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).
Recent high-quality photometry of many star clusters in the Magellanic Clouds with ages of 1$,-,$2 Gyr revealed main sequence turnoffs (MSTOs) that are significantly wider than can be accounted for by a simple stellar population (SSP). Such extended
MSTOs (eMSTOs) are often interpreted in terms of an age spread of several $10^8$ yr, challenging the traditional view of star clusters as being formed in a single star formation episode. Li et al. and Bastian & Niederhofer recently investigated the sub-giant branches (SGBs) of NGC 1651, NGC 1806, and NGC 1846, three star clusters in the Large Magellanic Cloud (LMC) that exhibit an eMSTO. They argued that the SGB of these star clusters can be explained only by a SSP. We study these and two other similar star clusters in the LMC, using extensive simulations of SSPs including unresolved binaries. We find that the shapes of the cross-SGB profiles of all star clusters in our sample are in fact consistent with their cross-MSTO profiles when the latter are interpreted as age distributions. Conversely, SGB morphologies of star clusters with eMSTOs are found to be inconsistent with those of simulated SSPs. Finally, we create PARSEC isochrones from tracks featuring a grid of convective overshoot levels and a very fine grid of stellar masses. A comparison of the observed photometry with these isochrones shows that the morphology of the red clump (RC) of such star clusters is also consistent with that implied by their MSTO in the age spread scenario. We conclude that the SGB and RC morphologies of star clusters featuring eMSTOs are consistent with the scenario in which the eMSTOs are caused by a distribution of stellar ages.
We present color-magnitude diagram analysis of deep Hubble Space Telescope imaging of a mass-limited sample of 18 intermediate-age (1 - 2 Gyr old) star clusters in the Magellanic Clouds, including 8 clusters for which new data was obtained. We find t
hat ${it all}$ star clusters in our sample feature extended main sequence turnoff (eMSTO) regions that are wider than can be accounted for by a simple stellar population (including unresolved binary stars). FWHM widths of the MSTOs indicate age spreads of 200-550 Myr. We evaluate dynamical evolution of clusters with and without initial mass segregation. Our main results are: (1) the fraction of red clump (RC) stars in secondary RCs in eMSTO clusters scales with the fraction of MSTO stars having pseudo-ages $leq 1.35$ Gyr; (2) the width of the pseudo-age distributions of eMSTO clusters is correlated with their central escape velocity $v_{rm esc}$, both currently and at an age of 10 Myr. We find that these two results are unlikely to be reproduced by the effects of interactive binary stars or a range of stellar rotation velocities. We therefore argue that the eMSTO phenomenon is mainly caused by extended star formation within the clusters; (3) we find that $v_{rm esc} geq 15$ km/s out to ages of at least 100 Myr for ${it all}$ clusters featuring eMSTOs, while $v_{rm esc} leq 12$ km/s at all ages for two lower-mass clusters in the same age range that do ${it not}$ show eMSTOs. We argue that eMSTOs only occur for clusters whose early escape velocities are higher than the wind velocities of stars that provide material from which second-generation stars can form. The threshold of 12-15 km/s is consistent with wind velocities of intermediate-mass AGB stars in the literature.
We use the Wide Field Camera 3 (WFC3) onboard the Hubble Space Telescope (HST) to obtain deep, high resolution images of two intermediate-age star clusters in the Large Magellanic Cloud of relatively low mass ($approx$ $10^4$ $M_{odot}$) and signific
antly different core radii, namely NGC2209 and NGC2249. For comparison purposes, we also re-analyzed archival HST images of NGC1795 and IC2146, two other relatively low mass star clusters. From the comparison of the observed color-magnitude diagrams with Monte Carlo simulations, we find that the main sequence turnoff (MSTO) regions in NGC2209 and NGC2249 are significantly wider than that derived from simulations of simple stellar populations, while those in NGC1795 and IC2146 are not. We determine the evolution of the clusters masses and escape velocities from an age of 10 Myr to the present age. We find that the differences among these clusters can be explained by dynamical evolution arguments if the currently extended clusters (NGC2209 and IC2146) experienced stronger levels of initial mass segregation than the currently compact ones (NGC2249 and NGC1795). Under this assumption, we find that NGC2209 and NGC2249 have estimated escape velocities $V_{rm esc}$ $geq$ 15 km s$^{-1}$ at an age of 10 Myr, large enough to retain material ejected by slow winds of first-generation stars, while the two clusters that do not feature extended MSTOs have $V_{rm esc}$ $leq$ 12 km s$^{-1}$ at that age. These results suggest that the extended MSTO phenomenon can be better explained by a range of stellar ages rather than a range of stellar rotation velocities or interacting binaries.
