The Young Star Cluster group  

Group members and research topics

Stars rarely form in isolation. In fact, star formation in galaxies generally occurs in extended regions, where the fragmentation of the giant molecular clouds (GMCs) making up a significant fraction of a galaxy's interstellar medium (ISM) leads to the (almost simultaneous) gravitational collapse of multiple GMC subclumps. It is well known that the vast majority of stars in the Milky Way, and in nearby galaxies out to distances where individual stars and a variety of star cluster-type objects can be resolved by high-resolution observations, are found in groups ranging from binary stars to "OB" or "T Tauri" associations (young star-forming regions dominated by a small number of massive stars), open cluster-type objects, compact, old "globular" and young massive clusters, to supermassive clusters often confusingly referred to as "super star clusters". The nearest examples of these latter objects include the Milky Way star-forming region NGC 3603, and the giant starburst region 30 Doradus with its central star cluster R136 in the Large Magellanic Cloud.

In addition to a fraction of the more massive unbound OB associations, our Milky Way galaxy contains two main populations of gravitationally bound clusters with masses exceeding ~10³ M. The Milky Way's globular cluster population, consisting of some 150 compact objects with a median mass of M_cl 3 × 10⁵ M, is predominantly old, with ages 8-10 billion years. The much larger open cluster population (with a likely Galactic total number ~10⁵), on the other hand, is dominated by significantly younger ages (although open clusters up to the lower age limit of the globular cluster population do exist) and lower masses (10-10⁴ M). Although the older open clusters are undoubtedly gravitationally bound objects, their lower masses and more diffuse structures make them much more vulnerable to disk (and bulge) shocking when they pass through the Milky Way disk (or close to the bulge) on their orbits, thus leading to enhanced cluster evaporation. These objects are therefore unlikely globular cluster progenitors. It appears that the conditions for the formation of compact, massive star clusters - that have the potential to eventually evolve into globular cluster-type objects by the time they reach a similar age - are currently not present in the Milky Way, or at best to a very limited extent.

The production of luminous, massive yet compact star clusters seems to be a key feature of the most intense star-forming episodes. Such so-called "starbursts" normally occur at least once during the lifetimes of the vast majority of galaxies. The defining properties of young massive star clusters (with masses often significantly in excess of M_cl = 10⁵ M, i.e., the median mass of the abundant old globular clusters in the local Universe) have been explored in intense starburst regions in several dozen galaxies, often involved in gravitational interactions of some sort.

An increasingly large body of observational evidence suggests that a large fraction of the star formation in starbursts actually takes place in the form of such concentrated clusters, rather than in small-scale star-forming "pockets". Young massive star clusters are therefore important as benchmarks of cluster formation and evolution. They are also important as tracers of the history of star formation of their host galaxies, their chemical evolution, the initial mass function (IMF; i.e., the proportion of low to high-mass stars at the time of star formation), and other physical characteristics in starbursts.

2. An evolutionary connection?

The (statistical) derivation of galaxy formation and evolution scenarios using their star cluster systems as tracers is limited to the study of integrated cluster properties (such as their luminosities, sizes, masses, ages and metallicities) for all but the nearest galaxies, even at Hubble Space Telescope spatial resolution.

The question remains, therefore, whether or not at least a fraction of the young compact star clusters seen in abundance in extragalactic starbursts, are potentially the progenitors of globular cluster-type objects in their host galaxies. If we could settle this issue convincingly, one way or the other, the implications of such a result would have profound and far-reaching implications for a wide range of astrophysical questions, including (but not limited to) our understanding of the process of galaxy formation and assembly, and the process and conditions required for star (cluster) formation. Because of the lack of a statistically significant sample of similar nearby objects, however, we need to resort to either statistical arguments or to the painstaking approach of one-by-one studies of individual objects in more distant galaxies, as outlined below. With the ever increasing number of large-aperture ground-based telescopes equipped with state-of-the-art high-resolution spectroscopic detectors and the wealth of observational data provided by the Hubble Space Telescope we may now be getting close to resolving this important issue. It is of paramount importance, however, that theoretical developements go hand in hand with observational advances.

The present state-of-the-art teaches us that the sizes, luminosities, and - in several cases - spectroscopic mass estimates of most (young) extragalactic star cluster systems are fully consistent with the expected properties of young Milky Way-type globular cluster progenitors. For instance, for the young massive star cluster system in the centre of the nearby starburst spiral galaxy NGC 3310, we find a median mass of < log( M_cl / M ) > = 5.24 ± 0.05; their mass distribution is characterised by a Gaussian width of _Gauss 0.33 dex. In view of the uncertainties introduced by the poorly known lower-mass slope of the stellar IMF (m 0.5 M; see below), our median mass estimate of the NGC 3310 cluster system - which was most likely formed in a (possibly extended) global burst of cluster formation ~ 3 × 10⁷ yr ago - is remarkably close to that of the Milky Way globular cluster system.

