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

Extragalactic Globular Clusters in the Near Infrared I: A comparison between M87 and NGC 4478

We compare optical and near infrared colours of globular clusters in M87, the central giant elliptical in Virgo, and NGC 4478, an intermediate luminosity galaxy in Virgo, close in projection to M87. Combining V and I photometry obtained with the WFPC2 on HST and Ks photometry obtained with the NIRC on Keck 1, we find the broad range in colour and previously detected bi-modality in M87. We confirm that NGC 4478 only hosts a blue sub-population of globular clusters and now show that these clusters' V-I and V-K colours are very similar to those of the halo globular clusters in Milky Way and M31. Most likely, a metal-rich sub-population never formed around this galaxy (rather than having formed and been destroyed later), probably because its metal-rich gas was stripped during its passage through the centre of the Virgo cluster. The V-I, V-K colours are close to the predicted colours from SSP models for old populations. However, M87 hosts a few red clusters that are best explained by intermediate ages (a few Gyr). Generally, there is evidence that the red, metal-rich sub-population has a complex colour structure and is itself composed of clusters spanning a large metallicity and, potentially, age range. This contrasts with the blue, metal-poor population which appears very homogeneous in all galaxies observed so far.Comment: accepted in A&A, 13 pages using the A&A macr

Hubble Space Telescope Imaging of the Globular Cluster System around NGC 5846

Bimodal globular cluster metallicity distributions have now been seen in a handful of large ellipticals. Here we report the discovery of a bimodal distribution in the dominant group elliptical NGC 5846, using the Hubble Space Telescope's Wide Field and Planetary Camera 2 (WFPC2). The two peaks are located at V-I = 0.96 and 1.17, which roughly correspond to metallicities of [Fe/H] = -1.2 and -0.2 respectively. The luminosity functions of the blue and red subpopulations appear to be the same, indicating that luminosity does not correlate with metallicity within an individual galaxy's globular cluster system. Our WFPC2 data cover three pointings allowing us to examine the spatial distribution of globular clusters out to 30 kpc (or 2.5 galaxy effective radii). We find a power law surface density with a very flat slope, and a tendency for globular clusters to align close to the galaxy minor axis. An extrapolation of the surface density profile, out to 50 kpc, gives a specific frequency S_N = 4.3 +/- 1.1. Thus NGC 5846 has a much lower specific frequency than other dominant ellipticals in clusters but is similar to those in groups. The central galaxy regions reveal some filamentary dust features, presumably from a past merger or accretion of a gas-rich galaxy. This dust reaches to the very nucleus and so provides an obvious source of fuel for the radio core. We have searched for proto-globular clusters that may have resulted from the merger/accretion and find none. Finally, we briefly discuss the implications of our results for globular cluster formation mechanisms.Comment: 22 pages, Latex. To be published in the Astronomical Journal. Full paper available at http://www.ucolick.org/~forbes/home.htm

Keck Imaging of the Globular Cluster Systems in the Early--type Galaxies NGC 1052 and NGC 7332

The presence of two globular cluster subpopulations in early-type galaxies is now the norm rather than the exception. Here we present two more examples for which the host galaxy appears to have undergone a recent merger. Using multi-colour Keck imaging of NGC 1052 and NGC 7332 we find evidence for a bimodal globular cluster colour distribution in both galaxies, with roughly equal numbers of blue and red globular clusters. The blue ones have similar colours to those in the Milky Way halo and are thus probably very old and metal-poor. If the red GC subpopulations are at least solar metallicity, then stellar population models indicate young ages. We discuss the origin of globular clusters within the framework of formation models. We conclude that recent merger events in these two galaxies have had little effect on their overall GC systems. We also derive globular cluster density profiles, global specific frequencies and in the case of NGC 1052, radial colour gradients and azimuthal distribution. In general these globular cluster properties are normal for early-type galaxies.Comment: 11 pages, Latex, 15 figures, 2 tables, accepted by MNRA

Keck Spectroscopy of Two Young Globular Clusters in the Merger Remnant NGC 3921

Low-resolution UV-to-visual spectra of two candidate globular clusters in the merger remnant NGC 3921 are presented. These two clusters of apparent magnitude V = 22.2 (Mv = -12.5) lie at projected distances of ~5 kpc from the center and move with halo-type radial velocities relative to the local galaxy background. Their spectra show strong Balmer absorption lines indicative of main-sequence turnoffs dominated by A-type stars. Comparisons with model-cluster spectra computed by Bruzual & Charlot and others yield cluster ages in the range of 200-530 Myr, and metallicities about solar to within a factor of three. Given their small half-light radii (Reff < 5 pc) and ages corresponding to ~100 core- crossing times, these clusters are gravitationally bound and, hence, indeed young globulars. Assuming that they had Chabrier-type initial mass functions, their estimated current masses are 2.3(+-0.1)x10^6 Msun and 1.5(+-0.1)x10^6 Msun, respectively, or roughly half the mass of omegaCen. Since NGC 3921 itself shows many signs of being a 0.7(+-0.3) Gyr old protoelliptical, these two young globulars of roughly solar metallicity and their many counterparts observed with the Hubble Space Telescope provide supporting evidence that, in the process of forming elliptical-like remnants, major mergers of gas-rich disks can also increase the number of metal-rich globular clusters. (Abridged)Comment: 22 pages, 6 figures, accepted for publication in AJ, July 200