We explore a suite of high-resolution cosmological simulations from the First Billion Years ( FiBY ) project at z \geq 6 to identify low-mass stellar systems , with a particular focus on globular clusters ( GCs ) .
Within the demographics of substructures found in the simulations , two distinct groups of objects emerge .
We associate the first group , which appear to have a high baryon fraction ( f _ { b } \geq 0.95 ) , with infant GC candidates .
The second group exhibit a high stellar fraction ( f _ { * } \geq 0.95 ) and show a resemblance to ultra-faint dwarf galaxies .
The infant GC candidates are characterised by a stellar content similar to the one observed in present-day GCs , but they still contain a high gas fraction ( f _ { gas } \sim 0.95 ) and a relatively low amount of dark matter .
They are very compact systems , with high stellar mass densities and sizes which are consistent with recent estimates based on the first observations of possible proto-GCs at high redshifts .
Such infant GCs appear to be more massive and more abundant in massive host galaxies , indicating that the assembly of galaxies via mergers may play an important role in shaping up several GC-host scaling relations .
Specifically , we express the relation between the mass of the most massive infant GC and its host stellar mass as \log ( M _ { cl } ) = ( 0.31 \pm 0.15 ) \log ( M _ { *,gal } ) + ( 4.17 \pm 1.06 ) .
Finally , we assess that the present-day GC mass – halo mass relation offers a satisfactory description of the behaviour of our infant GC candidates at high redshift , suggesting that such a relation may be set at formation .