Populations In Globular Clusters Research Articles

A Gemini/GMOS study of the bright elliptical galaxy NGC 3613 and its globular cluster system

ABSTRACT We present the first photometric study of the globular cluster system (GCS) of the E galaxy NGC 3613 (MV = −21.5, d ∼ 30.1 Mpc), as well as the surface photometry of the host galaxy, based on Gemini/GMOS images. Being considered the central galaxy of a group, NGC 3613 inhabits a low-density environment although its intrinsic brightness is similar to the expected one for galaxies in the centre of clusters. The following characteristics are obtained for this GCS. The colour distribution is bimodal, with metal-poor globular clusters (GCs) getting slightly bluer with increasing radius. The radial and azimuthal projected distributions show that metal-rich GCs are more concentrated towards the host galaxy and trace its light distribution very precisely, while metal-poor GCs present a more extended and uniform distribution. The GC luminosity function helps validate the adopted distance. The estimated total GC population of Ntot = 2075 ± 130 leads to a specific frequency SN = 5.2 ± 0.7, a value within the expected range for GCSs with host galaxies of similar luminosity. The surface photometry of NGC 3613 reveals a three-component profile and a noticeable substructure. Finally, a small sample of ultracompact dwarf candidates are identified in the surroundings of the host galaxy.

The Ophiuchus stream progenitor: a new type of globular cluster and its possible Sagittarius connection

ABSTRACT The Ophiuchus stream is a short arc-like stellar feature of uncertain origin located ∼5 kpc North of the Galactic centre. New proper motions from the second Gaia data release reconcile the direction of motion of stream members with the stream arc, resolving a puzzling mismatch reported in earlier work. We use N-body simulations to show that the stream is likely only on its second pericentric passage, and thus was formed recently. The simulations suggest that most of the disrupted progenitor is visible in the observed stream today, and that little further tidal debris is expected to lie beyond the ends of the stream. The luminosity, length, width, and velocity dispersion of the stream suggest a globular cluster (GC) progenitor substantially fainter and of lower surface brightness than estimated in previous work, and unlike any other known globulars in the Galaxy. This result suggests the existence of clusters that would extend the known GC population to fainter and more weakly bound systems than hitherto known. How such a weakly bound cluster of old stars survived until it was disrupted so recently, however, remains a mystery. Integrating backwards in time, we find that the orbits of Sagittarius and Ophiuchus passed within ∼5 kpc of each other about ∼100 Myr ago, an interaction that might help resolve this puzzle.

The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters – XXI. Binaries among multiple stellar populations

Abstract A number of scenarios for the formation of multiple populations in globular clusters (GCs) predict that second generation (2G) stars form in a compact and dense subsystem embedded in a more extended first-generation (1G) system. If these scenarios are accurate, a consequence of the denser 2G formation environment is that 2G binaries should be more significantly affected by stellar interactions and disrupted at a larger rate than 1G binaries. The fractions and properties of binary stars can thus provide a dynamical fingerprint of the formation epoch of multiple-population GCs and their subsequent dynamical evolution. We investigate the connection between binaries and multiple populations in five GCs, NGC 288, NGC 6121 (M 4), NGC 6352, NGC 6362, and NGC 6838 (M 71). To do this, we introduce a new method based on the comparison of Hubble Space Telescope observations of binaries in the F275W, F336W, F438W, F606W, and F814W filters with a large number of simulated binaries. In the inner regions probed by our data, we do not find large differences between the local 1G and the 2G binary incidences in four of the studied clusters, the only exception being M 4 where the 1G binary incidence is about three times larger than the 2G incidence. The results found are in general agreement with the results of simulations predicting significant differences in the global 1G and 2G incidences and in the local values in the clusters’ outer regions but similar incidences in the inner regions. The significant difference found in M 4 is consistent with simulations with a larger fraction of wider binaries. Our analysis also provides the first evidence of mixed (1G–2G) binaries, a population predicted by numerical simulations to form in a cluster’s inner regions as a result of stellar encounters during which one component of a binary is replaced by a star of a different population.

