ALBERT

Description: 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 NGC 1856 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 cannot 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 V esc ~= 17 km s –1 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 NGC 1856, in contradiction to the results of Bastian & Silva-Villa.

Description: 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 an 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.

Description: We present scaling relations between structural properties of nuclear star clusters and their host galaxies for a sample of early-type dwarf galaxies observed as part of the Hubble Space Telescope ( HST ) Advanced Camera for Surveys (ACS) Coma Cluster Survey. We have analysed the light profiles of 200 early-type dwarf galaxies in the magnitude range 16.0 〈  m F 814 W  〈 22.6 mag, corresponding to –19.0 〈  M F 814 W  〈 –12.4 mag. Nuclear star clusters are detected in 80 per cent of the galaxies, thus doubling the sample of HST -observed early-type dwarf galaxies with nuclear star clusters. We confirm that the nuclear star cluster detection fraction decreases strongly towards faint magnitudes. The luminosities of nuclear star clusters do not scale linearly with host galaxy luminosity. A linear fit yields $L_{\rm nuc} \sim L_{\rm gal}^{0.57\pm 0.05}$ . The nuclear star cluster–host galaxy luminosity scaling relation for low-mass early-type dwarf galaxies is consistent with formation by globular cluster (GC) accretion. We find that at similar luminosities, galaxies with higher Sérsic indices have slightly more luminous nuclear star clusters. Rounder galaxies have on average more luminous clusters. Some of the nuclear star clusters are resolved, despite the distance of Coma. We argue that the relation between nuclear star cluster mass and size is consistent with both formation by GC accretion and in situ formation. Our data are consistent with GC inspiralling being the dominant mechanism at low masses, although the observed trend with Sérsic index suggests that in situ star formation is an important second-order effect.

Description: We constrain the assembly history of galaxies in the projected central 0.5 Mpc of the Coma cluster by performing structural decomposition on 69 massive ( M *  ≥ 10 9 M ) galaxies using high-resolution F 814 W images from the Hubble Space Telescope ( HST ) Treasury Survey of Coma. Each galaxy is modelled with up to three Sérsic components having a free Sérsic index n . After excluding the two cDs in the projected central 0.5 Mpc of Coma, 57 per cent of the galactic stellar mass in the projected central 0.5 Mpc of Coma resides in classical bulges/ellipticals while 43 per cent resides in cold disc-dominated structures. Most of the stellar mass in Coma may have been assembled through major (and possibly minor) mergers. Hubble types are assigned based on the decompositions, and we find a strong morphology–density relation; the ratio of (E+S0):spirals is (91.0 per cent):9.0 per cent. In agreement with earlier work, the size of outer discs in Coma S0s/spirals is smaller compared with lower density environments captured with SDSS (Data Release 2). Among similar-mass clusters from a hierarchical semi-analytic model, no single cluster can simultaneously match all the global properties of the Coma cluster. The model strongly overpredicts the mass of cold gas and underpredicts the mean fraction of stellar mass locked in hot components over a wide range of galaxy masses. We suggest that these disagreements with the model result from missing cluster physics (e.g. ram-pressure stripping), and certain bulge assembly modes (e.g. mergers of clumps). Overall, our study of Coma underscores that galaxy evolution is not solely a function of stellar mass, but also of environment.

Description: We report the discovery of a very diverse set of five low-surface brightness (LSB) dwarf galaxy candidates in Hickson Compact Group 90 (HCG 90) detected in deep U - and I -band images obtained with Very Large Telescope/Visible Multi-Object Spectrograph. These are the first LSB dwarf galaxy candidates found in a compact group of galaxies. We measure spheroid half-light radii in the range 0.7 r eff /kpc 1.5 with luminosities of –11.65 M U –9.42 and –12.79 M I –10.58 mag, corresponding to a colour range of ( U – I ) 0 ~= 1.1–2.2 mag and surface brightness levels of μ U ~= 28.1 mag arcsec –2 and μ I ~= 27.4 mag arcsec –2 . Their colours and luminosities are consistent with a diverse set of stellar population properties. Assuming solar and 0.02 Z metallicities we obtain stellar masses in the range M * | Z ~= 10 5.7 – 6.3 M and $M_{\ast }|_{0.02\,\mathrm{Z}_{\odot }} \simeq 10^{6.3-8}\,\mathrm{M}_{{\odot }}$ . Three dwarfs are older than 1 Gyr, while the other two significantly bluer dwarfs are younger than ~2 Gyr at any mass/metallicity combination. Altogether, the new LSB dwarf galaxy candidates share properties with dwarf galaxies found throughout the Local Volume and in nearby galaxy clusters such as Fornax. We find a pair of candidates with ~2 kpc projected separation, which may represent one of the closest dwarf galaxy pairs found. We also find a nucleated dwarf candidate, with a nucleus size of r eff ~= 46–63 pc and magnitude M U , 0 = –7.42 mag and ( U – I ) 0 = 1.51 mag, which is consistent with a nuclear stellar disc with a stellar mass in the range 10 4.9 – 6.5 M .

