Metal-poor globular clusters and galaxy formation

We demonstrate a significant (>5 σ) correlation between the mean color of metal-poor globular cluster (GC) systems and parent galaxy luminosity. A Bayesian Markov Chain Monte Carlo method is introduced to find the mean color and is easily generalizable to quantify multimodality in other astronomical data sets. We derive a GC color-galaxy luminosity relation of the form Z ∝ L 0.15±0.03. When combined with evidence against a single primordial GC metallicity-galaxy luminosity relation for protogalactic fragments, the existence of such a correlation is evidence against both accretion and major merger scenarios as an explanation of the entire metal-poor GC systems of luminous galaxies. However, our relation arises naturally in an in situ picture of GC formation and is consistent with the truncation of metal-poor GC formation by reionization. A further implication is that the ages of metal-poor GCs in dwarf galaxies constrain the main epoch of galaxy formation in hierarchical models. If the ages of old metal-poor GCs in Local Group dwarfs (≳ 11 Gyr) are typical of those in dwarfs elsewhere, then the bulk of galaxy assembly (at least in clusters and groups) must have occurred at z≳2.5, contrary to the predictions of some structure formation models.