The [α/Fe]–[Fe/H] relation in the E-MOSAICS simulations: its connection to the birth place of globular clusters and the fraction of globular cluster field stars in the bulge

ABSTRACT The α-element abundances of the globular cluster (GC) and field star populations of galaxies encode information about the formation of each of these components. We use the E-MOSAICS cosmological simulations of ∼L* galaxies and their GCs to investigate the [α/Fe]–[Fe/H] distribution of field stars and GCs in 25 Milky Way–mass galaxies. The [α/Fe]–[Fe/H] distribution of GCs largely follows that of the field stars and can also therefore be used as tracers of the [α/Fe]–[Fe/H] evolution of the galaxy. Due to the difference in their star formation histories, GCs associated with stellar streams (i.e. which have recently been accreted) have systematically lower [α/Fe] at fixed [Fe/H]. Therefore, if a GC is observed to have low [α/Fe] for its [Fe/H] there is an increased possibility that this GC was accreted recently alongside a dwarf galaxy. There is a wide range of shapes for the field star [α/Fe]–[Fe/H] distribution, with a notable subset of galaxies exhibiting bimodal distributions, in which the high [α/Fe] sequence is mostly comprised of stars in the bulge, a high fraction of which are from disrupted GCs. We calculate the contribution of disrupted GCs to the bulge component of the 25 simulated galaxies and find values between 0.3 and 14 per cent, where this fraction correlates with the galaxy’s formation time. The upper range of these fractions is compatible with observationally inferred measurements for the Milky Way, suggesting that in this respect the Milky Way is not typical of L*galaxies, having experienced a phase of unusually rapid growth at early times.

Multiple Populations in Globular Clusters: Unified Efforts from Stellar Evolution and Chemical Evolution Models

Abstract Recent stellar evolution models for globular clusters (GCs) in a multiple population paradigm suggest that horizontal-branch (HB) morphology and the mean period of type ab RR Lyrae variables are mostly determined by He and CNO abundances and relative ages for subpopulations. These parameters are also provided by chemical evolution models constructed to reproduce the Na–O anticorrelation. Therefore, a consistency check is possible between the synthetic HB and chemical evolution models. Furthermore, by combining them, a better constraint might be attained for star formation history and chemical abundances of subpopulations in GCs. We find, from such efforts made for four GCs, M4, M5, M15, and M80, that consistent results can be obtained from these two independent studies. In our unified model, He and Na abundances gradually increase over the generation, and, therefore, the various extensions observed in both HB morphology and the Na–O chemical pattern depend on the presence of later generation stars after the second generation. It is schematically shown that this observed diversity, however, would not be naturally explained by the models requiring dilution. Further spectroscopic observations are required, for metal-poor GCs in particular, to obtain a more detailed constraint from this approach.

Cusp or core? Revisiting the globular cluster timing problem in Fornax

ABSTRACT We use N-body simulations to revisit the globular cluster (GC) ‘timing problem’ in the Fornax dwarf spheroidal (dSph). In agreement with earlier work, we find that, due to dynamical friction, GCs sink to the centre of dark matter haloes with a cuspy inner density profile but ‘stall’ at roughly 1/3 of the core radius (rcore) in haloes with constant-density cores. The time-scales to sink or stall depend strongly on the mass of the GC and on the initial orbital radius, but are essentially the same for either cuspy (Navarro–Frenk–White) or cored haloes normalized to have the same total mass within rcore. Arguing against a cusp on the basis that GCs have not sunk to the centre is thus no different from arguing against a core, unless all clusters are today at $\sim(1/3)\,r_{\rm core}$. This would imply a core radius exceeding ∼3 kpc, much larger than seems plausible in any core-formation scenario. (The average projected distance of Fornax GCs is 〈RGC, Fnx〉 ∼ 1 kpc and its effective radius is ∼700 pc.) A simpler explanation is that Fornax GCs have only been modestly affected by dynamical friction, as expected if clusters started orbiting at initial radii of the order of ∼1–2 kpc, just outside Fornax’s present-day half-light radius but well within the tidal radius imprinted by Galactic tides. This is not entirely unexpected. Fornax GCs are significantly older and more metal-poor than most Fornax stars, and such populations in dSphs tend to be more spatially extended than their younger and more metal-rich counterparts. Contrary to some earlier claims, our simulations further suggest that GCs do not truly ‘stall’ at $\sim 0.3\, r_{\rm core}$, but rather continue decaying towards the centre, albeit at reduced rates. We conclude that dismissing the presence of a cusp in Fornax based on the spatial distribution of its GC population is unwarranted.

