Formation of the Galactic globular clusters with He-rich stars in low-mass halos virialized at high redshift

TL;DR

This study links the formation epochs of globular clusters with helium-rich stars to their kinematic properties, suggesting earlier formation in high-redshift low-mass halos results in lower velocity dispersions observed today.

Contribution

It introduces a cosmological simulation-based model connecting GC formation epochs with their current kinematic and chemical properties, especially helium enrichment.

Findings

GCs in halos virialized at z>10 have lower velocity dispersions.

Lower velocity dispersions in GCs with EHBs are linked to earlier formation epochs.

The model offers insights into the second parameter problem of GCs.

Abstract

Recent observations have reported that the Galactic globular clusters (GCs) with unusually extended horizontal-branch (EHB) morphologies show a significantly lower velocity dispersion compared with that of the entire Galactic GC system. We consider that the observed distinctive kinematics of GCs with EHB has valuable information on the formation epochs of GCs and accordingly discuss this observational result based on cosmological N-body simulations with a model of GC formation. We assume that GCs in galaxies were initially formed in low-mass halos at high redshifts and we investigate final kinematics of GCs in their host halos at $z=0$. We find that GCs formed in halos virialized at z>10 show lower velocity dispersions on average than those formed at z>6 for halos with GCs at z=0. We thus suggest that the origin of the observed lower velocity dispersion for the Galactic GCs with EHBs is closely associated with earlier formation epochs (z>10) of halos initially hosting the GCs in the course of the Galaxy formation. Considering that the origin of EHBs can be due to the presence of helium-enhanced second-generation stars in GCs, we discuss the longstanding second parameter problem of GCs in the context of different degrees of chemical pollution in GC-forming gas clouds within low-mass halos virialized at different redshifts.