Stellar cluster formation in a Milky Way-sized galaxy at z > 4-I. The proto-globular cluster population and the imposter amongst us

The formation history of globular clusters (GCs) at redshift z > 4 remains an unsolved problem. In this work, we use the cosmological, N-body hydrodynamical 'zoom-in' simulation GigaEris to study the properties and formation environment of proto-GC candidates in the region surrounding the progenitor of a Milky Way-sized galaxy. The simulation employs a modern implementation of smoothed-particle hydrodynamics, including metal-line cooling and metal and thermal diffusion. We define proto-GC candidate systems as gravitationally bound stellar systems with baryonic mass fraction F-b >= 0.75 and stellar velocity dispersion sigma(*) < 20 km s(-1). At z = 4.4, we identify nine systems that satisfy our criteria, all of which form between 10 and 30 kpc from the centre of the main host. Their baryonic masses are in the range 10(5)-10(7) M-circle dot. By the end of the simulation, they still have a relatively low stellar mass (M-circle dot similar to 10(4)-10(5) M-circle dot) and a metallicity (-1.8 less than or similar to [Fe/H] less than or similar to -0.8) similar to the blue Galactic GCs. All of the identified systems except one appear to be associated with gas filaments accreting onto the main galaxy in the circum-galactic region and formed at z = 5-4. The exception is the oldest object, which appears to be a stripped compact dwarf galaxy that has interacted with the main halo between z = 5.8 and z = 5.2 and has lost its entire dark matter content due to tidal mass loss.