The E-MOSAICS project: tracing galaxy formation and assembly with the
  age-metallicity distribution of globular clusters
  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 time-scales, 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 $M_*\gtrsim10^8$ M$_\odot$ for redshifts
$1\leq z\leq2.5$. These results demonstrate that cosmological simulations of
the co-formation and co-evolution of GCs and their host galaxies successfully
unlock the potential of GCs as quantitative tracers of galaxy formation and
assembly.
