Early thin-disc assembly revealed by JWST edge-on galaxies

Abstract

The vertical structure of stellar discs provides key constraints on their formation and evolution. Nearby spirals, including the Milky Way, host thin and thick components that may arise either from an early turbulent phase or from the subsequent dynamical heating of an initially thin disc; measuring disc thickness across cosmic time therefore offers a direct test of these scenarios. We present a new methodology to measure the thickness of edge-on galaxies that explicitly accounts for departures from perfectly edge-on orientations by fitting a full three-dimensional model with forward modelling. This improves on traditional approaches that assume an inclination of $90^\circ$ and can bias thicknesses high. Applying the method to JWST imaging of galaxies at $1<z<3$ with stellar masses $\gtrsim 10^9~M_\odot$ from four major surveys, we measure a median scale height of $z_0 = 0.25\pm0.14$~kpc and a median ratio $h_r/z_0=8.4\pm3.7$. These values are consistent with the Milky Way and local thin discs, and indicate scale heights $\sim 1.6$ times smaller than those inferred for local galaxies from single-disc fits. This result implies that thin discs are already present at $z\sim3$. We further show that a thick disc contributing 10\% of the thin-disc luminosity would be detectable in the data considered in this work, implying that any thick disc present must be fainter and favouring a scenario in which thick discs build up progressively through dynamical heating at $z<1$.