Formation of proto-globular cluster candidates in cosmological simulations of dwarf galaxies at z > 4

We perform cosmological hydrodynamical simulations to study the formation of proto-globular cluster candidates in progenitors of present-day dwarf galaxies (M_vir ≈ 10¹⁰ M_☉ at z = 0) as part of the ‘Feedback in Realistic Environment’ (FIRE) project. Compact (r_1/2 < 30 pc), relatively massive (0.5 × 10⁵ ≲ M_*/M_☉ ≲ 5 × 10⁵), self-bound stellar clusters form at 11 ≳ z ≳ 5 in progenitors with M_vir ≈ 10⁹ M_☉. Cluster formation is triggered when at least 10⁷ M_☉ of dense, turbulent gas reaches Σ_gas ≈ 10⁴ M_☉ pc⁻² as a result of the compressive effects of supernova feedback or from cloud–cloud collisions. The clusters can survive for 2 − 3 Gyr; absent numerical effects, they could possibly survive substantially longer, perhaps to z = 0. The longest lived clusters are those that form at significant distance – several hundreds of pc – from their host galaxy. We therefore predict that globular clusters forming in progenitors of present-day dwarf galaxies will be offset from any pre-existing stars within their host dark matter haloes as opposed to deeply embedded within a well-defined galaxy. Properties of the nascent clusters are consistent with observations of some of the faintest and most compact high-redshift sources in Hubble Space Telescope lensing fields and are at the edge of what will be detectable as point sources in deep imaging of non-lensed fields with JWST. By contrast, the star clusters’ host galaxies will remain undetectable.