  Globular clusters (GCs) are typically old, with most having formed at z >~ 2.
This makes understanding their birth environments difficult, as they are
typically too distant to observe with sufficient angular resolution to resolve
GC birth sites. Using 25 cosmological zoom-in simulations of Milky Way-like
galaxies from the E-MOSAICS project, with physically-motivated models for star
formation, feedback, and the formation, evolution, and disruption of GCs, we
identify the birth environments of present-day GCs. We find roughly half of GCs
in these galaxies formed in-situ (52.0 +/- 1.0 per cent) between z ~ 2 - 4, in
turbulent, high-pressure discs fed by gas that was accreted without ever being
strongly heated through a virial shock or feedback. A minority of GCs form
during mergers (12.6 +/- 0.6 per cent in major mergers, and 7.2 +/- 0.5 per
cent in minor mergers), but we find that mergers are important for preserving
the GCs seen today by ejecting them from their natal, high density interstellar
medium (ISM), where proto-GCs are rapidly destroyed due to tidal shocks from
ISM substructure. This chaotic history of hierarchical galaxy assembly acts to
mix the spatial and kinematic distribution of GCs formed through different
channels, making it difficult to use observable GC properties to distinguish
GCs formed in mergers from ones formed by smooth accretion, and similarly GCs
formed in-situ from those formed ex-situ. These results suggest a simple
picture of GC formation, in which GCs are a natural outcome of normal star
formation in the typical, gas-rich galaxies that are the progenitors of
present-day galaxies.
