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

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 $\sim 10\mathrm{pc}$ resolution until $z\simeq 9$. The target galaxies span stellar masses of $M_{\star }\simeq 2\times 10^8$-$3\times 10^{10}{M}_{\odot }$ at $z=9$. The stellar half-mass radius grows steeply with stellar mass at low masses, turns over near $M_{\star }\simeq 5\times 10^9{M}_{\odot }$, 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 $M_{\star }\simeq 5\times 10^8{M}_{\odot }$, 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 $M_{\star }\simeq 5\times 10^9{M}_{\odot }$ coincides with rapid growth of rotational support to $v/\sigma \simeq 1.5$-$3$ 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.

arXiv comment30 pages, 15 figures, submitted to MNRAS, comments are welcome!