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

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 $\sim10\,\mathrm{pc}$ resolution until $z\simeq9$. The target galaxies span stellar masses of $M_\star\simeq2\times10^8$-$3\times10^{10}\,M_\odot$ at $z=9$. The stellar half-mass radius grows steeply with stellar mass at low masses, turns over near $M_\star\simeq5\times10^9\,M_\odot$, 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 $M_\star\simeq5\times10^8\,M_\odot$, 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 $M_\star\simeq5\times10^9\,M_\odot$ coincides with rapid growth of rotational support to $v/σ\simeq1.5$-$3$ 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.