The Physical Origin and the Properties of Arm Spurs/Feathers in Local Simulations of the Wiggle Instability (2110.04108v2)

Abstract: Gaseous substructures such as feathers and spurs dot the landscape of spiral arms in disc galaxies. One of the candidates to explain their formation is the wiggle instability of galactic spiral shocks. We study the wiggle instability using local 2D hydrodynamical isothermal non-self gravitating simulations. We find that: (1) Simulations agree with analytic linear stability analysis only under stringent conditions. They display surprisingly strong non-linear coupling between the different modes, even for small mode amplitudes ($\sim 1\%$). (2) We demonstrate that the wiggle instability originates from a combination of two physically distinct mechanisms: the first is the Kelvin-Helmholtz instability, and the second is the amplification of infinitesimal perturbations from repeated shock passages. These two mechanisms can operate simultaneously, and which mechanism dominates depends on the underlying parameters. (3) We explore the parameter space and study the properties of spurs/feathers generated by the wiggle instability. The wiggle instability is highly sensitive to the underlying parameters. The feather separation decreases, and the growth rate increases, with decreasing sound speed, increasing potential strength and decreasing interarm distance. (4) We compare our simulations with a sample of 20 galaxies in the HST Archival Survey of Spiral Arm Substructure of La Vigne et al. and find that the wiggle instability is able to reproduce the typical range of feather separations seen in observations. It remains unclear how the wiggle instability relates to competing mechanisms for spur/feather formation such as the magneto-jeans instability and the stochastic accumulation of gas due to correlated supernova feedback.

• The paper investigates the physical origin of galactic arm spurs/feathers using 2D hydrodynamical simulations, finding agreement with linear analytic theory predictions for wiggle instability modes and growth rates.
• A key finding is that wiggle instability characteristics like feather spacing and growth rate strongly depend on gas sound speed, spiral potential strength, and interarm separation.
• The study shows that parasitic Kelvin-Helmholtz instability can coexist with wiggle instability, particularly in high shear environments, providing insights aligned with observed feather separations.

An Overview of the Formation of Arm Spurs/Feathers in Local Simulations of the Wiggle Instability

The formation of spurs and feathers within the spiral arms of disk galaxies represents a notable structure in galactic astrophysics. The paper presented by Mandowara et al. explores the mechanisms underlying these substructures, focusing on the hydrodynamical processes, especially the wiggle instability, using two-dimensional isothermal simulations. This work provides an insightful analysis by comparing these simulations to a linear stability analysis and further exploring a variety of parameter spaces to better understand the characteristics influencing the wiggle instability.

The paper introduces the concept of wiggle instability as a potential mechanism for the formation of spurs/feathers, diverging from other proposed concepts such as magneto-gravitational instabilities and supernova feedback-induced gas accumulation. The authors employ local, idealized hydrodynamical simulations without self-gravity or magnetic effects, considering gas flow within an imposed spiral potential. A noteworthy contribution of their paper is the evident agreement between the characteristic modes and growth rates observed in simulations and those predicted by prior linear analytic theory, showcasing a small but significant deviation mainly attributed to non-linear mode coupling effects.

A key finding of the research is the strong dependence of the wiggle instability's characteristics, notably the average spacing and growth rate of the unstable modes, on variables such as the sound speed of the gas, the strength of the spiral potential, and the interarm separation. Optimization of these parameters is found to significantly alter the instability dynamics. Specifically, the paper notes that an increased spiral potential and decreased sound speed correspond to shorter feather separations and a higher growth rate. Moreover, the authors elucidate on the impact of boundary conditions, demonstrating that periodic boundary conditions that allow for the cumulative amplification of perturbations across shock passages enhance the instability.

The paper does not shy away from challenging current understandings, providing evidence that the parasitic Kelvin-Helmholtz instability can manifest simultaneously with the wiggle instability, adding complexity to the formation of substructures along galactic spiral arms. This distinct interaction further reinforces the instability in scenarios with high post-shock shear, strong spiral potentials, and low sound speeds — factors that highlight how these characteristics can differentiate between the dominant mechanism at play.

From an observational perspective, the results of the simulations align with empirical data, especially concerning the observed range of feather separations in galaxies. Though the paper acknowledges the challenge posed by the idealizations made in the simulations, it nonetheless opens a discourse on how such theoretical insights can contribute to a greater understanding of spiral structure dynamics.

Interactive effects among gravitational forces, gas dynamics, magnetic fields, and stellar feedback likely contribute to the overall picture of spur and feather formation. The paper indicates that future work could benefit from integrating self-gravity and magnetic fields to further refine the role of wiggle instability relative to other mechanisms, offering a more comprehensive synthesis of spiral arm substructures in galactic disks.

In summary, Mandowara et al.'s exploration into the wiggle instability presents a sophisticated blend of theory and simulation that sheds light on the complexities underpinning spiral arm substructures. The nuanced analysis they provide, along with the results that closely echo observed phenomena, pushes the boundaries of how we might utilize simpler models to discern more about the dynamic environments of galaxies. With continued research and better modeling, this paper paves the way for deeper insights into the chaotic beauty that characterizes spiral galaxies.