Effect of dark matter halo on global spiral modes in galaxies

TL;DR

This study investigates how dark matter halos influence the existence of grand-design spiral modes in galaxies, finding that such modes can exist even with dominant dark matter, contrasting previous assumptions about suppression.

Contribution

It provides a global mode analysis including dark matter halos, showing they do not necessarily suppress spiral modes in low surface brightness galaxies and the Milky Way.

Findings

Global spiral modes are permitted in both dark matter dominated and non-dominated galaxies.

Dark matter halos do not fully suppress global spiral modes, contrary to earlier local feature suppression.

Pattern speeds of global modes align with observed values in the Milky Way.

Abstract

Low surface brightness (LSB) galaxies form a major class of galaxies, and are characterized by low disc surface density and low star formation rate. These are known to be dominated by dark matter halo from the innermost regions. Here we study the role of dark matter halo on the grand-design, $m=2$, spiral modes in a galactic disc by carrying out a global mode analysis in the WKB approximation. The Bohr-Sommerfeld quantization rule is used to determine how many discrete global spiral modes are permitted. First a typical superthin LSB galaxy, UGC 7321 is studied by taking only the galactic disc, modelled as fluid; and then the disc embedded in a dark matter halo. We find that both cases permit the existence of global spiral modes. This is in contrast to earlier results where the inclusion of dark matter halo was shown to nearly fully suppress local, swing-amplified spiral features. Although technically global modes are permitted in the fluid model as shown here, we argue that due to lack of tidal interactions, these are not triggered in LSB galaxies. For comparison, we carried out a similar analysis for the Galaxy, for which the dark matter halo does not dominate in the inner regions. We show that here too the dark matter halo has little effect, hence the disc embedded in a halo is also able to support global modes. The derived pattern speed of the global mode agrees fairly well with the observed value for the Galaxy.