Globular cluster formation and evolution in the context of cosmological galaxy assembly: open questions

Abstract

We discuss some of the key open questions regarding the formation and evolution of globular clusters (GCs) during galaxy formation and assembly within a cosmological framework. The current state of the art for both observations and simulations is described, and we briefly mention directions for future research. The oldest GCs have ages greater than or equal to 12.5 Gyr and formed around the time of reionization. Resolved colour-magnitude diagrams of Milky Way GCs and direct imaging of lensed proto-GCs at z∼6 with the James Webb Space Telescope (JWST) promise further insight. GCs are known to host multiple populations of stars with variations in their chemical abundances. Recently, such multiple populations have been detected in ∼2 Gyr old compact, massive star clusters. This suggests a common, single pathway for the formation of GCs at high and low redshift. The shape of the initial mass function for GCs remains unknown; however, for massive galaxies a power-law mass function is favoured. Significant progress has been made recently modelling GC formation in the context of galaxy formation, with success in reproducing many of the observed GC-galaxy scaling relations.

Globular clusters in Coma cluster ultra-diffuse galaxies (UDGs): evidence for two types of UDG?

“…It is interesting to compare this GC specific frequency with that of the halo of our own Milky Way galaxy (given that UDGs appear to lack clear bulge or disk components). Indeed the formation of metal-poor GCs at early times may be a universal process (Forbes et al 2018b). Similar to the failed galaxy scenario for UDGs, the halo of our Galaxy formed at early times and is dominated by old metal-poor stars.…”

“…The local dwarf sample consists of GC systems for 14 Local Group dwarfs (including a re-assembled Sgr dwarf galaxy) and 29 Local Volume dwarfs in low density environments from Forbes et al (2018a). This therefore includes early (dE and dSph) and late-type (dIrr and Im) dwarfs.…”

“…Local dwarf galaxies reveal a wide range in their mean GC mass. Here we take the mass of the GC system directly from Forbes et al (2018a), who summed the observed individual GC masses. We note that some GC-poor local dwarfs have a mean GC mass closer to 10 4 M .…”

Structural analysis of disc super star clusters of M82: size and profile shape at intermediate ages

“…Understanding the formation and evolution of globular clusters (GCs) has been an active field of research in astrophysics over the last half century (Forbes et al 2018). The discovery of clusters as dense (ρ 10 3 M /pc 3 ) and massive (10 4 − 10 6 M ) as GCs, but relatively young, known as Young Massive Clusters or Super Star Clusters (SSCs), has given a new impetus to these studies in the last two decades (Bastian 2016).…”

A nitrogen-enhanced metal-poor star discovered in the globular cluster ESO280−SC06

“…The main driver is the star-to-star abundance dispersions that are observed for light elements in nearly 1 all ancient globular clusters. Spectroscopic, astrometric, and photometric observations have been undertaken with the aim of confirming, constraining, and ruling out the various proposed formation mechanisms of these multiple stellar populations (for an overview of the various proposals see the reviews of Charbonnel 2016; Bastian & Lardo 2018;Forbes et al 2018, and references therein). As the 'typical' clusters have not shown a clear path forward, there is now much interest in exploring the edges of the parameter space: the least massive clusters (e.g., Mucciarelli et al 2016;Simpson et al 2017b), the 'youngest' clusters (e.g., Valcheva et al 2014;Hollyhead et al 2016), and young massive clusters in other galaxies that are thought to be to precursors to the ancient globular clusters of the Milky Way (e.g., Cabrera-Ziri et al 2016, but see discussion in Renaud 2018Renaud , 2019 on whether these objects are truly analogues to present-day globular clusters).…”