Letters of Intent received in 2016

LoI 2018-1939
Constraining galaxy formation and evolution using massive star clusters

Topics

- The co-evolution of galaxies and their globular cluster populations
- Relation between field stars and globular clusters
- Formation of massive stellar clusters
- Are YMCs young GCs?
- Multiple populations within GCs
- Scaling relations between host galaxy properties (GCs, dark matter, etc.) and supermassive black holes and nuclear star clusters
- The origin of the metallicity distribution of old globular clusters
- Formation of the most massive stellar clusters (nuclear clusters, ultra-compact dwarf galaxies)
- Dynamical evolution of dense stellar systems
- Properties of old GCs: nature or nurture?

Rationale

The old GCs have a bi-modal colour distribution, and the current consensus is that this is due to a bi-modal metallicity (i.e. [Fe/H]) distribution. Their are various attempts to explain the origin of this modality which all find their origin in the hierarchical build-up of galaxies at high redshift (z>~3). The metal-poor (blue) GCs have a more extended distribution within their host galaxy than the field stars, while the metal-rich (red) GCs follow the star light. A possible explanation for this is that the blue GCs were accreted, while the red GCs formed in-situ. Further support for this comes from the age-metallicity relation of resolved GCs in the Local Group and chemical evolution models. The stellar populations of the most massive GCs in the Milky Way (i.e. Omega Cen, NGC 2419), and the similarity with M54 in the centre of the Sagittarius dwarf galaxy, suggests that these are the former nuclei of accreted dwarf galaxies. The details of the formation and evolution of nuclear star clusters, and their (massive) black hole, and the possible link with extra-galactic ultra-compact dwarf galaxies (UCDs) is a topic of ongoing debate and of importance for our understanding of galaxy formation.

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 exciting window into star formation at high redshift!

Additional support for the idea that 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 studies, 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 mostly found in old GCs. 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, and the first data was just released to the community. 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. Because this symposium has strong links with various astrophysical topics and IAU Divisions (ISM and Local Universe [Division H]; Galaxy formation [Division J]; Stars and stellar physics [Division G]; Computational astrophysics [Division B, Commission B1] and the inter-divisionG-H-J H4 [Commission Stellar Clusters throughout Cosmic Space and Time]) we believe this IAUS is ideally suited for the General Assembly, as we envision participation from a broad spectrum of the community.