The possible role of stellar mergers for the formation of multiple stellar populations in globular clusters

ABSTRACTMany possible scenarios for the formation of multiple stellar populations (MSPs) in globular clusters (GCs) have been discussed so far, including the involvement of asymptotic giant branch stars, fast-rotating main-sequence stars, very massive main-sequence stars and mass-transferring massive binaries based on stellar evolution modelling. But self-consistent, dynamical simulations of very young GCs are usually not considered. In this work, we perform direct N-body modelling of such systems with total masses up to 3.2 × 105 M⊙, taking into account the observationally constrained primordial binary properties, and discuss the stellar mergers driven both by binary stellar evolution and dynamical evolution of GCs. The occurrence of stellar mergers is enhanced significantly in binary-rich clusters such that stars forming from the gas polluted by merger-driven ejection/winds would appear as MSPs. We thus emphasize that stellar mergers can be an important process that connects MSP formation with star cluster dynamics, and that multiple MSP formation channels can naturally work together. The scenario studied here, also in view of a possible top-heavy initial mass function, may be particularly relevant for explaining the high mass fraction of MSPs (the mass budget problem) and the absence of MSPs in young and low-mass star clusters.

Multiple populations in globular clusters and their parent galaxies

ABSTRACT The ‘chromosome map’ diagram (ChM) proved a successful tool to identify and characterize multiple populations (MPs) in 59 Galactic globular clusters (GCs). Here, we construct ChMs for 11 GCs of both Magellanic Clouds (MCs) and with different ages to compare MPs in Galactic and extragalactic environments, and explore whether this phenomenon is universal through ‘place’ and ‘time’. MPs are detected in five clusters. The fractions of 1G stars, ranging from ∼50 per cent to >80 per cent, are significantly higher than those observed in Galactic GCs with similar present-day masses. By considering both Galactic and MC clusters, the fraction of 1G stars exhibits: (i) a strong anticorrelation with the present-day mass, and (ii) with the present-day mass of 2G stars; (iii) a mild anticorrelation with 1G present-day mass. All Galactic clusters without MPs have initial masses smaller than ∼1.5 · 105 M⊙ but a mass threshold governing the occurrence of MPs seems challenged by massive simple-population MC GCs; (iv) Milky Way clusters with large perigalactic distances typically host larger fractions of 1G stars, but the difference disappears when we use initial cluster masses. These facts are consistent with a scenario where the stars lost by GCs mostly belong to the 1G. By exploiting recent work based on Gaia, half of the known Type II GCs appear clustered in a distinct region of the integral of motions space, thus suggesting a common progenitor galaxy. Except for these Type II GCs, we do not find any significant difference in the MPs between clusters associated with different progenitors.

The Role of Cluster Mass in the Multiple Populations of Galactic and Extragalactic Globular Clusters

Abstract Studies based on Hubble Space Telescope (HST) photometry in F275W, F336W, and F438W have shown that the incidence and complexity of multiple populations (MPs) in globular clusters (GCs) depend on cluster mass. This result, however, is based on nearby, low-reddening Galactic GCs, for which we can obtain accurate F275W photometry. In this work, we exploit HST photometry in F336W, F438W, and F814W to characterize MPs in 68 Galactic and extragalactic GCs by using the pseudocolor C F336W,F438W,F814W, which is more efficient in terms of observation time than the pseudocolor C F275W,F336W,F438W adopted in previous works. We first analyze the Galactic GCs alone and find that the red giant branch (RGB) color width strongly correlates with [Fe/H]. After removing the dependence from metallicity, we obtain a strong correlation with cluster mass, thus confirming previous findings. We then exploit the RGB width to compare MPs in Galactic and extragalactic GCs. Similarly to Galactic GCs, the RGB width of Magellanic Cloud (MC) GCs correlates with cluster mass after removing the dependence from metallicity. This fact demonstrates that cluster mass is the main factor affecting the properties of MPs. The MC clusters exhibit, on average, narrower RGBs than Galactic GCs with similar mass and metallicity. We suggest that this difference is a signature of stellar mass loss in Galactic GCs. As an alternative, the mass–RGB–width relation would depend on the host galaxy. Finally, we use ground-based photometry to investigate Terzan 7 through the C U,B,I pseudocolor, ground-based analogs of C F336W,F438W,F814W and find that this cluster is consistent with a simple population.

The globular cluster population of NGC 1052-DF2: evidence for rotation

ABSTRACT Based upon the kinematics of 10 globular clusters, it has recently been claimed that the ultra-diffuse galaxy, NGC 1052-DF2, lacks a significant quantity of dark matter. Dynamical analyses have generally assumed that this galaxy is pressure supported, with the relatively small velocity dispersion of the globular cluster population indicating the deficit of dark matter. However, the presence of a significant rotation of the globular cluster population could substantially modify this conclusion. Here, we present the discovery of such a signature of rotation in the kinematics of NGC 1052-DF2’s globular clusters, with a velocity amplitude of ${\sim}12.44^{+4.40}_{-5.16}$ km s−1, which, through Bayesian model comparison, represents a marginally better fit to the available kinematic data; note that this rotation is distinct from, and approximately perpendicular to, the recently identified rotation of the stellar component of NGC 1052-DF2. Assuming this truly represents an underlying rotation, it is shown that the determined mass depends upon the inclination of the rotational component and, with a moderate inclination, the resultant mass-to-light ratio can exceed M/L ∼ 10.

