HI scale height in dwarf galaxies

Abstract

Assuming a vertical hydrostatic equilibrium in the baryonic discs, joint Poisson–Boltzmann equation was set up and solved numerically in a sample of 23 nearby dwarf galaxies from the LITTLE-THINGS survey. This is the largest sample to date for which detailed hydrostatic modelling is performed. The solutions of the Poisson–Boltzmann equation provide a complete three-dimensional distribution of the atomic hydrogen (H I) in these galaxies. Using these solutions, we estimate the vertical scale height (defined as the half width at half maxima of the density distribution) of the H I as a function of radius. We find that the scale height in our sample galaxies varies between a few hundred parsec at the centre to a few kiloparsec at the edge. These values are significantly higher than what is observed in spiral galaxies. We further estimate the axial ratios to investigate the thickness of the H I discs in dwarf galaxies. For our sample galaxies, we find a median axial ratio to be 0.40, which is much higher than the same observed in the Milky Way. This indicates that the vertical hydrostatic equilibrium results in thicker H I discs in dwarf galaxies naturally.