Dynamical effect of gas on spiral pattern speed in galaxies

TL;DR

This paper investigates how the presence of gas influences the stability and pattern speed of spiral arms in galaxies, showing that gas is essential for supporting stable density waves consistent with observations.

Contribution

It demonstrates that including gas in galactic disc models is crucial for supporting stable spiral density waves at observed pattern speeds, a novel insight into galactic dynamics.

Findings

Gas inclusion enables stable density waves at observed pattern speeds.

Even 15% gas by mass significantly affects spiral pattern stability.

Observed pattern speeds fall within the theoretically allowed range when gas is considered.

Abstract

In the density wave theory of spiral structure, the grand-design two-armed spiral pattern is taken to rotate rigidly in a galactic disc with a constant, definite pattern speed. The observational measurement of the pattern speed of the spiral arms, though difficult, has been achieved in a few galaxies such as NGC 6946, NGC 2997, and M 51 which we consider here. We examine whether the theoretical dispersion relation permits a real solution for wavenumber corresponding to a stable wave, for the observed rotation curve and the pattern speed values. We find that the disc when treated to consist of stars alone, as is usually done in literature, does not generally support a stable density wave for the observed pattern speed. Instead the inclusion of the low velocity dispersion component, namely, gas, is essential to obtain a stable density wave. Further, we obtain a theoretical range of allowed pattern speeds that correspond to a stable density wave at a certain radius, and check that for the three galaxies considered, the observed pattern speeds fall in the respective prescribed range. The inclusion of even a small amount (~ 15 %) of gas by mass fraction in the galactic disc is shown to have a significant dynamical effect on the dispersion relation and hence on the pattern speed that is likely to be seen in a real, gas-rich spiral galaxy.