The cosmic globular cluster formation history in the E-MOSAICS simulations

Abstract: We present a comprehensive analysis of globular cluster (GC) formation and evolution across the $34^3$ Mpc$^3$ volume of the E-MOSAICS galaxy formation simulations. Defining GCs as surviving, high-mass ($>10^5$ M$\odot$) clusters, we analyse their formation histories as a function of their metallicity and host galaxy mass, also distinguishing between central and satellite galaxies. The redshift of peak GC formation rate increases weakly with galaxy mass, decreases with metallicity, and does not differ between centrals and satellites. The epoch of peak GC formation precedes that of the stars by a factor of $1.1{-}1.6$, primarily due to `downsizing', i.e. low-mass galaxies form their stars later. Consequently, this offset decreases with galaxy mass, leading to nearly coeval stellar and GC populations in massive galaxies ($>10^{11}$ M$\odot$). GCs themselves do not exhibit strong downsizing, because they predominantly formed at early cosmic epochs conducive to the formation (through high gas pressures) and survival (through high galaxy merger and GC migration rates) of massive, compact stellar systems. The total GC formation rate in the volume peaks at $z\approx 2.5$, shortly before star formation peaks at $z\approx 2$, but well after the general cluster formation rate at $z\approx 4$, reflecting a survivor bias where surviving GCs formed more recently. We find that GC formation commenced early, at $z>10$, such that the results of this work may provide a framework for interpreting direct observations of proto-GC formation with the JWST, especially as these observations accumulate to enable statistical studies.