Based on new observations with the Wide Field Camera 3 onboard the Hubble Space Telescope, we report the discovery of an extended main sequence turn-off (eMSTO) in the intermediate-age star cluster NGC411. This is the second case of an eMSTO being id
entified in a star cluster belonging to the Small Magellanic Cloud (SMC), after NGC419. Despite the present masses of these two SMC clusters differ by a factor of 4, the comparison between their colour--magnitude diagrams (CMD) shows striking similarities, especially regarding the shape of their eMSTOs. The loci of main CMD features are so similar that they can be well described, in a first approximation, by the same mean metallicity, distance and extinction. NGC411, however, presents merely a trace of secondary red clump as opposed to its prominent manifestation in NGC419. This could be due either to the small number statistics in NGC411, or by the star formation in NGC419 having continued for 60 Myr longer than in NGC411. Under the assumption that the eMSTOs are caused by different generations of stars at increasing age, both clusters are nearly coeval in their first episodes of star formation. The initial period of star formation, however, is slightly more marked in NGC419 than in NGC411. We discuss these findings in the context of possible scenarios for the origin of eMSTOs.
We present a wide field study of the Globular Clusters/Low Mass X-ray Binary (LMXB) connection in the giant elliptical NGC1399. The large FOV of the ACS/WFC, combined with the HST and Chandra high resolution, allow us to constrain the LMXB formation
scenarios in elliptical galaxies. We confirm that NGC1399 has the highest LMXB fraction in GCs of all nearby elliptical galaxies studied so far, even though the exact value depends on galactocentric distance due to the interplay of a differential GC vs galaxy light distribution and the GC color dependence. In fact LMXBs are preferentially hosted by bright, red GCs out to $>5 R_{eff}$ of the galaxy light. The finding that GC hosting LMXBs follow the radial distribution of their parent GC population, argues against the hypothesis that the external dynamical influence of the galaxy affects LMXB formation in GCs. On the other hand field LMXBs closely match the host galaxy light, thus indicating that they are originally formed in situ and not inside GCs. We measure GC structural parameters, finding that the LMXB formation likelihood is influenced independently by mass, metallicity and GCs structural parameters. In particular the GC central density plays a major role in predicting which GC host accreting binaries. Finally our analysis shows that LMXBs in GCs are marginally brighter than those in the field, and in particular the only color-confirmed GC with $L_X>10^{39}$ erg s$^{-1}$ shows no variability, which may indicate a superposition of multiple LMXBs in these systems.
We discuss new photometry from high-resolution images of 7 intermediate-age (1-2 Gyr) star clusters in the Large Magellanic Cloud taken with the Advanced Camera for Surveys on board the Hubble Space Telescope. We fit color-magnitude diagrams (CMDs) w
ith several different sets of theoretical isochrones, and determine systematic uncertainties for population parameters when derived using any one set of isochrones. The cluster CMDs show several interesting features, including extended main sequence turnoff (MSTO) regions, narrow red giant branches, and clear sequences of unresolved binary stars. We show that the extended MSTOs are not caused by photometric uncertainties, contamination by field stars, or the presence of binary stars. Enhanced helium abundances in a fraction of cluster stars are also ruled out as the reason for the extended MSTOs. Quantitative comparisons with simulations indicate that the MSTO regions are better described by a spread in ages than by a bimodal age distribution, although we can not formally rule out the latter for the three lowest-mass clusters in our sample (which have masses lower than about 3E4 solar masses). This conclusion differs from that of some previous works which suggested that the age distribution in massive clusters in our sample is bimodal. This suggests that any secondary star formation occurred in an extended fashion rather than through short bursts. We discuss these results in the context of the nature of multiple stellar populations in star clusters.
