Slow Cooling in Low Metallicity Clouds: An Origin of Globular Cluster Bimodality?

TL;DR

This study investigates how low metallicity affects cloud collapse and fragmentation, proposing that slower cooling leads to global collapse, which may explain the formation of dense globular clusters with minimal fragmentation.

Contribution

It identifies a critical metallicity threshold below which low-metallicity clouds undergo global collapse without fragmenting, supported by numerical simulations including radiative heating effects.

Findings

Existence of a critical metallicity between 0.001 and 0.01 Z_sun for non-fragmenting collapse

Radiative heating increases the critical metallicity threshold

Global collapse without fragmentation can occur in low-metallicity clouds

Abstract

We explore the relative role of small-scale fragmentation and global collapse in low-metallicity clouds, pointing out that in such clouds the cooling time may be longer than the dynamical time, allowing the cloud to collapse globally before it can fragment. This, we suggest, may help to explain the formation of the low-metallicity globular cluster population, since such dense stellar systems need a large amount of gas to be collected in a small region (without significant feedback during the collapse). To explore this further, we carry out numerical simulations of low-metallicity Bonner-Ebert stable gas clouds, demonstrating that there exists a critical metallicity (between 0.001 and 0.01 $Z_\odot$) below which the cloud collapses globally without fragmentation. We also run simulations including a background radiative heating source, showing that this can also produce clouds that do not fragment, and that the critical metallicity -- which can exceed the no-radiation case -- increases with the heating rate.