Too shy to spin? Cosmic wallflowers as proto-globular clusters

Abstract We investigate the rotational properties of star-forming clusters at z ∼ 7.6 in the high-resolution simulation MassiveBlackPS, focusing on two formation channels: clusters forming in galactic discs via gravitational instability and isolated circumgalactic systems, referred to as cosmic wallflowers, born out of cosmic filaments. Using stellar kinematics, we compare their rotational velocities, vrot, and rotational support, v/σ, to study whether formation environment leaves a clear dynamical imprint. We find that cosmic wallflowers preferentially show lower rotational velocities, whereas the identified disc clusters extend systematically towards higher vrot. In contrast, the two populations overlap substantially in v/σ, although the disc clusters show a modest preference towards larger rotational support. When combined with stellar surface densities, a subset of the low-vrot cosmic wallflowers lie surprisingly close to the observed globular cluster population in the Milky Way, whereas disc clusters remain offset. Within the cosmic wallflower population, we identify two regimes: lower-density, weakly rotating systems that overlap with these globular cluster properties, and denser, more rotationally supported systems that likely follow a different evolutionary pathway, possibly linking them to the origin of massive black hole seeds at high redshift. We further find that the gas content correlates with this behaviour, with gas-rich cosmic wallflowers preferentially occupying this low-rotation regime. Together, these results suggest that formation environment is most clearly reflected in vrot, whereas gas content is linked to the variation within each population. In particular, weakly rotating, gas-rich cosmic wallflowers emerge as natural proto-globular cluster candidates, potentially evolving towards present-day systems through angular momentum loss and dynamical heating.