The MAGE Project: Size Evolution and Disk Formation in Massive Galaxies at Cosmic Dawn

Z. Yao, A. Dekel, R. Teyssier, R. S. Somerville, N. Mandelker, Z. L. Andalman, U. P. Steinwandel, & Z. Li

preprint • 2026

Abstract

We investigate size evolution and disk formation with a new suite of eleven cosmological zoom-in simulations, Massive Assembly of Galaxies in the Early universe (MAGE), run with RAMSES at ~10pc resolution until z~9. The target galaxies span stellar masses of M~2e8-3e10 Msol at z=9. The stellar half-mass radius grows steeply with stellar mass at low masses, turns over near M~5e9 Msol, and evolves weakly with redshift. The intrinsic UV sizes in MAGE overlap the compact end of observed distributions, including the compact strong-line systems, but not the more extended population at z>9. Dust, PSF-limited size measurements, and our overdense environments may contribute to this offset. In the simulations, stars dominate the mass within the half-mass radius above M~5e8 Msol, where the central circular speed exceeds a characteristic stellar-feedback scale. Outflows are then increasingly retained within the galaxy, consistent with gas recycling on galactic rather than halo scales, which sustains central gas supply and drives a compaction-like central buildup. The turnover in size at M~5e9 Msol coincides with rapid growth of rotational support to v/sig~1.5-3 in star-forming gas and stars younger than 10Myr, as the central circular speed exceeds the halo virial speed, which the star-forming gas velocity dispersion tracks. The subsequent low central gas fraction, declining central star formation, and younger outskirts suggest the onset of inside-out quenching.

go to arXiv open PDF

Schematic of the proposed evolutionary sequence for massive galaxies at cosmic dawn. The upper curves compare the total circular speed at the stellar half-mass radius, v_ctot(R_1/2) (solid black), the halo virial speed, v_v (dashed black), and the characteristic SN-feedback scale, v_SN~100 km/s (dotted red). The vertical guides mark their crossings near M~1e8 Msol, 5e8 Msol, and 5e9 Msol, separating the proposed gas-escape, halo-recycling, galactic-recycling, and young-disk regimes. Solid and dashed circles denote R_v and R_gal; the legend at the bottom right identifies the stellar and gas components, outflows, returning gas, and rotation. The lower curves illustrate the accompanying size turnover and increase in the rotational support of the star-forming gas and young stars. The fourth, unlabelled guide separates the sampled sequence from later disk growth; dashed continuations and the dashed regime outline indicate expectations beyond this study. Circled letters label the five phases. The right-hand column gives possible observational counterparts; the letters on each card mark the phases in which that type of system is preferentially, but not exclusively, found: compact systems with weak or strong UV lines (lulls or bursts), extended clumpy systems (mergers), and extended massive systems with high axis ratios (disk growth). These associations describe favoured phases rather than measured mass classes or exclusive assignments. All curves are illustrative, the transition masses characterize our simulated sample, and the drawings are not to scale.