The minimum metallicity of globular clusters and its physical origin --
  implications for the galaxy mass-metallicity relation and observations of
  proto-globular clusters at high redshift
  In the local Universe, globular clusters (GCs) with metallicities $[{\rm
Fe}/{\rm H}]<-2.5$ 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 $[{\rm Fe}/{\rm H}]\lesssim-2.5$
have too low masses to form GCs with initial masses $M_{\rm i}\gtrsim10^5~{\rm
M}_\odot$, needed to survive for a Hubble time. This translates the galaxy
mass-metallicity relation into a maximum initial cluster mass-metallicity
relation for $[{\rm Fe}/{\rm H}]\lesssim-1.8$, 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 $\alpha=0.4\pm0.1$ for $10^5\lesssim M_\star/{\rm
M}_\odot\lesssim10^7$ at $z\gtrsim2$. 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 ($M_\star\approx4\times10^7~{\rm M}_\odot$) formed at $z\lesssim3$.
