2019MNRAS.486L..20K

In the local Universe, globular clusters (GCs) with metallicities [Fe/H] < -2.5 are extremely rare. In this Letter, the close connection between GC formation and galaxy evolution is used to show that this GC metallicity 'floor' results from the galaxy mass-metallicity relation of ultra low-luminosity galaxies (ULLGs) at high redshift, where the most metal-poor GCs must have formed. Galaxies with metallicities [Fe/H] <= -2.5 have too low masses to form GCs with initial masses M_i >= 10⁵ M_☉ needed to survive for a Hubble time. This translates the galaxy mass-metallicity relation into a maximum initial cluster mass-metallicity relation for [Fe/H] <= -1.8, which naturally leads to the observed colour-magnitude relation of metal-poor GCs at z = 0 (the 'blue tilt'). Its strength traces the slope of the gas phase mass-metallicity relation of ULLGs. Based on the observed blue tilt of GCs in the Virgo and Fornax Clusters, the galaxy mass-metallicity relation is predicted to have a slope of α = 0.4 ± 0.1 for 10⁵ <= M_*/M_☉ <= 10⁷ at z >= 2. The GC metallicity floor implies a minimum host galaxy mass and a maximum redshift for GC formation. Any proto-GCs that may be detected at z > 9 are most likely to end up in galaxies presently more massive than the Milky Way, whereas GCs in low-mass galaxies such as the Fornax dSph (M_* ≃ 4 x 10⁷ M_☉) formed at z <= 3.