Wiggle Instability of Galactic Spiral Shocks: Effects of Magnetic Fields

TL;DR

This study investigates how magnetic fields influence the wiggle instability of galactic spiral shocks, revealing that magnetic fields partially suppress the instability and affect the formation of gaseous feathers in spiral galaxies.

Contribution

It provides a linear stability analysis and simulations showing magnetic fields' stabilizing effect on spiral shock wiggle instability, with implications for observed feather spacing.

Findings

Magnetic fields partially suppress the wiggle instability.

Unperturbed magnetic fields reduce density compression in shocks.

Most unstable mode wavelength matches observed feather spacing.

Abstract

It has been suggested that the wiggle instability (WI) of spiral shocks in a galactic disk is responsible for the formation of gaseous feathers observed in grand-design spiral galaxies. We perform both a linear stability analysis and numerical simulations to investigate the effect of magnetic fields on the WI. The disk is assumed to be infinitesimally-thin, isothermal, and non-self-gravitating. We control the strengths of magnetic fields and spiral-arm forcing using the dimensionless parameters $\beta$ and $\mathcal{F}$, respectively. By solving the perturbation equations as a boundary-eigenvalue problem, we obtain dispersion relations of the WI for various values of $\beta=1-\infty$ and $\mathcal{F}=5\%$ and $10\%$. We find that the WI arising from the accumulation of potential vorticity at disturbed shocks is suppressed, albeit not completely, by magnetic fields. The stabilizing effect of magnetic fields is not from the perturbed fields but from the unperturbed fields that reduce the density compression factor in the background shocks. When $\mathcal{F}=5\%$ and $\beta\lesssim 10$ or $\mathcal{F}=10\%$ and $\beta\sim5-10$, the most unstable mode has a wavelength of $\sim0.1-0.2$ times the arm-to-arm separation, which appears consistent with a mean spacing of observed feathers.