Realistic H I scale heights of Milky Way-mass galaxies in the FIREbox cosmological volume

Accurately reproducing the thin cold gas discs observed in nearby spiral galaxies has been a long standing issue in cosmological simulations. Here, we present measurements of the radially resolved H I scale height in 22 non-interacting Milky Way-mass galaxies from the FIREbox cosmological volume. We measure the H I scale heights using five different approaches commonly used in the literature: fitting the vertical volume density distribution with a Gaussian, the distance between maximum and half-maximum of the vertical volume density distribution, a semi-empirical description using the velocity dispersion and the galactic gravitational potential, the analytic assumption of hydrostatic equilibrium, and the distance from the midplane which encloses similar to 60 per cent of the H I mass. We find median H I scale heights, measured using the vertical volume distribution, that range from similar to 100 pc in the galactic centres to similar to 800 pc in the outskirts and are in excellent agreement with recent observational results. We speculate that the presence of a realistic multiphase interstellar medium, including cold gas, and realistic stellar feedback are the drivers behind the realistic H I scale heights.