Transforming galaxies with EASE: widespread structural changes enabled by short-lived spirals

Abstract

We propose that galaxy structural changes - and the rapid rise of a population of galaxies with early-type morphologies at cosmic noon ($1<z<3$) - can be explained with EASE: Early, Accelerated, Secular Evolution. The mechanism relies on the torques exerted by stellar spirals in late-type galaxies that are present and active at $z>1.5$ as revealed by JWST/NIRCam. The process is at once secular, because the transformative structural changes (heating, compaction, bulge formation) occur over many ($\approx10-30$) orbital periods, but accelerated, because orbital times were significantly shorter than at the present day. In a first application, we take galaxy effective radius as a proxy for galaxy structure and, using new measurements of the abundance and properties of stellar spirals observed in a collection of JWST deep fields, show that EASE predicts a distribution of early-type sizes that is smaller than late-type galaxies and consistent with what is observed. The success of EASE relies on an updated picture of the influence of spiral arms, in which transience plays a key role. We present a new calculation of the characteristic wave equation in the fluid approximation that applies to steady and non-steady open spirals beyond the more traditional tight-winding limit. This shows open, transient spirals above the Jeans length growing and decaying on the order of a dynamical time in a wider region around and inside corotation than canonical steady spirals. We show that this transient activity spreads out angular momentum gains and losses, and the associated dynamical heating, giving spirals a more extended influence than a single steady spiral. The ubiquity of spirals in star-forming galaxies with $M_*>10^{10.5}M_{\odot}$ across the redshifts where early-type galaxies appear in large numbers suggests that EASE can play an important role in the morphological transformation of galaxies.