Research summary
Young, massive star
clusters are the most notable and significant end products of violent
star-forming episodes triggered by galaxy collisions, mergers, and close
encounters. 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 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. 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, but
also that of cluster disruption in the time-dependent gravitational
potentials of interacting galaxies. Initial results seem to indicate that
proto-globular clusters indeed continue to form today, which would support
hierarchical galaxy formation scenarios.
1. Extreme environmental conditions
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 Mcl 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
Mcl = 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(
Mcl / 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
NYSC(L) dL
L dL, where
NYSC(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.