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Abstract Theories of spiral structure traditionally separate into tight-winding Lin–Shu spiral density waves and the swing-amplified material patterns of Goldreich & Lynden-Bell and Julian & Toomre. In this paper we consolidate these two types of spirals into a unified description, treating density waves beyond the tight-winding limit, in the regime of shearing and nonsteady open spirals. This shearing wave scenario novelly captures swing amplification that enables structure formation above conventional Q thresholds. However, it also highlights the fundamental role of spiral forcing on the amplification process in general, whether the wave is shearing or not. Thus it captures resonant and nonresonant mode growth through the donkey effect described by Lynden-Bell & Kalnajs and, critically, the cessation of growth when donkey behavior is no longer permitted. Our calculations predict growth exclusive to trailing spirals above the Jeans length, the prominence of spirals across a range of orientations that increases with decreasing arm multiplicity, and a critical orientation where growth is fastest that is the same for both modes and material patterns. Predicted structures are consistent with highly regular, high-multiplicity gaseous spur features and long filaments spaced close to the Jeans scale in spirals and bars. Applied to stellar disks, conditions favor low multiplicity ( m < 5) open trailing spirals with pitch angles in the observed range 10° < i p < 50°. The results of this work serve as a basis for describing spirals as a unified class of transient waves, abundantly stimulated but narrowly selected for growth depending on local conditions.
Meidt et al. (Mon,) studied this question.