Abstract

In this talk I will discuss a new view of structure formation in galactic disks mediated by gravity and rotation in the regime beyond where the Toomre criterion applies. The description is an extension of the Lin-Shu framework to the regime of ‘open spiral’ waves, well outside the conventional ‘tight-winding’ limit. In this regime, structures grow readily but at specific `critical’ orientations (pitch angles) that help, given local conditions, the growing wave capitalize on azimuthal gravitational forcing. From this perspective the ‘swing amplification’ of shearing material patterns results from the spiral pattern’s swing through the critical pitch angle. The same physical process also stimulates the growth of wave modes at corotation. In both cases, the spiral structures that go on to dominate a disk’s appearance are predicted to appear on scales nearer to the Jeans length than the Toomre length. In gas disks, this makes them consistent with the highly regular, remarkably long filamentary features newly revealed by JWST across the neutral gas disks of nearby galaxies.