The oblateness of dark matter halos of nearby galaxies and its correlation with gas mass fractions

TL;DR

This study introduces a method to measure the oblateness of dark matter halos in gas-rich galaxies and finds a correlation between halo shape and galaxy mass, revealing that gas-dominated galaxies tend to have oblate halos.

Contribution

The paper presents a novel technique to determine dark matter halo oblateness using HI data and applies it to a diverse galaxy sample, uncovering a correlation with stellar mass.

Findings

Gas-rich, low-mass galaxies have oblate halos (q<0.55).

Stellar mass positively correlates with halo sphericity.

Halo shape influences galaxy disk properties.

Abstract

We present a method to measure the the oblateness parameter q of the dark matter halos of gas rich galaxies that have extended HI disks. We have applied our model to a sample of 20 nearby galaxies that are gas rich and close to face-on, of which 6 are large disk galaxies, 8 have moderate stellar masses and 6 are low surface brightness (LSB) dwarf galaxies. We have used the stacked HI velocity dispersion and HI surface densities to derive q in the outer disk regions. Our most important result is that gas dominated galaxies (such as LSB dwarfs) that have M(gas)/M(baryons)>0.5 have oblate halos (q<0.55), whereas stellar dominated galaxies have a range of q values from 0.2 to 1.3. We also find a significant positive correlation between q and stellar mass, which indicates that galaxies with massive stellar disks have a higher probability of having halos that are spherical or slightly prolate, whereas low mass galaxies preferably have oblate halos. We briefly also discuss how the halo shape affects the disks of galaxies, especially the oblate halos.