The Formation of Globular Clusters

Abstract: Globular clusters (GCs) are among the oldest and most luminous stellar systems in the Universe, offering unique insights into galaxy formation and evolution. While the physical processes behind their origin have long remained elusive, major theoretical and observational developments in the past decade have led to a new understanding of GCs as the natural outcome of high-pressure star formation in high-redshift galaxies. This review synthesizes recent advancements in our understanding of GC formation and aims to provide a comprehensive point of reference for leveraging the revolutionary capabilities of the current and upcoming generation of telescopes. The latest generation of GC models combine our understanding of their formation and destruction with advanced galaxy formation simulations. The next decade will provide the first-ever opportunity to test such models across their full evolutionary history, from GC formation at high redshift as seen with the James Webb Telescope, to snapshots of GC demographics at intermediate redshifts obtained with 30m-class telescopes, and eventually to the well-characterized GC populations observed at the present day. We identify the major questions that we should expect to address this way.

• The paper reviews recent progress in understanding globular cluster formation, integrating advanced theoretical models with modern observational astronomy.
• Globular clusters form in high-pressure environments prevalent in high-redshift galaxies but require migration to survive tidal destruction in their nascent environments.
• Modern models successfully reproduce observed globular cluster demographics and make predictions testable with next-generation telescopes like JWST and ELT.

Insights into Globular Cluster Formation

The study of globular clusters (GCs) provides significant insight into the processes of galaxy formation and evolution. Among the oldest and most luminous stellar systems in the universe, GCs have long been enigmatic due to the elusive nature of their origins. Recent advances in theoretical modeling and observational astronomy have enriched our understanding, positioning GCs as a natural consequence of high-pressure star formation in high-redshift galaxies.

This comprehensive review synthesizes recent progress in the understanding of GC formation and charts a path forward. Modern GC models integrate advanced simulations of galaxy formation, drawing a cohesive picture from GC formation at high redshift, as observable with next-generation telescopes like the James Webb Space Telescope (JWST), to the comprehensive demographic snapshots available for current-day GCs through 30-meter class telescopes.

Crucial to this synthesis are several key components:

1. GC Formation Environment: High-redshift galaxies, characterized by high interstellar medium (ISM) pressures, provide the necessary conditions for the formation of young massive clusters (YMCs). These environments are analogous to the pressure conditions observed locally in galaxy mergers and interactions, but they were prevalent in the "normal" star-forming disks of early galaxies.
2. Cluster Survival and Evolution: While high-pressure environments facilitate the formation of YMCs, these clusters face rapid destruction by tidal processes in their nascent environments. A notable phase of GC evolution includes a migration out of these destructive environments, often facilitated by galaxy mergers and interactions – a process that eventually allows their long-term survival as GCs.
3. Modeling GC Demographics: Current models successfully reproduce observed GC system demographics, including the mass function, specific frequency, metallicity distributions, and spatial and kinematic properties. They also offer significant predictions for future observation, like the proto-GC formation timeline and characteristics observable with JWST.

Theoretical insights emphasize that the formation of GCs does not necessitate unique or exotic conditions but rather occurs as part of regular star formation under the right conditions at specific cosmic epochs. This viewpoint unifies multiple strands of GC research, viewing GCs across cosmic time as witnesses to both their immediate formation environment and the larger galactic structure and evolution.

As we look forward, empirical validation of these models' predictions will be critical. Instruments like JWST and the upcoming ELT will extend our observational reach into the high-redshift universe, allowing us to test these models and refine our understanding of GC evolution. They will enable the observation of GCs across a range of redshifts, providing empirical anchor points for theoretical models and ensuring that our understanding of GC formation is robust.

In particular, addressing unresolved questions such as the extent of mass loss over a GC's lifetime, the mechanisms preserving the characteristic mass function of GCs, and the detailed formation histories of GCs in various galaxy environments will be pivotal. These investigations hold the potential not only to resolve fundamental questions about GC formation but also to enhance our broader understanding of galactic and cosmic evolution.