We explore the evolution of galaxy sizes at high redshift (3 < z < 13) using the high-resolution THESAN-ZOOM radiation-hydrodynamics simulations, focusing on the mass range of 106M⊙ < M* < 1010M⊙.
Our analysis reveals that galaxy size growth is tightly coupled to bursty star formation. Galaxies above the star-forming main sequence experience rapid central compaction during starbursts, followed by inside-out quenching and spatially extended star formation that leads to expansion, causing oscillatory behavior around the size-mass relation.
Notably, we find a positive intrinsic size-mass relation at high redshift, consistent with observations but in tension with large-volume simulations. We attribute this discrepancy to the bursty star formation captured by our multi-phase interstellar medium framework, but missing from simulations using the effective equation-of-state approach with hydrodynamically decoupled feedback.
We also find that the normalization of the size-mass relation follows a double power law as a function of redshift, with a break at z≈6, because the majority of galaxies at z > 6 show rising star-formation histories, and therefore are in a compaction phase.
We demonstrate that Hα emission is systematically extended relative to the UV continuum by a median factor of 1.7, consistent with recent JWST studies. However, in contrast to previous interpretations that link extended Hα sizes to inside-out growth, we find that Lyman-continuum (LyC) emission is spatially disconnected from Hα. Instead, a simple Strömgren sphere argument reproduces observed trends, suggesting that extreme LyC production during central starbursts is the primary driver of extended nebular emission.