A Family Picture: Tracing the Dynamical Path of the Structural Properties of Multiple Populations in Globular Clusters

Abstract We studied the spatial distributions of multiple stellar populations (MPs) in a sample of 20 globular clusters (GCs) spanning a broad range of dynamical ages. The differences between first-population (FP) and second-population (SP) stars were measured by means of the parameter A +, defined as the area enclosed between their cumulative radial distributions. We provide the first purely observational evidence of the dynamical path followed by MPs from initial conditions toward a complete FP–SP spatial mixing. Less dynamically evolved clusters have SP stars more centrally concentrated than FPs, while in more dynamically evolved systems the spatial differences between FP and SP stars decrease and eventually disappear. By means of an appropriate comparison with a set of numerical simulations, we show that these observational results are consistent with the evolutionary sequence expected by the long-term dynamical evolution of clusters forming with an initially more centrally concentrated SP subsystem. This result is further supported by the evidence of a trend between A + and the stage of GC dynamical evolution inferred by the ratio between the present-day and the initial mass of the cluster.

Two major accretion epochs in M31 from two distinct populations of globular clusters.

Large galaxies grow through the accumulation of dwarf galaxies1,2. In principle it is possible to trace this growth history viathe properties of a galaxy's stellar halo3-5. Previous investigations of the galaxy Messier 31 (M31, Andromeda) have shown that outside a galactocentric radius of 25kiloparsecs the population of halo globular clusters is rotating in alignment with the stellar disk6,7, as are more centrally located clusters8,9. The M31 halo also contains coherent stellar substructures, along with a smoothly distributed stellar component10-12. Many of the globular clusters outside a radius of 25 kiloparsecs are associated with the most prominent substructures, but some are part of the smooth halo13. Here we report an analysis of the kinematics of these globular clusters. We find two distinct populations rotating perpendicular to each other. The rotation axis for the population associated with the smooth halo is aligned with the rotation axis for the plane of dwarf galaxies14 that encircles M31. We interpret these separate cluster populations as arising from two major accretion epochs, probably separated by billions of years. Stellar substructures from the first epoch are gone, but those from the more recent second epoch still remain.

Multiple Stellar Populations of Globular Clusters from Homogeneous Ca–CN–CH Photometry. V. as a Surrogate cnJWL Index and NGC 6723∗ †

Abstract We introduce new color indices (=Ca CTIO − Ca JWL) and ch JWL [=(JWL43 − b) − (b − y)], accurate photometric measures of the CN band at λ3883 and the CH G band, respectively, in the study of the multiple populations (MPs) in globular clusters (GCs). Our photometric CN–CH relation for a large number of red-giant branch (RGB) in M5 shows that the evolutions of the CN and CH between the CN-w and CN-s populations are not continuous. Armed with our new color indices, we investigate the MPs of NGC 6723, finding the RGB populational number ratio of n(CN-w):n(CN-s) ≈ 35.5:64.5 (±2.8) with no radial gradient. Similar to other normal GCs with MPs, the helium abundance of the CN-s population inferred from the RGB bump magnitude is slightly enhanced by ΔY = 0.012 ± 0.012. Our cn JWL and color–magnitude diagrams clearly show the discrete double asymptotic giant branch (AGB) populations in NGC 6723, whose bright AGB populational number ratio is in marginal agreement with that of the RGB stars within the statistical errors. Finally, our synthetic cn JWL index is in good agreement with observations, except for the CN-w AGB. Mitigation of the discrepancy in the CN-w AGB may require a mild nitrogen enhancement and/or a large decrement in the 12C/13C ratio with respect to the bright RGB.

