Transforming Galaxies with EASE: Widespread structural changes enabled by short-lived spirals

Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium

^⋆ Corresponding author: arjen.vanderwel@ugent.be

Received: 5 August 2025
Accepted: 2 September 2025

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 both secular – because the transformative structural changes (heating, compaction, and bulge formation) occur over many (≈10 − 30) orbital periods – and accelerated, because orbital times were significantly shorter than at the present day. In a first application, we took 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, we show that EASE predicts a distribution of early-type sizes that is smaller than late-type galaxies and consistent with that 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 nonsteady 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, as well as the associated dynamical heating, giving spirals a more extended influence than a single steady spiral. The ubiquity of spirals in star-forming galaxies with stellar masses M_⋆ > 10^10.5 M_⊙ across the entire redshift range where early-type galaxies appear in large numbers suggests that EASE can play an important, or even dominant, role in morphological transformations across cosmic time.