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&

Nitrogen abundances and multiple stellar populations in the globular clusters of the Fornax dSph

We use measurements of nitrogen abundances in red giants to search for multiple stellar populations in the four most metal-poor globular clusters (GCs) in the Fornax dwarf spheroidal galaxy (Fornax 1, 2, 3, and 5). New imaging in the F343N filter, obtained with the Wide Field Camera 3 on the Hubble Space Telescope, is combined with archival F555W and F814W observations to determine the strength of the NH band near 3370 AA. After accounting for observational errors, the spread in the F343N-F555W colors of red giants in the Fornax GCs is similar to that in M15 and corresponds to an abundance range of Delta([N/Fe])=2 dex, as observed also in several Galactic GCs. The spread in F555W-F814W is, instead, fully accounted for by observational errors. The stars with the reddest F343N-F555W colors (indicative of N-enhanced composition) have more centrally concentrated radial distributions in all four clusters, although the difference is not highly statistically significant within any individual cluster. From double-Gaussian fits to the color distributions we find roughly equal numbers of "N-normal" and "N-enhanced" stars (formally about 40% N-normal stars in Fornax 1, 3, and 5 and 60% in Fornax 2). We conclude that GC formation, in particular regarding the processes responsible for the origin of multiple stellar populations, appears to have operated similarly in the Milky Way and in the Fornax dSph. Combined with the high ratio of metal-poor GCs to field stars in the Fornax dSph, this places an important constraint on scenarios for the origin of multiple stellar populations in GCs.Comment: 16 pages, 7 figures, accepted for publication in Ap

Radial distributions of sub-populations in the globular cluster M15: a more centrally concentrated primordial population

We examine the radial distributions of stellar populations in the globular cluster (GC) M15, using HST/WFC3 photometry of red giants in the nitrogen-sensitive F343N-F555W color. Surprisingly, we find that giants with "primordial" composition (i.e., N abundances similar to those in field stars) are the most centrally concentrated within the WFC3 field. We then combine our WFC3 data with SDSS u, g photometry and find that the trend reverses for radii >1' (3 pc) where the ratio of primordial to N-enhanced giants increases outwards, as already found by Lardo et al. The ratio of primordial to enriched stars thus has a U-shaped dependency on radius with a minimum near the half-light radius. N-body simulations show that mass segregation might produce a trend resembling the observed one, but only if the N-enhanced giants are ~0.25 Mo less massive than the primordial giants, which requires extreme He enhancement (Y~0.40). However, such a large difference in Y is incompatible with the negligible optical color differences between primordial and enriched giants which suggest Delta Y < 0.03 and thus a difference in turn-off mass of Delta M < 0.04 Mo between the different populations. The radial trends in M15 are thus unlikely to be of dynamical origin and presumably reflect initial conditions, a result that challenges all current GC formation scenarios. We note that population gradients in the central regions of GCs remain poorly investigated and may show a more diverse behavior than hitherto thought.Comment: 16 pages, accepted for Ap