Two Thresholds for Globular Cluster Formation and the Common Occurrence of Massive Clusters in the Early Universe

Abstract

Young massive clusters (YMCs) are usually accompanied by lower-mass clusters and unbound stars with a total mass equal to several tens times the mass of the YMC. If this was also true when globular clusters (GCs) formed, then their cosmic density implies that most star formation before redshift ∼ 2 made a GC that lasted until today. Star-forming regions had to change after this time for the modern universe to be making very few YMCs. Here we consider the conditions needed for the formation of a ∼ 10 6 M ⊙ cluster. These include a star formation rate inside each independent region that exceeds ∼ 1 M ⊙ yr −1 to sample the cluster mass function up to such a high mass, and a star formation rate per unit area of Σ SFR ∼ 1 M ⊙ kpc −2 yr −1 to get the required high gas surface density from the Kennicutt-Schmidt relation, and therefore the required high pressure from the weight of the gas. High pressures are implied by the virial theorem at cluster densities. The ratio of these two quantities gives the area of a GC-forming region, ∼ 1 kpc 2 , and the young stellar mass converted to a cloud mass gives the typical gas surface density of 500 − 1000 M ⊙ pc −2 . Observations of star-forming clumps in young galaxies are consistent with these numbers, suggesting they formed today's GCs. Observations of the cluster cut-off mass in local galaxies agree with the maximum mass calculated from Σ SFR . Metal-poor stellar populations in local dwarf irregular galaxies confirm the dominant role of GC formation in building their young disks.

The GRIFFIN Project—Formation of Star Clusters with Individual Massive Stars in a Simulated Dwarf Galaxy Starburst

“…The most massive clusters forming during the starburst fall along a trend between the young clusters in the Magellanic clouds and the upper extreme set by the Antennae. We also compare to a model relation for self-gravitating clouds from Elmegreen (2018), which shows a slope similar (β = 0.86) to our trend.…”

“…The black crosses show observed values for the most massive young clusters in the LMC and SMC (two age bins labeled on the left,Hunter et al 2003) and in the Antennae merger (cluster age of the order of 1 Myr,Whitmore et al 2010) where the ΣSFR values are fromChandar et al (2017) andJohnson et al (2017) respectively. The black diagonal line shows a model relation fromElmegreen (2018).…”

The Molecular Cloud Lifecycle

“…The most extreme values are found the interacting Antennae galaxies, where the gas is experiencing elevated external pressures induced by the merger and is undergoing violent compression, possibly leading to gravitational collapse (e.g. Elmegreen 2018;. The GMC population detected in main sequence galaxies at z ∼ 1 (Dessauges-Zavadsky et al 2019) has physical properties similar to those detected in local starbursts (e.g.…”

Self-consistent proto-globular cluster formation in cosmological simulations of high-redshift galaxies

“…Galaxies in the early universe preferentially meet such conditions, while low-mass galaxies at later times are likely less efficient in forming bound clusters (e.g. Elmegreen 2018, and reference therein). We defer a detailed investigation on cluster formation efficiency over a broad range of galaxy mass, redshift, and ISM properties to a future study.…”