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), afte
r 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).
The most massive elliptical galaxies show a prominent multi-modality in their globular cluster system color distributions. Understanding the mechanisms which lead to multiple globular cluster sub-populations is essential for a complete picture of mas
sive galaxy formation. By assuming that globular cluster formation traces the total star formation and taking into account the radial variations in the composite stellar populations predicted by the Pipino & Matteucci (2004) multi-zone photo-chemical evolution code, we compute the distribution of globular cluster properties as a function of galactocentric radius. We compare our results to the spectroscopic measurements of globular clusters in nearby early-type galaxies by Puzia et al. (2006) and show that the observed multi-modality in globular cluster systems of massive ellipticals can be, at least partly, ascribed to the radial variation in the mix of stellar populations. Our model predicts the presence of a super-metal-rich population of globular clusters in the most massive elliptical galaxies, which is in very good agreement with the spectroscopic observations. Furthermore, we investigate the impact of other non-linear mechanisms that shape the metallicity distribution of globular cluster systems, in particular the role of merger-induced globular cluster formation and a non-linear color-metallicity transformation, and discuss their influence in the context of our model (abridged)
