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Nuclear star clusters (NSCs) are massive star clusters found in the innermost region of most galaxies. While recent studies suggest that low-mass NSCs in dwarf galaxies form largely out of the merger of globular clusters and NSCs in massive galaxies accumulate mass primarily through central star formation, the formation channel of the Milky Way’s NSC is still uncertain. In this work, we use GigaEris, a high-resolution N-body, hydrodynamical, cosmological ‘zoom-in’ simulation, to investigate a possible formation path of the NSC in the progenitor of a Milky Way-sized galaxy, as well as its relation to the assembly and evolution of the galactic nuclear region. We study the possibility that bound, young, gas-rich, stellar clusters within a radius of 1.5 kpc of the main galaxy’s centre at z > 4 are the predecessors of the old, metal-poor stellar population of the Milky Way’s NSC. We identify 47 systems which satisfy our criteria, with a total stellar mass of 107.5 M. We demonstrate that both stellar cluster accretion and in situ star formation will contribute to the formation of the NSC, providing evidence for a hybrid formation scenario for the first time in an N-body, hydrodynamical, cosmological ‘zoom-in’ simulation. Additionally, we find that the gas required for in situ star formation can originate from two pathways: gas-rich stellar clusters and gas influx driven by large-scale non-axisymmetric structures within the galaxy. This is partly supported by the presence of a stellar ring, resulting from gas dynamics, with properties similar to those of the Milky Way’s nuclear stellar disc.

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.