light ratio and cluster mass. This correlation, in contrast to the correlation with metallicity, is well-explained by the accepted model of dynamical evolution of GCs through mass segregation and the preferential loss of low-mass stars, and these data are among the best available to constrain this process.Comment: AJ in press. 11 pages, 7 figures (not including tables) in emulate forma

Galaxy Disruption in a Halo of Dark Matter

The relics of disrupted satellite galaxies around the Milky Way and Andromeda have been found, but direct evidence of a satellite galaxy in the early stages of being disrupted has remained elusive. We have discovered a dwarf satellite galaxy in the process of being torn apart by gravitational tidal forces as it merges with a larger galaxy's dark matter halo. Our results illustrate the morphological transformation of dwarf galaxies by tidal interaction and the continued build-up of galaxy halos.Comment: 7 pages, Latex, 4 figures, to appear in Science v301 p563