We investigate the origin of the shape of the globular cluster (GC) system scaling parameters as a function of galaxy mass, i.e. specific frequency (S N ), specific luminosity (S L ), specific mass (S M ) and specific number (T ) of GCs. In the low-mass galaxy regime (M V −16 mag), our analysis is based on Hubble Space Telescope/Advanced Camera for Surveys observations of GC populations of faint, mainly late-type dwarf galaxies in low-density environments. In order to sample the entire range in galaxy mass (M V = −11 to −23 mag =10 6 -10 11 L ), environment and morphology we augment our sample with data of spiral and elliptical galaxies from the literature, in which old GCs are reliably detected. This large data set confirms (irrespective of the galaxy type) the increase in the specific frequencies of GCs above and below a galaxy magnitude of M V −20 mag. Over the full mass range, the S L value of early-type galaxies is, on average, twice that of late types. To investigate the observed trends, we derive theoretical predictions of GC system scaling parameters as a function of host galaxy mass based on the models of Dekel and Birnboim in which star formation processes are regulated by stellar and supernova feedback below a stellar mass of 3 × 10 10 M and by virial shocks above it. We find that the analytical model describes remarkably well the shape of the GC system scaling parameter distributions with a universal specific GC formation efficiency, η, which relates the total mass in GCs to the total galaxy halo mass. Early-type and late-type galaxies show a similar mean value of η = 5.5 × 10 −5 , with an increasing scatter towards lower galaxy masses. This can be due to the enhanced stochastic nature of the star and star-cluster formation processes for such systems. Some massive galaxies have excess η values compared to what is expected from the mean model prediction for galaxies more luminous than M V −20 mag (L V 10 10 L ). This may be attributed to a very efficient early GC formation, less efficient production of field stars, accretion of predominantly low-mass/luminosity high-η galaxies or a mixture of all these effects.

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 that 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 1.35 Gyr; (2) the width of the pseudo-age distributions of eMSTO clusters is correlated with their central escape velocity v 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 esc ≥ 15 km s −1 out to ages of at least 100 Myr for all clusters featuring eMSTOs, while v esc ≤ 12 km s −1 at all ages for two lower-mass clusters in the same age range that do 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 −1 is consistent with wind velocities of intermediate-mass AGB stars and massive binary stars in the literature.

We report on multi‐object spectroscopy in the red spectral region of 37 candidate star clusters in an ∼8×8 arcmin2 field centred on the giant early‐type radio galaxy NGC 1316 (Fornax A), the brightest galaxy in the Fornax cluster. Out of this sample, 24 targets are found to be genuine star clusters associated with NGC 1316, and 13 targets are Galactic foreground stars. For the star cluster sample, we measure a mean heliocentric velocity vhel=1698±46 km s−1 and a velocity dispersion σ=227±33 km s−1 within a galactocentric radius of 24 kpc. Partly responsible for the velocity dispersion is a significant rotation in the star cluster system, with a mean velocity of ∼175±70 km s−1 along a position angle of ∼6°±18°. Using the projected mass estimator and assuming isotropic orbits, the estimated total mass is (6.6±1.7)×1011 M⊙ within a radius of 24 kpc. The mass is uncertain by about a factor of 2, depending on the orbital assumptions. The implied ℳ/LB ratio is in the range 3–6. Four star clusters in our sample are exceptionally luminous (MV<−12.3). This means that (1) at least this many clusters in NGC 1316 are up to an order of magnitude more luminous than the most luminous star cluster in our Galaxy or M31, and that (2) the S/N ratio of their spectra allows us to measure line strengths with good accuracy. By comparing the measured colours and equivalent widths of Hα and the Ca ii triplet (λλ 8498, 8542, 8662 Å) absorption lines for those bright star clusters in our sample with those of single‐burst population models (the Bruzual & Charlot models), we find that they are coeval with an age of 3.0±0.5 Gyr. Their metallicities are found to be solar to within ±0.15 dex. We discuss the properties of the main body of NGC 1316, and conclude that they are consistent with having hosted a major merger 3 Gyr ago as well. The presence of intermediate‐age globular clusters in NGC 1316 shows once again that globular clusters with near‐solar metallicity do form during galactic mergers, and, moreover, that they can survive disruption processes taking place during the merger (e.g., dynamical friction, tidal disruption), as well as evaporation. In this respect, NGC 1316 provides a hitherto ‘missing’ evolutionary link between young merger remnants of age ∼0.5 Gyr such as NGC 3597, 3921 and 7252 on one side and older giant ellipticals featuring bimodal colour distributions on the other side.

We present an overview of the GHOSTS survey, the largest study to date of the resolved stellar populations in the outskirts of disk galaxies. The sample consists of 14 disk galaxies within 17 Mpc, whose outer disks and halos are imaged with the Hubble Space Telescope Advanced Camera for Surveys (ACS). In the first paper of this series, we describe the sample, explore the benefits of using resolved stellar populations, and discuss our ACS F606W and F814W photometry. We use artificial star tests to assess completeness and use overlapping regions to estimate photometric uncertainties. The median depth of the survey at 50% completeness is 2.7 mag below the tip of the red giant branch (TRGB). We comprehensively explore and parameterize contamination from unresolved background galaxies and foreground stars using archival fields of high-redshift ACS observations. Left uncorrected, these would account for 10 0.65×F814W−19.0 detections per mag per arcsec 2. We therefore identify several selection criteria that typically remove 95% of the contaminants. Even with these culls, background galaxies are a significant limitation to the surface brightness detection limit which, for this survey, is typically V ∼ 30 mag arcsec −2. The resulting photometric catalogs are publicly available and contain some 3.1 million stars across 76 ACS fields, predominantly of low extinction. The uniform magnitudes of TRGB stars in these fields enable galaxy distance estimates with 2%-7% accuracy.

Abstract. We combine near-infrared photometry obtained with the VLT/ISAAC instrument and archival HST/WFPC2 optical images to determine VIK magnitudes and colours of globular clusters in two early-type galaxies, NGC 3115 and NGC 4365. The combination of near-IR and optical photometry provides a way to lift the age-metallicity degeneracy. For NGC 3115, the globular cluster colours reveal two major sub-populations, consistent with previous studies. By comparing the V − I, V − K colours of the NGC 3115 globular clusters with stellar populations models, we find that the colour difference between the two > ∼ 10 Gyr old major sub-populations is primarily due to a difference in metallicity. We find ∆[Fe/H] = 1.0 ± 0.3 dex and the blue (metal-poor) and red (metal-rich) globular cluster sub-populations being coeval within 3 Gyr. In contrast to the NGC 3115 globular clusters, the globular cluster system in NGC 4365 exhibits a more complex age and metallicity structure. We find a significant population of intermediate-age very metal-rich globular clusters along with an old population of both metal-rich and metal-poor clusters. Specifically, we observe a large population of globular clusters with red V − K colours but intermediate V − I colours, for which all current stellar population models give ages and metallicities in the range ∼2−8 Gyr and ∼0.5 Z −3 Z , respectively. After 10 Gyr of passive evolution, the intermediate-age globular clusters in NGC 4365 will have colours which are consistent with the very metal-rich population of globular clusters in giant elliptical galaxies, such as M 87. Our results for both globular cluster systems are consistent with previous age and metallicity studies of the diffuse galactic light. In addition to the major globular cluster populations in NGC 3115 and NGC 4365 we report on the detection of objects with extremely red colours (V − K > ∼ 3.8 mag), whose nature could not ultimately be revealed with the present data.