Nexae in caverna: the secular evolution of disks via collectively excited, transient spiral structure
Using the hydrodynamical (fluid) approximation, we present a self-consistent theoretical framework that couples the origin, evolution and decay of spiral structures to the secular dynamical evolution of their host galactic disks. Our approach highlights non-resonant spiral excitation through azimuthal forcing that leverages mild, pervasive gradients in the disk's mass and angular momentum distributions, structural features we term cavernae. These cavernae are weaker but more widespread than the sharp features behind groove mode excitation and commonplace in exponential disks. We discuss how non-resonant features combine with other responses -- resonant dressing, steady waves, groove modes -- to produce a global, evolving spiral nexum that transports angular momentum and reshapes the disk. Using expressions for torques, angular momentum transport and heating, we demonstrate that global spirals are intrinsically self-limiting; the angular momentum changes and heating they generate quenches their own growth, dictating a finite lifetime for any single spiral episode. A succession of transient episodes, each with properties adjusted to the changed disk conditions, lays the pathway to long-lived spiral activity. This behavior suggests that the character of secular evolution shifts over time. We find that the short-lived, high-multiplicity (high-m) spirals that dominate in dynamically cold disks induce widespread, impulse-like non-resonant heating, yet with a low ratio of heating to radial migration. As the disk warms, high-m features are suppressed, leading to steadier, lower-m spirals that heat progressively more efficiently near resonances. In this light, the dynamical coldness of disk galaxies today requires a past dominated by high-m transient perturbations, whereas warmer, more compact systems reflect an advanced stage of evolution regulated by transient, low-m spirals.