Globular clusters formed within dark haloes I: Present-day abundance, distribution, and kinematics

Abstract

We explore a scenario where metal-poor globular clusters (GCs) form at the centres of their own dark matter haloes in the early Universe before reionization. This hypothesis leads to predictions about the abundance, distribution, and kinematics of GCs today that we explore using cosmological N-body simulations and analytical modelling. We find that selecting the massive tail of collapsed objects at z ≳ 9 as GCs formation sites leads to four main predictions: (i) a highly clustered population of GCs around galaxies today, (ii) a natural scaling between number of GCs and halo virial mass that follows roughly the observed trend, (iii) a very low number of free-floating GCs outside massive haloes, and (iv) GCs should be embedded within massive and extended dark matter (sub)haloes. We find that the strongest constraint to the model is given by the combination of (i) and (ii): a mass cut to tagged GCs haloes which accounts for the number density of metal-poor GCs today predicts a radial distribution that is too extended compared to recent observations. On the other hand, a mass cut sufficient to match the observed half-number radius could only explain 60 per cent of the metal-poor population. In all cases, observations favour early redshifts for GC formation (z ≥ 15).