Slowly Breaking Waves: The Longevity of Tidally Induced Spiral Structure

TL;DR

This paper presents evidence of long-lived, tidally induced spiral waves in galaxy models, supported by an analytic theory, suggesting such waves can persist for billions of years and influence galaxy evolution.

Contribution

It introduces a new analytic theory explaining long-lived spiral waves as caustic waves, supported by numerical models, extending previous work on colliding ring galaxies.

Findings

Spiral waves can persist for up to a few billion years.

Waves are maintained by coherent stellar oscillations, not transient phenomena.

Mechanism explains long-lived, tightly wound spirals in early-type galaxies.

Abstract

We have discovered long-lived waves in two sets of numerical models of fast (marginally bound or unbound) flyby galaxy collisions, carried out independently with two different codes. In neither simulation set are the spirals the result of a collision-induced bar formation. Although there is variation in the appearance of the waves with time, they do not disappear and reform recurrently, as seen in other cases described in the literature. We also present an analytic theory that can account for the wave structure, not as propagating transients, nor as a fixed pattern propagating through the disc. While these waves propagate through the disc, they are maintained by the coherent oscillations initiated by the impulsive disturbance. Specifically, the analytic theory suggests that they are caustic waves in ensembles of stars pursuing correlated epicyclic orbits after the disturbance. This theory is an extension of that developed by Struck and collaborators for colliding ring galaxies. The models suggest that this type of wave may persist for a couple of Gyr., and galaxy interactions occur on comparable timescales, so waves produced by the mechanism may be well represented in observed spirals. In particular, this mechanism can account for the tightly wound, and presumably long-lived spirals, seen in some nearby early-type galaxies. These spirals are also likely to be common in groups and clusters, where fast encounters between galaxies occur relatively frequently. However, as the spirals become tightly wound, and evolve to modest amplitudes, they may be difficult to resolve unless they are nearby. Nonetheless, the effect may be one of several processes that result from galaxy harassment, and via wave-enhanced star formation contribute to the Butcher-Oemler effect.