Keck imaging of the globular cluster systems in the early-type galaxies NGC 1052 and 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 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 globular cluster subpopulations are at least of 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 globular cluster 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.     

The globular cluster system of the young elliptical NGC 6702

We study the globular cluster (GC) system of the dust-lane elliptical galaxy NGC 6702, using B -, V - and I -band imaging observations carried out at the Keck telescope. This galaxy has a spectroscopic age of ≈2 Gyr suggesting recent star formation. We find strong evidence for a bimodal GC colour distribution, with the blue peak having a colour similar to that of the Galactic halo GCs. Assuming that the blue GCs are indeed old and metal-poor, we estimate an age of 2–5 Gyr and supersolar metallicity for the red GC subpopulation. Despite the large uncertainties, this is in reasonable agreement with the spectroscopic galaxy age. Additionally, we estimate a specific frequency of S _N =2.3±1.1 for NGC 6702. We predict that passive evolution of NGC 6702 will further increase its specific frequency to S _N ≈2.7 within 10 Gyr, in closer agreement to that of typical present-day ellipticals. We also discuss evidence that the merger/accretion event that took place a few Gyr ago involved a high gas fraction.     

UBRI photometry of globular clusters in the Leo group galaxy NGC 3379

We present wide-area UBRI photometry for globular clusters around the Leo group galaxy NGC 3379. Globular cluster candidates are selected from their B -band magnitudes and their  ( U − B ) _o   versus  ( B − I ) _o   colours. A colour–colour selection region was defined from photometry of the Milky Way and M31 globular cluster systems. We detect 133 globular cluster candidates, which supports previous claims of a low specific frequency for NGC 3379.
The Milky Way and M31 reveal blue and red subpopulations, with  ( U − B ) _o   and  ( B − I ) _o   colours indicating mean metallicities similar to those expected based on previous spectroscopic work. The stellar population models of Maraston and Brocato et al. are consistent with both subpopulations being old, and with metallicities of  [Fe/H]∼−1.5  and −0.6 for the blue and red subpopulations, respectively. The models of Worthey do not reproduce the  ( U − B ) _o   colours of the red (metal-rich) subpopulation for any modelled age.
For NGC 3379 we detect a blue subpopulation with similar colours, and presumably age/metallicity, to that of the Milky Way and M31 globular cluster systems. The red subpopulation is less well defined, perhaps due to increased photometric errors, but indicates a mean metallicity of [Fe/H]∼−0.6.     

Gemini/GMOS spectra of globular clusters in the Virgo giant elliptical NGC 4649

NGC 4649 (M60) is one of a handful of giant Virgo ellipticals. We have obtained Gemini/GMOS (Gemini North Multi-Object Spectrograph) spectra for 38 globular clusters (GCs) associated with this galaxy. Applying the multi-index  χ²  minimization technique of Proctor and Sansom with the single stellar population models of Thomas, Maraston and Korn, we derive ages, metallicities and α-element abundance ratios. We find several young (2–3 Gyr old) supersolar metallicity GCs, while the majority are old (>10 Gyr), spanning a range of metallicities from solar to  [Z/H]=−2  . At least two of these young GCs are at large projected radii of 17–20 kpc. The galaxy itself shows no obvious signs of a recent starburst, interaction or merger. A trend of decreasing α-element ratio with increasing metallicity is found.     

GMOS spectroscopy of the S0 galaxy NGC 3115

We present Gemini Multi-Object Spectrograph longslit spectroscopy of the isolated S0 galaxy NGC 3115. We have determined kinematical data and Lick/IDS absorption line-strength indices for the major axis out to around 9 kpc and for the minor axis out to around 5 kpc (around 2R _e ). Using stellar population models which include the effects of variable [α/Fe] ratios, we derive metallicities, abundance ratios and ages for the stellar population of NGC 3115. We find that [α/Fe] remains fairly constant with increasing radius at around  [α/Fe]= 0.17  for the major axis but increases rapidly for the minor axis to around  [α/Fe]= 0.3  . We also find that to first order, this behaviour can be explained by a simple spheroid + disc model, where the spheroid has  [α/Fe]= 0.3  and the disc shows close to solar abundance ratios. The disc also appears considerably younger than the spheroid, having an age of around 6 Gyr compared to 12 Gyr for the spheroid. We compare these results to those previously presented for the globular cluster system of NGC 3115.     

The binary star population of the young cluster NGC 1818 in the Large Magellanic Cloud

We determine the binary star fraction as a function of radius in NGC 1818, a young rich cluster in the Large Magellanic Cloud, using Hubble Space Telescope images in bands F336W (∼ U ) and F555W (∼ V ). Our sample includes binaries with M _primary ∼ 2–5.5 M_⊙ and M _secondary ≳ 0.7 M_primary. The binary fraction increases towards the cluster centre, from ∼ 20 ± 5 per cent in the outer parts, to ∼ 35 ± 5 per cent inside the core. This increase is consistent with dynamical mass segregation and need not be primordial. We compare our results with expectations from N -body models, and discuss the implications for the formation and early evolution of such clusters.     

Nuclear star formation in the hotspot galaxy NGC 2903

We present high-resolution near-infrared imaging obtained using adaptive optics and HST /NICMOS, and ground-based spectroscopy of the hotspot galaxy NGC 2903. Our near-infrared resolution imaging enables us to resolve the infrared hotspots into individual young stellar clusters or groups of these. The spatial distribution of the stellar clusters is not coincident with that of the bright H  ii regions, as revealed by the HST /NICMOS Pa α image. Overall, the circumnuclear star formation in NGC 2903 shows a ring-like morphology with an approximate diameter of 625 pc.
The star formation properties of the stellar clusters and H  ii regions have been studied using the photometric and spectroscopic information in conjunction with evolutionary synthesis models. The population of bright stellar clusters shows a very narrow range of ages, 4–7×10⁶ yr after the peak of star formation, or absolute ages 6.5–9.5×10⁶ yr (for the assumed short-duration Gaussian bursts), and luminosities similar to the clusters found in the Antennae interacting galaxy. This population of young stellar clusters accounts for some 7–12 per cent of the total stellar mass in the central 625 pc of NGC 2903. The H  ii regions in the ring of star formation have luminosities close to that of the supergiant H  ii region 30 Doradus, they are younger than the stellar clusters, and they will probably evolve into bright infrared stellar clusters similar to those observed today. We find that the star formation efficiency in the central regions of NGC 2903 is higher than in normal galaxies, approaching the lower end of infrared luminous galaxies.