The SLUGGS survey: measuring globular cluster ages using both photometry and spectroscopy

ABSTRACT Globular cluster ages provide both an important test of models of globular cluster formation and a powerful method to constrain the assembly history of galaxies. Unfortunately, measuring the ages of unresolved old stellar populations has proven challenging. Here, we present a novel technique that combines optical photometry with metallicity constraints from near-infrared spectroscopy in order to measure ages. After testing the method on globular clusters in the Milky Way and its satellite galaxies, we apply our technique to three massive early-type galaxies using data from the SAGES Legacy Unifying Globulars and GalaxieS (SLUGGS) survey. The three SLUGGS galaxies and the Milky Way show dramatically different globular cluster age and metallicity distributions, with NGC 1407 and the Milky Way showing mostly old globular clusters, while NGC 3115 and NGC 3377 show a range of globular ages. This diversity implies different galaxy formation histories and that the globular cluster optical colour–metallicity relation is not universal as is commonly assumed in globular cluster studies. We find a correlation between the median age of the metal-rich globular cluster populations and the age of the field star populations, in line with models where globular cluster formation is a natural outcome of high-intensity star formation.

Characteristic radii of the Milky Way globular clusters

ABSTRACT We report on the extent of the effects of the Milky Way gravitational field in shaping the structural parameters and internal dynamics of its globular cluster population. We make use of a homogeneous, up-to-date data set with kinematics, structural properties, current and initial masses of 156 globular clusters. In general, cluster radii increase as the Milky Way potential weakens; with the core and Jacobi radii being those which increase at the slowest and fastest rate, respectively. We interpret this result as the innermost regions of globular clusters being less sensitive to changes in the tidal forces with the Galactocentric distance. The Milky Way gravitational field also seems to have differentially accelerated the internal dynamical evolution of individual clusters, with those toward the bulge appearing dynamically older. Finally, we find a subpopulation consisting of both compact and extended globular clusters (as defined by their rh/rJ ratio) beyond 8 kpc that appear to have lost a large fraction of their initial mass lost via disruption. Moreover, we identify a third group with rh/rJ > 0.4, which have lost an even larger fraction of their initial mass by disruption. In both cases the high fraction of mass lost is likely due to their large orbital eccentricities and inclination angles, which lead to them experiencing more tidal shocks at perigalacticon and during disc crossings. Comparing the structural and orbital parameters of individual clusters allows for constraints to be placed on whether or not their evolution was relaxation or tidally dominated.

Formation Imprints in the Kinematics of the Milky Way Globular Cluster System

We report results on the kinematics of Milky Way (MW) globular clusters (GCs) based on updated space velocities for nearly the entire GC population. We found that a 3D space with the semi-major axis, the eccentricity and the inclination of the orbit with respect to the MW plane as its axes is helpful in order to dig into the formation of the GC system. We find that GCs formed in-situ show a clear correlation between their eccentricities and their orbital inclination in the sense that clusters with large eccentricities also have large inclinations. These GCs also show a correlation between their distance to the MW center and their eccentricity. Accreted GCs do not exhibit a relationship between eccentricity and inclination, but span a wide variety of inclinations at eccentricities larger than ~ 0.5. Finally, we computed the velocity anisotropy "beta" of the GC system and found for GCs formed in-situ that "beta" decreases from ~ 0.8 down to 0.3 from the outermost regions towards the MW center, but remains fairly constant (0.7-0.9) for accreted ones. These findings can be explained if GCs formed from gas that collapsed radially in the outskirts, with preference for relative high infall angles. As the material reached the rotating forming disk, it became more circular and moved with lower inclination relative to the disk. A half of the GC population was accreted and deposited in orbits covering the entire range of energies from the outer halo to the bulge.

Hypervelocity stars from star clusters hosting intermediate-mass black holes

ABSTRACT Hypervelocity stars (HVSs) represent a unique population of stars in the Galaxy reflecting properties of the whole Galactic potential. Determining their origin is of fundamental importance to constrain the shape and mass of the dark halo. The leading scenario for the ejection of HVSs is an encounter with the supermassive black hole in the Galactic centre. However, new proper motions from the Gaia mission indicate that only the fastest HVSs can be traced back to the Galactic centre and the remaining stars originate in the disc or halo. In this paper, we study HVSs generated by encounters of stellar binaries with an intermediate-mass black hole (IMBH) in the core of a star cluster. For the first time, we model the effect of the cluster orbit in the Galactic potential on the observable properties of the ejected population. HVSs generated by this mechanism do not travel on radial orbits consistent with a Galactic centre origin, but rather point back to their parent cluster, thus providing observational evidence for the presence of an IMBH. We also model the ejection of high-velocity stars from the Galactic population of globular clusters, assuming that they all contain an IMBH, including the effects of the cluster’s orbit and propagation of the star in the Galactic potential up to detection. We find that high-velocity stars ejected by IMBHs have distinctive distributions in velocity, Galactocentric distance and Galactic latitude, which can be used to distinguish them from runaway stars and stars ejected from the Galactic Centre.

