Angular momentum transfer to a Milky Way disc at high redshift

An Adaptive Mesh Refinement cosmological resimulation is analysed in order to test whether filamentary flows of cold gas are responsible for the build-up of angular momentum within a Milky Way-like disc at z 3. A set of algorithms is presented that takes advantage of the high spatial resolution of the simulation (12 pc) to identify: (i) the central gas disc and its plane of orientation; (ii) the complex individual filament trajectories that connect to the disc; and (iii) the infalling satellites. The results show that two filaments at z 5.5, which later merge to form a single filament at z 4, drive the angular momentum and mass budget of the disc throughout its evolution, whereas luminous satellite mergers make negligible fractional contributions. Combined with the ubiquitous presence of such filaments in all large-scale cosmological simulations that include hydrodynamics, we argue that these findings provide strong quantitative evidence that the growth of a large fraction of the thin discs in haloes with masses below 10 12 M , which host the vast majority of galaxies, is supported via inflowing streams of cold gas at intermediate and high redshifts.