However, the postulated evolutionary connection between the recently formed massive star clusters in regions of violent star formation and starburst galaxies, and old globular clusters similar to those in the Milky Way, the Andromeda galaxy, the giant elliptical galaxy M87 at the centre of the Virgo cluster, and other old elliptical galaxies is still a contentious issue. The evolution and survivability of young clusters depend crucially on the stellar IMF of their constituent stars: if the IMF is too shallow, i.e., if the clusters are significantly depleted in low-mass stars compared to (for instance) the solar neighbourhood, they will disperse within a few orbital periods around their host galaxy's centre, and likely within about a billion years of their formation.

Ideally, one would need to obtain (i) high-resolution spectroscopy (e.g., with 8m-class ground-based telescopes) of all clusters in a given cluster sample in order to obtain dynamical mass estimates (we will assume, for the purpose of the present discussion, that our young clusters can be approximated as systems in full virial equilibrium, so that the widths of their absorption lines reflect the clusters' internal velocity dispersions and therefore their masses) and (ii) high-resolution imaging (e.g., with the Hubble Space Telescope) to measure their luminosities. One could then estimate the mass-to-light (M/L) ratios for each cluster, and their ages from the features in their spectra. The final, crucial analysis would involve a direct comparison between the clusters' locations in the M/L ratio vs. age diagramme with models of so-called "simple stellar populations" (i.e., stellar populations of a single metallicity formed in an instantaneous burst of star formation) governed by a variety of IMF descriptions.

However, individual young star cluster spectroscopy, feasible today with 8m-class telescopes for the nearest systems, is very time-consuming, since observations of large numbers of clusters are required to obtain statistically significant results. Instead, one of the most important and most widely used diagnostics, both to infer the star (cluster) formation history of a given galaxy, and to constrain scenarios for its expected future evolution, is the distribution of cluster luminosities, or - alternatively - their associated masses, commonly referred to as the cluster luminosity and mass functions (CLF, CMF), respectively.

Starting with the seminal work by Elson & Fall (1985: PASP, 97, 692) on the young cluster system in the Large Magellanic Cloud (with ages 2 × 10⁹ yr), an ever increasing body of evidence, mostly obtained with the Hubble Space Telescope, seems to imply that the CLF of young star clusters (YSCs) is well described by a power law of the form N_YSC(L) dL L dL, where N_YSC(L) dL is the number of YSCs with luminosities between L and L + dL, and -2 -1.5. On the other hand, for the old globular cluster systems in the local Universe, with ages 10 billion years, the CLF shape is well established to be roughly Gaussian. This shape (characterised by its peak - or turn-over - magnitude and width) is almost universal, showing only a weak dependence on the metallicity and mass of the host galaxy.

This type of observational evidence has led to the popular - but thus far mostly speculative - theoretical prediction that not only a power-law, but any initial CLF (and CMF) will be rapidly transformed into a Gaussian distribution because of (i) stellar evolutionary fading of the lowest-luminosity (and therefore lowest-mass) clusters to below the detection limit; and (ii) disruption of the low-mass clusters due both to interactions with the gravitational field of the host galaxy, and to cluster-internal two-body relaxation effects (such as caused by star-star collisions and the resulting redistribution of mass inside the cluster) leading to enhanced cluster evaporation.

In summary, young, massive star clusters are the most significant end products of violent star-forming episodes (starbursts) triggered by galaxy collisions and gravitational interactions in general. Their contribution to the total luminosity induced by such extreme conditions dominates, by far, the overall energy output due to the gravitationally-induced star-formation. The general characteristics of these newly-formed clusters (such as their ages, masses, luminosities, and sizes) suggest that at least a fraction may eventually evolve into equal, or perhaps slightly more massive, counterparts of the abundant old globular cluster systems in the local Universe, although they will likely be more metal rich than the present generation of globular clusters. Establishing whether or not such an evolutionary connection exists requires our detailed knowledge of not only the physics underlying the evolution of "simple" stellar populations (i.e., idealised model clusters), but also that of cluster disruption in the time-dependent gravitational potentials of interacting galaxies. Initial results seem to indicate that proto-globular clusters do indeed continue to form today, which would support hierarchical galaxy formation scenarios. Settling this issue conclusively will have far-reaching consequences for our understanding of the process of galaxy formation and assembly, and of star formation itself, both of which processes are as yet poorly understood.