Wiggle Instability of Galactic Spiral Shocks: Effects of Magnetic Fields

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 $\beta \sim 5-10$, the most unstable mode has a wavelength of $\sim 0.1-0.2$ times the arm-to-arm separation, which appears consistent with a mean spacing of observed feathers.

41 pages, 12 figures, 3 tables, Accepted for publication in ApJ

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Cited by in corpus (11)

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• Regularly Spaced Infrared Peaks in the Dusty Spirals of Messier 100
• Magnetic field evolution and reversals in spiral galaxies
• Periodicity makes galactic shocks unstable - I. Linear analysis
• Globular Clusters and Spur Clusters in NGC 4921, the Brightest Spiral Galaxy in the Coma Cluster
• Global enhancement and structure formation of the magnetic field in spiral galaxies
• The Physical Origin and the Properties of Arm Spurs/Feathers in Local Simulations of the Wiggle Instability
• Equilibrium Sequences and Gravitational Instability of Rotating Isothermal Rings
• The role of magnetic fields in disc galaxies: spiral arm instability
• Synthetic HI observations of spiral structure in the outer disk in galaxies
• Formation of filaments and feathers in disc galaxies: Is self-gravity enough?