HI scale height in spiral galaxies

TL;DR

This study models the vertical structure of HI gas in seven spiral galaxies, revealing that HI discs flare linearly with radius, are thinner than in dwarf galaxies, and that HI holes often break out into the circumgalactic medium.

Contribution

It introduces a self-consistent method to estimate the HI velocity dispersion profile and vertical scale height in spiral galaxies using joint Poisson-Boltzmann equations.

Findings

HI scale height varies from a few hundred parsecs to 1-2 kpc.

HI discs are significantly thinner than in dwarf galaxies.

Most HI holes in NGC 6946 break out into the circumgalactic medium.

Abstract

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 three-dimensional distribution of HI in these galaxies. While solving the Poisson-Boltzmann equation, we do not consider a constant HI velocity dispersion ($\sigma_{\rm HI}$); rather, we develop an iterative method to self-consistently estimate the $\sigma_{\rm HI}$ profile in a galaxy by using the observed second-moment profile of the HI spectral cube. Using the density solutions, we determine the HI vertical scale height in our galaxies. We find that the HI discs flare in a linear fashion as a function of radius. HI scale height in our galaxies is found to vary between a few hundred parsecs at the center to $\sim 1-2$ kpc at the outskirts. We estimate the axial ratio of the HI 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 HI 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 HI distribution and the HI hole sizes in NGC 6946, we find that most of the HI 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.