2020MNRAS.499.2063P

We model the galactic discs of seven nearby large spiral galaxies as three-component systems consist of stars, molecular gas, and atomic gas in vertical hydrostatic equilibrium. We set up the corresponding joint Poisson-Boltzmann equation and solve it numerically to estimate the 3D distribution of H I in these galaxies. While solving the Poisson-Boltzmann equation, we do not consider a constant H I velocity dispersion (σ_HI); rather, we develop an iterative method to self-consistently estimate the σ_HI profile in a galaxy by using the observed second-moment profile of the H I spectral cube. Using the density solutions, we determine the H I vertical scale height in our galaxies. We find that the H I discs flare in a linear fashion as a function of radius. H I scale height in our galaxies is found to vary between a few hundred parsecs at the centre to ∼1-2 kpc at the outskirts. We estimate the axial ratio of the H I discs in our sample galaxies and find a median ratio of 0.1, which is much lower than what is found for dwarf galaxies, indicating much thinner H I discs in spiral galaxies. Very low axial ratios in three of our sample galaxies (NGC 5055, NGC 6946, and NGC 7331) suggest them to be potential superthin galaxies. Using the H I distribution and the H I hole sizes in NGC 6946, we find that most of the H I holes in this galaxy are broken out into the circumgalactic medium and this breaking out is more effective in the inner radii as compared to the outer radii.