The E-MOSAICS project: tracing galaxy formation and assembly with the age–metallicity distribution of globular clusters

Abstract

We present 25 cosmological zoom-in simulations of Milky Way-mass galaxies in the 'MOdelling Star cluster population Assembly In Cosmological Simulations within EAGLE' (E-MOSAICS) project. E-MOSAICS couples a detailed physical model for the formation, evolution, and disruption of star clusters to the EAGLE galaxy formation simulations. This enables following the co-formation and co-evolution of galaxies and their star cluster populations, thus realising the long-standing promise of using globular clusters (GCs) as tracers of galaxy formation and assembly. The simulations show that the age-metallicity distributions of GC populations exhibit strong galaxy-to-galaxy variations, resulting from differences in their evolutionary histories. We develop a formalism for systematically constraining the assembly histories of galaxies using GC age-metallicity distributions. These distributions are characterised through 13 metrics that we correlate with 30 quantities describing galaxy formation and assembly (e.g. halo properties, formation/assembly redshifts, stellar mass assembly timescales, galaxy merger statistics), resulting in 20 statistically (highly) significant correlations. The GC age-metallicity distribution is a sensitive probe of the mass growth, metal enrichment, and minor merger history of the host galaxy. No such relation is found between GCs and major mergers, which play a sub-dominant role in GC formation for Milky Way-mass galaxies. Finally, we show how the GC age-metallicity distribution enables the reconstruction of the host galaxy's merger tree, allowing us to identify all progenitors with masses M * 10 8 M for redshifts 1 z 2.5. These results demonstrate that cosmological simulations of the coformation and co-evolution of GCs and their host galaxies successfully unlock the potential of GCs as quantitative tracers of galaxy formation and assembly.

The mass fraction of halo stars contributed by the disruption of globular clusters in the E-MOSAICS simulations

“…In a companion paper, we look at the mass contribution of GCs to the bulge in the suite of 25 present-day Milky Way-mass galaxies from the E-MOSAICS simulations Kruijssen et al 2019a). We find that the disruption of GCs contributes between 0.3-14 per cent of the bulge mass, in agreement with recent observational estimates .…”

“…The stellar clusters lose mass due to stellar evolution (Wiersma et al 2009), tidal shocks, two-body relaxation (Kruijssen et al 2011) and dynamical friction , the latter being necessarily applied in post-processing. Such a description for cluster formation and evolution has been found to reproduce a wide variety of observed cluster populations Kruijssen et al 2019a;Usher et al 2018;Pfeffer et al 2019b), as well as to predict links between the cluster population and its host galaxy Hughes et al 2019b;Pfeffer et al 2019a;Reina-Campos et al 2019).…”

“…This enables a self-consistent study of the formation and co-evolution of galaxies and their stellar cluster populations, in which GCs emerge from the cluster population after a Hubble time of evolution. For a detailed description of the physical models adopted in E-MOSAICS and details of the simulations, we refer the reader to Pfeffer et al (2018) and Kruijssen et al (2019a). Here we briefly summarize the most relevant prescriptions used.…”

The Fornax 3D project: Non-linear colour–metallicity relation of globular clusters

“…Ashman & Zepf 1992;Forbes et al 1997;Côté et al 1998;Beasley et al 2002;Lee & Jang 2016;Beasley et al 2018) and is also expected in the hierarchical merger scheme of galaxy formation (e.g. Muratov & Gnedin 2010;Tonini 2013;Li & Gnedin 2014;Choksi et al 2018;Kruijssen et al 2019). It is assumed that the red, metal-rich GCs either form in-situ in massive halos around the peak of star formation or during major mergers of gas-rich galaxies together with the bulk of in-situ stars.…”

“…If GCs trace the metallicity of their birthplace, the diverse merger histories of major galaxies as predicted from cosmological simulations (e.g. Article Kruijssen et al 2019) translate into diverse shapes of the MDF and consequently, the shape of the MDF can put constraints on the merger history. In many galaxies, the MDF was found to have a bimodal shape with a metal-poor ([Fe/H] ∼ −1.5 dex) and a metal-rich component ([Fe/H] ∼ −0.5 dex), for example in the Milky Way (MW, e.g.…”