The origin of the 'blue tilt' of globular cluster populations in the E-MOSAICS simulations

TL;DR

The study uses E-MOSAICS simulations to explain the 'blue tilt' in globular clusters as a result of galaxy formation conditions, rather than self-enrichment, linking it to galaxy mass and gas density.

Contribution

It demonstrates that the blue tilt arises from the relationship between galaxy mass, gas density, and GC formation, challenging the self-enrichment hypothesis.

Findings

The blue tilt can be explained without mass-dependent self-enrichment.

Massive, metal-poor GCs are scarce due to formation conditions.

High gas surface densities in massive galaxies foster massive GC formation.

Abstract

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.