The ultra-diffuse dwarf galaxies NGC 1052-DF2 and 1052-DF4 are in conflict with standard cosmology

ABSTRACT Recently van Dokkum et al. reported that the galaxy NGC 1052-DF2 (DF2) lacks dark matter if located at 20 Mpc from Earth. In contrast, DF2 is a dark-matter-dominated dwarf galaxy with a normal globular cluster population if it has a much shorter distance near 10 Mpc. However, DF2 then has a high peculiar velocity wrt. the cosmic microwave background of 886 $\rm {km\, s^{-1}}$, which differs from that of the Local Group (LG) velocity vector by 1298 $\rm {km\, s^{-1}}$ with an angle of $117 \, ^{\circ }$. Taking into account the dynamical M/L ratio, the stellar mass, half-light radius, peculiar velocity, motion relative to the LG, and the luminosities of the globular clusters, we show that the probability of finding DF2-like galaxies in the lambda cold dark matter (ΛCDM) TNG100-1 simulation is at most 1.0 × 10−4 at 11.5 Mpc and is 4.8 × 10−7 at 20.0 Mpc. At 11.5 Mpc, the peculiar velocity is in significant tension in the TNG100-1, TNG300-1, and Millennium simulations, but naturally in a Milgromian cosmology. At 20.0 Mpc, the unusual globular cluster population would challenge any cosmological model. Estimating that precise measurements of the internal velocity dispersion, stellar mass, and distance exist for 100 galaxies, DF2 is in 2.6σ (11.5 Mpc) and 4.1σ (20.0 Mpc) tension with standard cosmology. Adopting the former distance for DF2 and assuming that NGC 1052-DF4 is at 20.0 Mpc, the existence of both is in tension at ≥4.8σ with the ΛCDM model. If both galaxies are at 20.0 Mpc the ΛCDM cosmology has to be rejected by ≥5.8σ.

Is there a fundamental upper limit to the mass of a star cluster?

Abstract The discovery around the turn of the millennium of a population of very massive (M⋆ > 2 × 106 M⊙) compact stellar systems (CSS) with physical properties (radius, velocity dispersion, stellar mass etc.) that are intermediate between those of the classical globular cluster (GC) population and galaxies led to questions about their exact nature. Recently a consensus has emerged that these objects, usually called ultracompact dwarfs (UCDs), are a mass-dependent mixture of high-mass star clusters and remnant nuclei of tidally disrupted galaxies. The existence of genuine star clusters with stellar masses >107 M⊙ naturally leads to questions about the upper mass limit of the star cluster formation process. In this work we compile a comprehensive catalogue of CSS, and reinforce the evidence that the true ancient star cluster population has a maximum mass of M⋆ ∼ 5 × 107 M⊙, corresponding to a stellar mass at birth of close to 108 M⊙. We then discuss several physical and statistical mechanisms potentially responsible for creating this limiting mass.

A model for the minimum mass of bound stellar clusters and its dependence on the galactic environment

ABSTRACT We present a simple physical model for the minimum mass of bound stellar clusters as a function of the galactic environment. The model evaluates which parts of a hierarchically clustered star-forming region remain bound given the time-scales for gravitational collapse, star formation, and stellar feedback. We predict the initial cluster mass functions (ICMFs) for a variety of galaxies and we show that these predictions are consistent with observations of the solar neighbourhood and nearby galaxies, including the Large Magellanic Cloud and M31. In these galaxies, the low minimum cluster mass of ∼102 M⊙ is caused by sampling statistics, representing the lowest mass at which massive (feedback-generating) stars are expected to form. At the high gas density and shear found in the Milky Way’s Central Molecular Zone and the nucleus of M82, the model predicts that a mass >102 M⊙ must collapse into a single cluster prior to feedback-driven dispersal, resulting in narrow ICMFs with elevated characteristic masses. We find that the minimum cluster mass is a sensitive probe of star formation physics due to its steep dependence on the star formation efficiency per free-fall time. Finally, we provide predictions for globular cluster (GC) populations, finding a narrow ICMF for dwarf galaxy progenitors at high redshift, which can explain the high specific frequency of GCs at low metallicities observed in Local Group dwarfs like Fornax and WLM. The predicted ICMFs in high-redshift galaxies constitute a critical test of the model, ideally suited for the upcoming generation of telescopes.