We present new analysis of 11 intermediate-age (1-2 Gyr) star clusters in the Large Magellanic Cloud based on Hubble Space Telescope imaging data. Seven of the clusters feature main sequence turnoff (MSTO) regions that are wider than can be accounted
for by a simple stellar population, whereas their red giant branches indicate a single value of [Fe/H]. The star clusters cover a range in present-day mass from about 1E4 to 2E5 solar masses. We compare radial distributions of stars in the upper and lower parts of the MSTO region, and calculate cluster masses and escape velocities from the present time back to a cluster age of 10 Myr. Our main result is that for all clusters in our sample with estimated escape velocities > 15 km/s at an age of 10 Myr, the stars in the brightest half of the MSTO region are significantly more centrally concentrated than the stars in the faintest half AND more massive red giant branch and asymptotic giant branch stars. This is not the case for clusters with escape velocities < 10 km/s at an age of 10 Myr. We argue that the wide MSTO region of such clusters is mainly caused by to a 200 - 500 Myr range in the ages of cluster stars due to extended star formation within the cluster from material shed by first-generation stars featuring slow stellar winds. Dilution of this enriched material by accretion of ambient interstellar matter is deemed plausible if the spread of [Fe/H] in this ambient gas was very small when the second-generation stars were formed in the cluster.
Intracluster stellar populations are a natural result of tidal interactions in galaxy clusters. Measuring these populations is difficult, but important for understanding the assembly of the most massive galaxies. The Coma cluster is one of the neares
t truly massive galaxy clusters, and is host to a correspondingly large system of globular clusters (GCs). We use imaging from the HST/ACS Coma Cluster Survey to present the first definitive detection of a large population of intracluster GCs (IGCs) that fills the Coma cluster core and is not associated with individual galaxies. The GC surface density profile around the central massive elliptical galaxy, NGC 4874, is dominated at large radii by a population of IGCs that extend to the limit of our data (R<520 kpc). We estimate that there are 47000+/-1600 (random) +4000/-5000 (systematic) IGCs out to this radius, and that they make up ~70% of the central GC system, making this the largest GC system in the nearby Universe. Even including the GC systems of other cluster galaxies, IGCs still make up ~30-45% of the GCs in the cluster core. Observational limits from previous studies of the intracluster light (ICL) suggest that the IGC population has a high specific frequency. If the IGC population has a specific frequency similar to high-S_N dwarf galaxies, then the ICL has a total stellar mass of ~10^12 M_sun within the cluster core. The ICL makes up approximately half of the stellar luminosity and one-third of the stellar mass of the central (NGC4874+ICL) system. The color distribution of the IGC population is bimodal, with blue, metal-poor GCs outnumbering red, metal-rich GCs by a ratio of 4:1. The fraction of red IGCs (20%), and the red color of those GCs, implies that IGCs can originate from the halos of relatively massive, L* galaxies, and not solely from the disruption of dwarf galaxies. (Abridged)
We present a study of the old globular clusters (GC) using archival F606W and F814W HST/ACS images of 19 Magellanic-type dwarf Irregular (dIrr) galaxies found in nearby (2 - 8 Mpc) associations of only dwarf galaxies. All dIrrs have absolute magnitud
es fainter than or equal to the SMC (Mv = -16.2 mag). We detect 50 GC candidates in 13 dIrrs, of which 37 have (V-I) colors consistent with blue (old, metal-poor) GCs (bGC). The luminosity function (LF) of the bGCs in our sample peaks at Mv = -7.41 +/- 0.22 mag, consistent with other galaxy types. The width of the LF is sigma = 1.79 +/- 0.31 which is typical for dIrrs, but broader than the typical width in massive galaxies. The half-light radii and ellipticities of the GCs in our sample (rh ~ 3.3 pc, e ~ 0.1) are similar to those of old GCs in the Magellanic Clouds and to those of Old Halo (OH) GCs in our Galaxy, but not as extended and spherical as the Galactic Young Halo (YH) GCs (rh ~ 7.7 pc, e ~ 0.06). The e distribution shows a turnover rather than a power law as observed for the Galactic GCs. This might suggest that GCs in dIrrs are kinematically young and not fully relaxed yet. The present-day specific frequencies (SN) span a broad range: 0.3 < SN < 11. Assuming a dissipationless age fading of the galaxy light, the SN values would increase by a factor of ~ 2.5 to 16, comparable with values for early-type dwarfs (dE/dSphs). A bright central GC candidate, similar to nuclear clusters of dEs, is observed in one of our dIrrs: NGC 1959. This nuclear GC has luminosity, color, and structural parameters similar to that of wCen and M54, suggesting that the latter might have their origin in the central regions of similar Galactic building blocks. A comparison between properties of bGCs and Galactic YH GCs, suspected to have originated from similar dIrrs, is performed.
