The Shape of Dark Matter Haloes, V. Analysis of observations of edge-on galaxies

TL;DR

This study analyzes the dark matter halo shapes of edge-on galaxies through rotation curve and vertical force field gradient decomposition, revealing differences in halo density, scale length, and shape, with implications for dark matter modeling.

Contribution

It introduces a combined analysis of rotation curves and vertical force gradients to model dark matter halos in edge-on galaxies, considering HI self-absorption effects.

Findings

Dark matter halos have less dense cores when considering HI self-absorption.

Median halo shape is spherical for self-absorbing HI, prolate for optically thin models.

Galaxies show diverse halo shapes, including oblate and prolate configurations.

Abstract

In the previous papers in this series, we have measured the stellar and \hi content in a sample of edge-on galaxies. In the present paper, we perform a simultaneous rotation curve and vertical force field gradient decomposition for five of these edge-on galaxies. The rotation curve decomposition provides a measure of the radial dark matter potential, while the vertical force field gradient provide a measure of the vertical dark matter potential. We fit dark matter halo models to these potentials. Using our \hi self-absorption results, we find that a typical dark matter halo has a less dense core ($0.094\pm0.230$\,M$_\odot$/pc$^3$) compared to an optically thin \hi model ($0.150\pm0.124$\,M$_\odot$/pc$^3$). The HI self-absorption dark matter halo has a longer scale length $R_c$ of $1.42\pm 3.48$\,kpc, versus $1.10\pm 1.81$\,kpc for the optically thin HI model. The median halo shape is spherical, at $q=1.0\pm0.6$ (self-absorbing \hi), while it is prolate at $q=1.5\pm0.6$ for the optically thin. Our best results were obtained for ESO\,274-G001 and UGC\,7321, for which we were able to measure the velocity dispersion in Paper III. These two galaxies have drastically different halo shapes, with one oblate and one strongly prolate. Overall, we find that the many assumptions required make this type of analysis susceptible to errors.