Structure and rotation of young massive star clusters in a simulated dwarf starburst

TL;DR

This study uses high-resolution simulations to analyze the shapes and kinematics of young star clusters in a dwarf galaxy, revealing two sub-populations with distinct angular momentum and rotation properties, similar to observed clusters.

Contribution

It provides new insights into the formation and angular momentum characteristics of young massive star clusters through detailed simulation analysis.

Findings

Massive clusters show ordered rotation and high specific angular momentum.

Lower mass clusters have more varied shapes and less alignment with rotation.

Simulated clusters' kinematic properties match observations of young and old clusters.

Abstract

We analyze the three-dimensional shapes and kinematics of the young star cluster population forming in a high-resolution GRIFFIN project simulation of a metal-poor dwarf galaxy starburst. The star clusters, which follow a power-law mass distribution, form from the cold ISM phase with an IMF sampled with individual stars down to 4 solar masses at sub-parsec spatial resolution. Massive stars and their important feedback mechanisms are modelled in detail. The simulated clusters follow a surprisingly tight relation between the specific angular momentum and mass with indications of two sub-populations. Massive clusters ($M_\mathrm{cl}\gtrsim 3\times 10^4 M_{\odot})$ have the highest specific angular momenta at low ellipticities ($\epsilon\sim 0.2$) and show alignment between their shapes and rotation. Lower mass clusters have lower specific angular momenta with larger scatter, show a broader range of elongations, and are typically misaligned indicating that they are not shaped by rotation. The most massive clusters $(M \gtrsim 10^5\,M_{\odot})$ accrete gas and proto-clusters from a $ \lesssim 100\,\rm pc$ scale local galactic environment on a $t \lesssim 10\,\rm Myr$ timescale, inheriting the ambient angular momentum properties. Their two-dimensional kinematic maps show ordered rotation at formation, up to $v \sim 8.5\,\rm km s^{-1}$, consistent with observed young massive clusters and old globular clusters, which they might evolve into. The massive clusters have angular momentum parameters $\lambda_R\lesssim 0.5$ and show Gauss-Hermite coefficients $h_3$ that are anti-correlated with the velocity, indicating asymmetric line-of-sight velocity distributions as a signature of a dissipative formation process.