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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 _cl ≳ 3 × 10⁴ M _⊙) have the highes specific angular momenta at low ellipticities (ϵ∼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≳10⁵M_⊙) accrete gas and proto-clusters from a ≲100pc scale local galactic environment on a t≲10Myr timescale, inheriting the ambient angular momentum properties. Their two-dimensional kinematic maps show ordered rotation at formation, up to v∼8.5kms⁻¹, consistent with observed young massive clusters and old globular clusters, which they might evolve into. The massive clusters have angular momentum parameters λ_R≲0.5 and show Gauss-Hermite coefficients h₃ that are anti-correlated with the velocity, indicating asymmetric line-of-sight velocity distributions as a signature of a dissipative formation process.