The formation and hierarchical assembly of globular cluster populations

Abstract

We use a semi-analytic model for globular cluster (GC) formation built on dark matter merger trees to explore the relative role of formation physics and hierarchical assembly in determining the properties of GC populations. Many previous works have argued that the observed linear relation between total GC mass and halo mass points to a fundamental GC–dark matter connection or indicates that GCs formed at very high redshift before feedback processes introduced non-linearity in the baryon-to-dark matter mass relation. We demonstrate that at Mvir(z = 0) 1011.5 M, a constant ratio between halo mass and total GC mass is in fact an almost inevitable consequence of hierarchical assembly: by the central limit theorem, it is expected at z = 0 independent of the GC-to-halo mass relation at the time of GC formation. The GC-to-halo mass relation at Mvir(z = 0) < 1011.5 M is more sensitive to the details of the GC formation process. In our fiducial model, GC formation occurs in galaxies when the gas surface density exceeds a critical value. This model naturally predicts bimodal GC colour distributions similar to those observed in nearby galaxies and reproduces the observed relation between GC system metallicity and halo mass. It predicts that the cosmic GC formation rate peaked at z ∼ 4, too late for GCs to contribute significantly to the UV luminosity density during reionization.