Star Clusters Near and Far

Building on Canadian expertise in studies of massive star clusters, we outline outstanding prospects in this field to which we can make fundamental contributions in the next decade and beyond. These include:

1. understanding cluster formation in the early Universe and interpreting future observations of globular cluster progenitors at high redshifts,
2. settling the question of the presence of intermediate-mass black holes in the cores of globular clusters,
3. mapping the kinematics and abundances in the outer parts of Galactic globular clusters to understand their formation and the interplay with Galactic tides,
4. solving the puzzle of multiple stellar populations in globular clusters,
5. unveiling and characterizing compact objects in dense clusters and their implications of gravitational-wave astrophysics, and
6. bringing studies of cluster populations as fossil records of the formation and evolution of external galaxies to the next level with large statistical samples. We highlight synergies between these science goals and upcoming and proposed ground- and space-based facilities.

Following up on globular cluster progenitor candidates at high redshift (identified by JWST) with a 30-m class ground-based telescope and adaptive optics will be crucial to characterize these systems and will provide important clues on the relative timing of GC formation in the context of galaxy assembly as well as the possible role of GCs in cosmic reionization. In the meantime, we have reached a stage where our computational methods can model GC formation, and these models will be important to interpret observations. The hydro+dynamics simulations needed are however very expensive, and will require Compute Canada to have capacity for such intensive simulations for our community to take full advantage of its expertise in the field of (massive) cluster formation and evolution.

Extreme Multi-Conjugate Adaptive Optics (MCAO) on a 30-m class telescope also promises important breakthroughs in this field. It is the most promising option for reaching a definitive answer on the presence of IMBHs in globular clusters and their importance in the growth of supermassive black holes. It would provide order-of-magnitude improvements in angular resolution and in the precision of proper motion measurements needed for dynamical analyses. Imaging and spectroscopy with 30-m class telescopes would also enable careful studies of the companions of compact objects in clusters, especially accreting BHs, allowing to study of the link between black holes in globular clusters and gravitational sources.

Important questions in the field can be tackled with deep, space-based, wide-field imaging at HST-like resolution, preferably in the blue optical/UV region. CASTOR represents a unique opportunity to fill this niche. The statistical samples of proper motion measurements over large regions that this would enable would be ideal to map the plane-of-the-sky kinematics of nearby clusters over their entire extent, for example to measure velocity anisotropy and its impact on the inferred mass/presence of IMBHs, or map the kinematics of clusters near the tidal boundary and disentangle scenarios about their formation and interaction with the Galactic tidal field. Moreover, CASTOR imaging would offer opportunities to probe the host environments of neutron star mergers and constrain their dynamical formation in GCs. At larger distances, CASTOR’s UV and blue-optical coverage would complete the SED coverage of other missions over the critical UVOIR region, allowing complete photometric characterization of potentially many tens of thousands of galaxies and their cluster systems in the local volume.

MSE with its large field of view, would also be ideally suited for mapping the kinematics (line-of-sight velocities) in the outskirts of globular clusters, out to the tidal radius. But crucially, it would yield detailed chemical composition for large samples of stars in globular clusters, significantly improving upon current surveys. As such, it would be the most important next-generation facility to address the origin of multiple (chemical) populations in these systems, a great unsolved problems in the study of stellar populations. MSE would also make it possible to build large spectroscopic databases of star cluster populations in external galaxies.

Finally, upcoming radio facilities like ngVLA and SKA will provide a significant step forward towards a better determination of BH populations in GCs, with substantial improvement in sensitivity that would allow the identification of typical weakly-accreting black hole candidates in clusters, as well as thoroughly test the presence of weakly-accreting intermediate-mass black holes. Significant improvements in the study of compact objects in clusters will also be possible with future advanced X-ray telescopes with higher spectral resolution and sensitivity, such as ESA’s Athena and NASA’s Lynx missions.