The origin of the 'blue tilt' of globular cluster populations in the
  E-MOSAICS simulations
  The metal-poor sub-population of globular cluster (GC) systems exhibits a
correlation between the GC average colour and luminosity, especially in those
systems associated with massive elliptical galaxies. More luminous (more
massive) GCs are typically redder and hence more metal-rich. This 'blue tilt'
is often interpreted as a mass-metallicity relation stemming from GC
self-enrichment, whereby more massive GCs retain a greater fraction of the
enriched gas ejected by their evolving stars, fostering the formation of more
metal-rich secondary generations. We examine the E-MOSAICS simulations of the
formation and evolution of galaxies and their GC populations, and find that
their GCs exhibit a colour-luminosity relation similar to that observed in
local galaxies, without the need to invoke mass-dependent self-enrichment. We
find that the blue tilt is most appropriately interpreted as a dearth of
massive, metal-poor GCs: the formation of massive GCs requires high
interstellar gas surface densities, conditions that are most commonly fostered
by the most massive, and hence most metal rich, galaxies, at the peak epoch of
GC formation. The blue tilt is therefore a consequence of the intimate coupling
between the small-scale physics of GC formation and the evolving properties of
interstellar gas hosted by hierarchically-assembling galaxies.
