Globular clusters are spheroidal groups of stars self-bound by gravity. They are ubiquitous around massive galaxies, with some ~150 orbiting the Milky Way and tens of thousands around the brightest galaxies in massive galaxy clusters. They have long been used as tools in astrophysics, allowing Shapley (1918) to conclude that the Sun was not at the centre of the Milky Way based on distances to globular clusters. The origin of globular clusters has remained a long-standing problem in astrophysics. Their typically old ages (>10 gigayears) and small radii (few parsecs) currently precludes direct observations of their formation. Theoretical work has faced similar difficulties. For the foreseeable future, it remains computationally impossible to simultaneously simulate the small scales of star cluster formation and large scales of the host galaxy formation from the early universe to the present day.
The key to understanding the origin of globular clusters may come from young, massive stellar clusters observed to be forming today in the gaseous discs of local galaxies. These young clusters have masses and densities similar to globular clusters and are thought to be analogous to proto-globular clusters. One of the main goals of the E-MOSAICS (MOdelling of Star cluster population Assembly In Cosmological Simulations within EAGLE) simulations is to determine whether young stellar clusters and globular clusters can be described by a single formation mechanism.
The project has been made possible by major advances in hydrodynamical galaxy formation simulations such that they can now follow a cosmologically-representative volume and reproduce galaxy populations similar to those we observe. The MOSAICS star cluster model is coupled to the EAGLE (Evolution and Assembly of GaLaxies and their Environments) model for galaxy formation. EAGLE is a series of state-of-the-art, cosmological, hydrodynamical simulations of the Lambda-Cold Dark Matter universe, the largest of which is 100 megaparsecs on a side and contains 6.8 billion particles. The EAGLE simulations successfully reproduce many properties of the observed evolving galaxy population, including galaxy stellar masses, sizes and gas properties.
Because the current resolution of cosmological simulations of the galaxy population is by far insufficient to resolve star clusters, MOSAICS adopts a semi-analytic approach where the properties of star clusters are governed by analytic expressions that depend on resolved quantities within the hydrodynamical simulation to which it is coupled. Such an approach enables us to follow both the evolution of the star cluster population and the evolving cosmological environment of the host galaxy. The physical prescriptions of the MOSAICS model include:
- Cluster formation efficiency (fraction of stars formed in bound clusters) that depends on the local gas and dynamical properties,
- Schechter cluster mass function (power-law with an exponential truncation mass),
- Exponential truncation mass determined by local gas and dynamical properties,
- Cluster mass-loss by stellar evolution, tidal shocks and evaporation using the evolving local tidal field of each ‘cluster particle’,
- Dynamical friction in post-processing.
These physical prescriptions are based on models which reproduce observed properties of young star clusters and direct N-body simulations of star cluster mass-loss.
The E-MOSAICS suite of simulations currently includes zoom-in simulations of ten Milky Way-like galaxies. The nature of the simulations is visualised in the figure below. The ten Milky Way-like galaxies were chosen from the full box of the high resolution (“Recal”) EAGLE box. The main panel shows the dark matter distribution in the box, with yellow circles highlighting the positions of the galaxies resolved in the zoom-in simulations. The panels towards the right show for a single zoom-in simulation the gas density (top) and simulated optical images of the face-on (middle) and edge-on (bottom) views of the final galaxy. The five panels in the bottom row show the evolution of the gas density in the galaxy and its star cluster population from high redshift (z=10) to today (z=0). For the first time, we can follow the formation and evolution of the entire star cluster population through full cosmic history in realistic galaxy formation simulations.
The first results from the E-MOSAICS simulations are presented by Pfeffer et al. (2018) and Kruijssen et al. (2019). We find that both old (globular) and young star clusters can form by single formation mechanism, i.e. globular clusters are simply evolved versions of clusters forming today. The differences between the populations arises simply from the evolution of the properties of star-forming gas over cosmic time. In addition, we find that globular cluster populations carry quantitative imprints of their host galaxy’s formation history, enabling their practical use as tracers of galaxy formation and assembly.
In the near future we will investigate predictions from the simulations in more detail. The simulations will also be extended to galaxy groups and clusters hosting massive elliptical galaxies, environments which are the target of the majority of observational surveys of globular clusters.