Kavli Affiliate: Mark Vogelsberger
| Summary:
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 $10^6,mathrmM_odot < mathrmM_ast < 10^10,mathrmM_odot$. 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 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 $zapprox6$, because the majority of galaxies at $z > 6$ show rising star-formation histories, and therefore are in a compaction phase. We demonstrate that H$α$ 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 H$α$ sizes to inside-out growth, we find that Lyman-continuum (LyC) emission is spatially disconnected from H$α$. 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.
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