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 halos 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 halos and iv) GCs should be embedded within massive and extended dark matter (sub)halos. We find that the strongest constraint to the model is given by the combination of i) and ii): a mass cut to tagged GCs halos 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% of the metal poor population. In all cases, observations favour early redshifts for GC formation (z ≥ 15).

Simulating the spatial distribution and kinematics of globular clusters within galaxy clusters in illustris

“…Furthermore, an independent analysis of the GCs around M87 presented in Zhu et al (2014) argues for a β < 0 beyond r ≥ 100 kpc also in Virgo. Such preferentially tangential orbits for GCs are difficult to reconcile with the accretion origin for the ICGC component explored here (see Creasey et al 2019, for a similar conclusion).…”

“…Some of the observed trends in GCs can be explained in models where they trace the most turbulent, high density and gas rich star formation episodes in galaxies (Gnedin et al 2004;Prieto & Gnedin 2008), and analytical calculations support such view (Kruijssen & Cooper 2012;Kruijssen 2015;Elmegreen 2017). Other scenarios where GCs are placed at the centers of their own dark matter halo (Peebles 1984;Rosenblatt et al 1988) and form completely independent of their host galaxy are, although compelling, currently disfavoured due to the large degree of tidal stripping expected and the observationally constrained abundances and radial distribution of GCs around galaxies (Carlberg 2018;Creasey et al 2019).…”

Star Clusters Near and Far

“…Kim et al 2018;Li and Gnedin 2019;Madau et al 2019), or special conditions during reionisation (e.g. Katz and Ricotti 2014;Trenti et al 2015;Kimm et al 2016;Ricotti et al 2016;Creasey et al 2019). All of these scenarios predict increased GC formation towards high redshift, because of an increase in gas pressure, merger rate, or specific early-Universe conditions, but the specific redshift range in which GCs are predicted to have formed still varies, from slightly preceding (but largely tracing) the cosmic star formation history (e.g.…”

A MODEST review

“…In other words, are GCs old young massive clusters that have survived? To address these open issues, several groups are targeting the challenging problem of the formation of GCs in a cosmological context, with a variety of computational approaches (e.g., see the recent results by Ricotti et al 2016;Renaud et al 2017;Li et al 2017Li et al , 2018Creasey et al 2018;Carlberg 2018), or within broader galaxy formation projects, such as E-MOSAICS-MOdelling Star cluster population Assembly In Cosmological Simulations within EAGLE (Schaye et al 2015;Crain et al 2015;Pfeffer et al 2018;, FIRE-Feedback In Realistic Environments simulations (e.g., Hopkins et al 2014;Grudić et al 2017;Kim et al 2018), CosmoGrid (Ishiyama et al 2013;Rieder et al 2013).…”