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

ABSTRACT We perform cosmological hydrodynamical simulations to study the formation of proto-globular cluster candidates in progenitors of present-day dwarf galaxies (M ₕ₈ₑ 10^10\, M_ at z = 0) as part of the ‘Feedback in Realistic Environment’ (FIRE) project. Compact (r1/2 30 pc), relatively massive (0. 5 × 105 ≲ M⋆/M⊙ ≲ 5 × 105), self-bound stellar clusters form at 11 ≳ z ≳ 5 in progenitors with M ₕ₈ₑ 10⁹\, M. Cluster formation is triggered when at least 10⁷\, M of dense, turbulent gas reaches ₆₀ₒ 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.