Structure and Rotation of Young Massive Star Clusters in a Simulated Dwarf Starburst

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 phase interstellar medium with an initial mass function sampled with individual stars down to four solar masses at sub-parsec spatial resolution. Massive stars and their important feedback mechanisms are modeled 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 greater than or similar to 3 x 10(4) M) have the highest specific angular momenta at low ellipticities (epsilon similar to 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 greater than or similar to 10(5) M) accrete gas and protoclusters from a less than or similar to 100 pc scale local galactic environment on a t less than or similar to 10 Myr timescale, inheriting the ambient angular momentum properties. Their two-dimensional kinematic maps show ordered rotation at formation, up to v similar to 8.5 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) less than or similar to 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.