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 (Mᵥᵢᵣ ≈ 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 Mᵥᵢᵣ ≈ 10⁹ M_⊙. Cluster formation is triggered when at least 10⁷ M_⊙ of dense, turbulent gas reaches Σ₉ₐₛ ≈ 10⁴. M_⊙ pc⁻² as a result of the compressive effects of supernova feedback or from cloud-cloud collisions. The clusters can survive for 2−3Gyr; absent numerical effects, they would likely 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 halos 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 the James Webb Space Telescope. By contrast, the star clusters' host galaxies will remain undetectable.