Dwarf irregular galaxies with extended HI gas disks: Suppression of small-scale spiral structure by dark matter halo

TL;DR

This study shows that the dominant dark matter halos in dwarf irregular galaxies suppress small-scale spiral structures, explaining their observed lack of such features despite being gas-rich.

Contribution

It provides a detailed analysis of how dark matter halos inhibit small-scale spiral features in dwarf irregular galaxies, a novel insight into galaxy morphology.

Findings

Dark matter halos increase the Toomre Q parameter, preventing spiral amplification.

Small-scale spiral structures are absent despite high gas content.

Dark matter dominance explains the lack of spiral arms in these galaxies.

Abstract

Dwarf irregular galaxies with extended HI disk distributions, such as DDO 154, allow measurement of rotation curves, hence deduction of dark matter halo properties to large radial distances, up to several times the optical radius. These galaxies contain a huge reservoir of dark matter halo, which dominates over most of disk. We study the effect of the dark matter halo on small-scale spiral features by carrying out the local, non-axisymmetric perturbation analysis in the disks of five such late-type, gas-rich dwarf irregular galaxies, namely, DDO 154, NGC 3741, DDO 43, NGC 2366, and DDO 168 which host a dense and compact dark matter halo. We show that when the gas disk is treated alone, it allows a finite swing amplification; which would result in small-scale spiral structure in the outer gas disk, but the addition of dark matter halo in the analysis results in a higher Toomre Q parameter which prevents the amplification almost completely. This trend is also seen to be true in regions inside the optical radius. This implies absence of strong small-scale spiral arms in these galaxies, which is in agreement with observations. Hence despite being gas-rich, and in fact having gas as the main baryonic component, these galaxies cannot support small-scale spiral structure which would otherwise have been expected in normal gas-rich galaxies.