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 approximate to 10(10) M-circle dot at z = 0) as part of the 'Feedback in Realistic Environment' (FIRE) project. Compact ( r (1/2) < 30 pc), relatively massive (0.5 x 10 (5) less than or similar to M-*/M-circle dot less than or similar to 5 x 10 (5) ), self-bound stellar clusters form at 11 greater than or similar to 5 in progenitors with M (vi)r approximate to 10(9) M-circle dot. Cluster formation is triggered when at least 10 7 Me of dense, turbulent gas reaches Egas 104 Me 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.