Description: 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 NGC 411. This is the second case of an eMSTO being identified in a star cluster belonging to the Small Magellanic Cloud (SMC), after NGC 419. Despite that the present masses of these two SMC clusters differ by a factor of ~4, the comparison between their colour–magnitude diagrams (CMDs) 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. NGC 411, however, presents merely a trace of secondary red clump as opposed to its prominent manifestation in NGC 419. This could be either due to the small number statistics in NGC 411 or due to the star formation in NGC 419 having continued for ~60 Myr longer than in NGC 411. 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 NGC 419 than in NGC 411. We discuss these findings in the context of possible scenarios for the origin of eMSTOs.

Description: We present Hubble Space Telescope ( HST ) Wide-Field Camera 3 (WFC3) images of the merger remnant NGC 7252. In particular, we focus on the surface brightness profiles and effective radii R eff of 36 young massive clusters (YMCs) within the galaxy. All the clusters have masses exceeding 10 5  M and are, despite the 64 Mpc distance to the galaxy, (partly) resolved on the HST  images. Effective radii can be measured down to ~2.5 pc, and the largest clusters have R eff approaching 20 pc. The median R eff of our sample clusters is ${\sim }6\text{--}7$ pc, which is larger than typical radii of YMCs (~2.5 pc). This could be due to our sample selection (only selecting resolved sources) or to an intrinsic mass–radius relation within the cluster population. We find at least three clusters that have power-law profiles of the Elson, Fall and Freeman (EFF) type extending out to 150 pc. Among them are the two most massive clusters, W3 and W30, which have profiles that extend to at least 500 and 250 pc, respectively. Despite their extended profiles, the effective radii of the three clusters are 17.2, 12.6 and 9.1 pc for W3, W26 and W30, respectively. We compare these extended profiles with those of YMCs in the Large Magellanic Cloud (R136 in 30 Dor), the Antennae galaxies (Knot S) and in the nearby spiral galaxy NGC 6946. Extended profiles seem to be a somewhat common feature, even though many nearby YMCs show distinct truncations. A continuous distribution between these two extremes, i.e. truncated or extremely extended, is the most likely interpretation. We suggest that the presence or absence of an extended envelope in very young clusters may be due to the gas distribution of the proto-cluster giant molecular cloud, in particular if the proto-cluster core becomes distinct from the surrounding gas before star formation begins.

Description: NGC 1846 and NGC 1783 are two massive star clusters in the Large Magellanic Cloud, hosting both an extended main-sequence turn-off and a dual clump of red giants. They present similar masses but differ mainly in angular size. Starting from their high-quality ACS data in the F435W, F555W and F814W filters, and updated sets of stellar evolutionary tracks, we derive their star formation rates as a function of age, SFR( t ), by means of the classical method of colour–magnitude diagram reconstruction which is usually applied to nearby galaxies. The method confirms the extended periods of star formation derived from previous analysis of the same data. When the analysis is performed for a finer resolution in age, we find clear evidence for a ~50-Myr long hiatus between the oldest peak in the SFR( t ), and a second prolonged period of star formation, in both clusters. For the more compact cluster NGC 1846, there seems to be no significant difference between the SFR( t ) in the cluster centre and in an annulus with radii between 20 and 60 arcsec (from 4.8 to 15.4 pc). The same does not occur in the more extended NGC 1783 cluster, where the outer ring (between 33 and 107 arcsec, from 8.0 to 25.9 pc) is found to be slightly younger than the centre. We also explore the best-fitting slope of the present-day mass function and binary fraction for the different cluster regions, finding hints of a varying mass function between centre and outer ring in NGC 1783. These findings are discussed within the present scenarios for the formation of clusters with multiple turn-offs.