Letters of Intent received in 2015

LoI 2017-289
Constraining galaxy formation and evolution using massive star clusters

Topics

- Relation between field stars and globular clusters
- Formation of massive stellar clusters
- Are YMCs young GCs?
- Multiple populations within GCs
- Scaling relations between GCs, dark matter and super massive black holes
- The origin of the metallicity distribution of old globular clusters
- Dynamical evolution of dense stellar systems
- Finding and characterising massive proto-YMC molecular clouds - Properties of old GCs: nature or nurture?
- Current state of cluster formation simulation

Rationale

Globular clusters (GCs) are among the first baryonic structures to form at high redshift and they are intimately linked with the formation and earliest phases of galaxy evolution. Their spatial distribution resembles that of the dark matter distribution of their host galaxy and tight scaling relations between the mass of the central super-massive black hole and the number of GCs hold over many orders of magnitude. Despite their importance for our understanding of the stellar initial mass function, star formation, re-ionisation and chemical evolution of the Universe, their origin and exact role in galaxy formation is still shrouded in mystery.

Owing to the superb spatial resolution of the Hubble Space Telescope (HST), GCs can be resolved up to distances of several 10s of Mpc. In the last decades, star clusters with masses similar to the old GCs in the Milky Way, but with ages as young as a few Myr, have been discovered by HST. These so-called young massive clusters (YMCs) are predominantly found in star-bursting galaxies, but also in more quiescent spiral and irregular galaxies in the Local Universe. More recently, the sensitivity and resolution of the Atacama Large Millimeter Array (ALMA) allowed us to chart the giant molecular clouds from which YMCs are thought to form, both in the centre of the Milky Way, and as far out as the merging Antennae galaxies. The YMCs seem to form in the high-density tail of the density distribution of interstellar medium, suggesting that YMCs are simply the tip of the iceberg and no special conditions are required for their formation. If this is true, then the environments of YMCs, and their progenitor clouds, offer an exiting window into star formation at high redshift!

Additional support for the idea that both old GCs and YMCs are part of the same family comes from the similarity between individual YMCs and old GCs in terms of their structural properties. YMC populations, however, are notably different from GC populations. Most striking is the absence of a mass scale: the luminosity functions of YMCs are (scale-free) power-laws with indications for an environment dependent cut-off. If GCs formed with a similar mass function the low-mass clusters need to be preferentially lost because of evaporation in the tidal field. The near universal peak in the luminosity function of GCs puts extremely strong constraints on the evolution of GCs and it is still an open question whether dynamical evolution in realistic external boundary conditions allows to evolve the properties of YMCs into what is observed for the ancient GCs. The initial GC mass function is of importance for our understanding of star formation of high redshift, but its shape also has implications for the number of disrupted GCs that populate the halos of galaxies. The disruption of GCs leads to cold tidal streams, which are power-full tools in unraveling the (dark) mass distribution in the Milky Way. The success of mass-modelling methods relying on cold streams depends sensitively on the number of cold streams, and their distribution within the halo.

Besides the extra-galactic discoveries, the HST also revolutionised our view on resolved GCs in the Milky Way. These were considered to be the archetypical simple stellar population, containing stars of the same age and chemical composition. Thanks to HST’s ability to select member stars based on their proper motions, multiple parallel main sequences were discovered in the colour-magnitude diagrams of GCs. In order to explain these observations, stellar evolution models require the majority of the stars in GCs to be significantly enriched in Helium, with respect to the canonical Big Bang nucleosynthesis value. In addition, spreads in light elements were found from spectroscopic studied, which have been attributed to hydrogen burning at high temperatures, but so far no model has been able to successfully explain the peculiar features for all observed GCs. The light element anomalies and multiple main sequences appear to be unique to old GCs, and have despite various attempts, not been found in YMCs so far. This intriguing difference between old GCs and YMCs suggests that there may after all be something unique about the old GCs. The clue to this difference could lie in understanding the conditions in the early Universe and the scales we need to consider to solve the “multiple population problem” are potentially much larger than hitherto considered.

Stellar cluster research will see a tremendous growth in the next decade. ALMA is operational and putting constraints on the formation of YMCs and the ESA-Gaia mission is charting the distribution of a billion Milky Way stars. Ambitious ground based surveys have been designed to complement the Gaia astrometry and proper motions with metallicity and abundance data (HERMES, Gaia-ESO, etc.) and will provide a gold mine of data on the formation of the Milky Way and its different components, including the GCs. Extra-galactic surveys are exploring the properties of GCs in galaxies of all Hubble type and the James Webb Space Telescope will be able to resolve GC formation at redshifts of z~5. Finally, numerical simulations solving for gravity and hydro-dynamics are getting close to forming Milky Way-type galaxies with realistic properties within the Lambda Cold Dark Matter (LCDM) framework and are soon reaching the resolution limit that allows to resolve the formation of individual GCs. In light of the arrival of all these unprecedented observational data and the tremendous progress on the computational side, this IAU symposium is extremely timely. We will address the above questions and aim to understand the formation of GCs within the cosmological context by reaching out and bring together both the galaxy formation community and the massive star cluster community.