The THESAN-ZOOM project: central starbursts and inside-out quenching govern galaxy sizes in the early Universe

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 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 $z\approx6$, 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.

💡 Research Summary

The paper presents a comprehensive study of galaxy size evolution in the early Universe (3 < z < 13) using the THESAN‑ZOOM suite of high‑resolution radiation‑hydrodynamics zoom‑in simulations. These simulations improve upon the parent THESAN volume by employing a multi‑phase interstellar medium (ISM) model and on‑the‑fly radiative transfer, rather than the effective equation‑of‑state (EOS) approach used in large‑volume runs. The authors analyse a sample of 130 762 subhaloes spanning stellar masses 10⁶–10¹⁰ M⊙, with at least 100 star particles per galaxy, allowing robust measurements of half‑mass radii, star formation rates (SFR), and nebular emission (LyC and Hα).

A key result is that galaxy size growth is tightly linked to bursty star formation. When a galaxy lies above the star‑forming main sequence, rapid central gas compression triggers a “central starburst”. This event dramatically raises the SFR, but because star formation is concentrated in the core, the stellar half‑mass radius shrinks temporarily. The intense feedback (photo‑ionisation, radiation pressure, stellar winds, supernovae) then drives gas outward, quenching star formation from the inside out. Subsequent star formation occurs preferentially in the outskirts, causing the radius to expand again. The galaxy therefore oscillates around the size–mass relation, moving down during compaction and up during inside‑out quenching.

At redshifts z > 6 most galaxies exhibit rising star‑formation histories, placing them in a prolonged compaction phase. The normalization of the size–mass relation follows a double power‑law in redshift, with a clear break at z ≈ 6. Below this redshift the inside‑out quenching dominates, leading to larger sizes at fixed mass; above it, central compaction keeps galaxies compact. This two‑regime behaviour reproduces the observed positive slope of the size–mass relation at high‑z, in contrast to many large‑volume simulations that predict a negative slope. The authors attribute this discrepancy to the inability of effective‑EOS models to capture bursty star formation and realistic multi‑phase feedback, which are essential for the compaction‑quenching cycle.

The paper also examines wavelength‑dependent sizes. Using on‑the‑fly radiative transfer, they generate mock Hα maps and UV continuum images. Hα emission is systematically more extended than the UV by a median factor of 1.7, matching recent JWST measurements. Contrary to the common interpretation that extended Hα signals inside‑out growth, the authors find that LyC photons are highly concentrated in the central starburst, while the resulting ionised nebula (the Strömgren sphere) expands to larger radii. A simple Strömgren sphere scaling, Rₛ ∝ (Qₕ / nₑ²)¹ᐟ³, where Qₕ is the LyC photon production rate and nₑ the electron density, reproduces the observed Hα‑to‑UV size ratios. Thus, the extended nebular emission is driven primarily by radiative transfer effects linked to bursty central star formation, not by a gradual outward migration of star‑forming regions.

Finally, the authors highlight the spatial disconnect between LyC and Hα emission: LyC remains centrally peaked, while Hα is spread over the larger ionised volume. This provides a diagnostic for identifying galaxies undergoing intense, centrally‑located starbursts in the early Universe.

Overall, the study demonstrates that galaxy sizes at z > 3 are governed by a cyclical interplay of central starbursts and inside‑out quenching, leading to an oscillatory size‑mass relation and a redshift‑dependent normalization. The work underscores the importance of high‑resolution, multi‑phase simulations for interpreting JWST observations and for resolving tensions between theory and data regarding the size evolution of early galaxies.