On Collision Course: The Nature of the Binary Star Cluster NGC 2006 / SL 538

The LMC hosts a rich variety of star clusters seen in close projected proximity. Ages have been derived for few of them showing differences up to few million years, hinting at being binary star clusters. However, final confirmation needs to be done through spectroscopic analysis. Here we focus on the LMC cluster pair NGC2006-SL538 and aim to determine whether the star cluster pair is a bound entity (binary star cluster) or a chance alignment. Using the MIKE echelle spectrograph at LCO we have acquired integrated-light spectra for each cluster. We have measured radial velocities by two methods: a) direct line profile measurement yields v$_r=300.3\pm5\pm6$ km/s for NGC2006 and $v_r=310.2\pm4\pm6$ km/s for SL538. b) By comparing observed spectra with synthetic bootstrapped spectra yielding $v_r=311.0\pm0.6$ km/s for NGC2006 and $v_r=309.4\pm0.5$ km/s for SL538. Finally when spectra are directly compared, we find a ${\Delta}v=1.08\pm0.47$ km/s. Full-spectrum SED fits reveal that the stellar population ages lie in the range 13-21 Myr with a metallicity of Z=0.008. We find indications for differences in the chemical abundance patterns as revealed by the helium absorption lines between the two clusters. The dynamical analysis shows that the two clusters are likely to merge within the next $\sim$150 Myr. The NGC2006-SL538 cluster pair shows radial velocities, stellar population and dynamical parameters consistent with a gravitational bound entity. We conclude that this is a genuine binary cluster pair, and we propose that their differences in ages and stellar population chemistry is most likely due to variances in their chemical enrichment history within their environment. We suggest that their formation may have taken place in a loosely bound star-formation complex which saw initial fragmentation but then had its clusters become a gravitationally bound pair by tidal capture.Comment: Accepted for publication in Astronomy & Astrophysics. 15 pages, 10 figures in low resolutio

Extremely alpha-Enriched Globular Clusters in Early-Type Galaxies: A Step towards the Dawn of Stellar Populations?

We compare [alpha/Fe], metallicity, and age distributions of globular clusters in elliptical, lenticular, and spiral galaxies, which we derive from Lick line index measurements. We find a large number of globular clusters in elliptical galaxies that reach significantly higher [alpha/Fe] values (>0.5) than any clusters in lenticular and spiral galaxies. Most of these extremely alpha-enriched globular clusters are old (t > 8 Gyr) and cover the metallicity range -1 <~ [Z/H] <~ 0. A comparison with supernova yield models suggests that the progenitor gas clouds of these globular clusters must have been predominantly enriched by massive stars (>~20 M_sol) with little contribution from lower-mass stars. The measured [alpha/Fe] ratios are also consistent with yields of very massive pair-instability supernovae (~130-190 M_sol). Both scenarios imply that the chemical enrichment of the progenitor gas was completed on extremely short timescales of the order of a few Myr. Given the lower [alpha/Fe] average ratios of the diffuse stellar population in early-type galaxies, our results suggest that these extremely alpha-enhanced globular clusters could be members of the very first generation of star clusters formed, and that their formation epochs would predate the formation of the majority of stars in giant early-type galaxies.Comment: 6 pages, 3 figures, accepted for publication in the Astrophysical Journa

Wide-Field Hubble Space Telescope Observations of the Globular Cluster System in NGC1399

We present a comprehensive high spatial-resolution imaging study of globular clusters (GCs) in NGC1399, the central giant elliptical cD galaxy in the Fornax galaxy cluster, conducted with HST/ACS. Using a novel technique to construct drizzled PSF libraries for HST/ACS data, we accurately determine the fidelity of GC structural parameter measurements from detailed artificial star cluster experiments. The measurement of rh for the major fraction of the NGC1399 GC system reveals a trend of increasing rh versus galactocentric distance, Rgal, out to about 10 kpc and a flat relation beyond. This trend is very similar for blue and red GCs which are found to have a mean size ratio of rh(red)/rh(blue)=0.82+/-0.11 at all galactocentric radii from the core regions of the galaxy out to ~40 kpc. This suggests that the size difference between blue and red GCs is due to internal mechanisms related to the evolution of their constituent stellar populations. Modeling the mass density profile of NGC1399 shows that additional external dynamical mechanisms are required to limit the GC size in the galaxy halo regions to rh~2 pc. We suggest that this may be realized by an exotic GC orbit distribution function, an extended dark matter halo, and/or tidal stress induced by the increased stochasticity in the dwarf halo substructure at larger Rgal. We match our GC rh measurements with radial velocity data from the literature and find that compact GCs show a significantly smaller line-of-sight velocity dispersion, =225+/-25 km/s, than their extended counterparts, =317+/-21 km/s. Considering the weaker statistical correlation in the GC rh-color and the GC rh-Rgal relations, the more significant GC size-dynamics relation appears to be astrophysically more relevant and hints at the dominant influence of the GC orbit distribution function on the evolution of GC structural parameters.Comment: 22 pages, 17 figures, accepted for publication in The Astrophysical Journal, a high-quality PDF version is available at http://www.astro.puc.cl/~tpuzia/PUC/Home.htm

New Constraints on the Star Formation History of the Star Cluster NGC 1856

We use the Wide Field Camera 3 onboard the Hubble Space Telescope to obtain deep, high-resolution photometry of the young (age ~ 300 Myr) star cluster NGC1856 in the Large Magellanic Cloud. We compare the observed colour-magnitude diagram (CMD), after having applied a correction for differential reddening, with Monte Carlo simulations of simple stellar populations (SSPs) of various ages. We find that the main sequence turn-off (MSTO) region is wider than that derived from the simulation of a single SSP. Using constraints based on the distribution of stars in the MSTO region and the red clump, we find that the CMD is best reproduced using a combination of two different SSPs with ages separated by 80 Myr (0.30 and 0.38 Gyr, respectively). However, we can not formally exclude that the width of the MSTO could be due to a range of stellar rotation velocities if the efficiency of rotational mixing is higher than typically assumed. Using a King-model fit to the surface number density profile in conjunction with dynamical evolution models, we determine the evolution of cluster mass and escape velocity from an age of 10 Myr to the present age, taking into account the possible effects of primordial mass segregation. We find that the cluster has an escape velocity Vesc ~ 17 km/s at an age of 10 Myr, and it remains high enough during a period of ~ 100 Myr to retain material ejected by slow winds of first-generation stars. Our results are consistent with the presence of an age spread in NGC1856, in contradiction to the results of Bastian & Silva-Villa (2013).Comment: 17 pages, 14 figures. Re-submitted to MNRAS after addressing all the comments by the refere

A Genuine Intermediate-Age Globular Cluster in M33

We present deep integrated-light spectroscopy of nine M33 globular clusters taken with the Hectospec instrument at the MMT Observatory. Based on our spectroscopy and previous deep color-magnitude diagrams obtained with HST/WFPC2, we present evidence for the presence of a genuine intermediate-age globular cluster in M33. The analysis of Lick line indices indicates that all globular clusters are metal-poor ([Z/H] <~ -1.0) and that cluster M33-C38 is about 5-8 Gyr younger than the rest of the sample M33 star clusters. We find no evidence for a population of blue horizontal branch stars in the CMD of M33-C38, which rules out the possibility of an artificially young spectroscopic age due to the presence of hot stars. We infer a total mass of 5-9 x 10^4 M_sol for M33-C38, which implies that M33-C38 has survived ~2-3 times longer than some dynamical evolution model predictions for star clusters in M33, although it is not yet clear to which dynamical component of M33 - thin disk, thick disk, halo - the cluster is associated.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter