Theoretical determination of HI vertical scale heights in the dwarf galaxies: DDO 154, HoII, IC2574 & NGC2366

TL;DR

This study models the vertical scale heights of atomic hydrogen in dwarf galaxies by considering gravitational coupling with stars and dark matter, revealing that gas self-gravity significantly influences disk thickness and flaring behavior.

Contribution

It introduces a numerical model incorporating gas self-gravity and rising rotation curves to accurately determine HI scale heights in dwarf galaxies, improving upon previous estimates.

Findings

Most galaxies show HI disk flaring with radius.

Gas self-gravity reduces estimated scale heights.

One galaxy has a consistently thick HI disk.

Abstract

In this paper, we model dwarf galaxies as a two-component system of gravitationally coupled stars and atomic hydrogen gas in the external force field of a pseudo-isothermal dark matter halo, and numerically obtain the radial distribution of {H\,{\sc i}} vertical scale heights. This is done for a group of four dwarf galaxies (DDO\,154, Ho\,II, IC\,2574 and NGC\,2366) for which most necessary input parameters are available from observations. The formulation of the equations takes into account the rising rotation curves generally observed in dwarf galaxies. The inclusion of self-gravity of the gas into the model at par with that of the stars results in scale heights that are smaller than what was obtained by previous authors. This is important as the gas scale height is often used for deriving other physical quantities. The inclusion of gas self-gravity is particularly relevant in the case of dwarf galaxies where the gas cannot be considered a minor perturbation to the mass distribution of the stars. We find that three out of four galaxies studied show a flaring of their {H\,{\sc i}} disks with increasing radius, by a factor of a few within several disk scale lengths. The fourth galaxy has a thick {H\,{\sc i}} disk throughout. This arises as a result of the gas velocity dispersion remaining constant or decreasing only slightly while the disk mass distribution declines exponentially as a function of radius.