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 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.
