The <scp>thesan-zoom</scp> project: central starbursts and inside-out quenching govern galaxy sizes in the early Universe

ABSTRACT We explore the evolution of galaxy sizes at high redshift ($3&amp;lt; z &amp;lt; 13$) using the high-resolution thesan-zoom radiation-hydrodynamics simulations, focusing on the mass range of $10^6\, \mathrm{M}_{\odot } &amp;lt; \mathit{M}_{\ast } &amp;lt; 10^{10}\, \mathrm{M}_{\odot }$. Our analysis reveals that galaxy size growth is tightly coupled to bursty star formation. Galaxies above the star-forming main sequence tend to form stars in a central starburst, which decreases their radial size. These galaxies quench inside-out, causing spatially extended star formation and increasing their radial size, leading to oscillatory behaviour 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 multiphase 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 $z\approx 6$, because the majority of galaxies at $z&amp;gt;6$ show rising star-formation histories, and therefore are in a compaction phase. We demonstrate that H $\alpha$ emission is systematically extended relative to the UV continuum by a median factor of 1.7, consistent with recent James Webb Space Telescope studies. However, in contrast to previous interpretations that link extended H $\alpha$ sizes to inside-out growth, we find that Lyman-continuum (LyC) emission is spatially disconnected from H $\alpha$. 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.