Abstract
The causes of spiral structure in galaxies remain uncertain. Leaving aside the grand bisymmetric spirals with their own well-known complications, here we consider the possibility that multi-armed spiral features originate from density inhomogeneities orbiting within disks. Using high-resolution N-body simulations, we follow the motions of stars under the influence of gravity, and show that mass concentrations with properties similar to those of giant molecular clouds can induce the development of spiral arms through a process termed swing amplification. However, unlike in earlier work, we demonstrate that the eventual response of the disk can be highly non-linear, significantly modifying the formation and longevity of the resulting patterns. Contrary to expectations, ragged spiral structures can thus survive at least in a statistical sense long after the original perturbing influence has been removed.
Highlights
Seventy percent of galaxies in the nearby Universe are characterized by a disk with prominent spiral arms, but our understanding of the origin of these patterns is incomplete, even after decades of theoretical study (Toomre 1977; Athanassoula 1984; Binney & Tremaine 2008; Sellwood 2011)
Theories based on a linear approximation describing the local dynamic response, e.g. a disk patch, predict that the emerging spiral patterns should be a superposition of swinging wavelets, shearing with the general flow, but exhibiting transient growth as they swing from leading to trailing
The outcome is surprising, and we find that the response is highly non-linear in the sense that the wakes that initially formed around the perturbers, e.g. the giant molecular clouds, depart from them after only about a tenth of a galactic year and become new perturbers that continuously excite the disk
Summary
Seventy percent of galaxies in the nearby Universe are characterized by a disk with prominent spiral arms, but our understanding of the origin of these patterns is incomplete, even after decades of theoretical study (Toomre 1977; Athanassoula 1984; Binney & Tremaine 2008; Sellwood 2011). One model posits that these features are large-scale density waves continuing to propagate in a differentially rotating disk This theory argues that the matter in the galaxy (stars and gas) can maintain a density wave through gravitational interactions even in the presence of shear. An alternative theory proposes that spiral arms are stochastically produced by local gravitational amplification in a differentially rotating disk (Goldreich & Lynden-Bell 1965; Julian & Toomre 1966) The mechanism behind this process is known as swing amplification and it can be seeded either by preexisting leading waves or else by the response of a disk to the presence of a corotating overdensity, such as a giant molecular cloud. This dynamical response takes the form of wakelets in the surrounding medium, each amplified by its own self-gravity through the swinging of leading features into trailing ones owing to the shear
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