Kavli Affiliate: Mark Vogelsberger
| First 5 Authors: William McClymont, Sandro Tacchella, Aaron Smith, Rahul Kannan, Ewald Puchwein
| 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 $10^6,mathrm{M}_{odot} < mathrm{M}_{ast} <
10^{10},mathrm{M}_{odot}$. Our analysis reveals that galaxy size growth is
tightly coupled to bursty star formation. Galaxies above the star-forming main
sequence 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 $zapprox6$, because the
majority of galaxies at $z > 6$ show rising star-formation histories, and
therefore are in a compaction phase. We demonstrate that H$alpha$ 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$alpha$ sizes to inside-out growth, we
find that Lyman-continuum (LyC) emission is spatially disconnected from
H$alpha$. Instead, a simple Str"{o}mgren 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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