Metal-Poor Globular Clusters and Galaxy Formation

Abstract

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 ∝ L0.15±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.