The THESAN project: galaxy sizes during the epoch of reionization

We investigate galaxy sizes at redshift z≳ 6 with the cosmological radiation-magneto-hydrodynamic simulation suite THESAN(-HR). These simulations simultaneously capture the reionization of the large-scale intergalactic medium and resolved galaxy properties. The intrinsic size (r^∗_1/2) of simulated galaxies increases moderately with stellar mass at M_∗≲ 10^8 M_⊙ and decreases fast at larger masses, resulting in a hump feature at M_∗∼ 10^8 M_⊙ that is insensitive to redshift. Low-mass galaxies are in the initial phase of size growth and are better described by a spherical shell model with feedback-driven gas outflows competing with the cold inflows. In contrast, massive galaxies fit better with the disk formation model. They generally experience a phase of rapid compaction and gas depletion, likely driven by internal disk instability rather than external processes. We identify four compact quenched galaxies in the (95.5 cMpc)^3 volume of THESAN-1 at z≃ 6, and their quenching follows reaching a characteristic stellar surface density akin to the massive compact galaxies at cosmic noon. Compared to observations, we find that the median UV effective radius (R^ UV_ eff) of simulated galaxies is at least three times larger than the observed ones at M_∗≲ 10^9 M_⊙ or M_ UV≳ -20 at 6 ≲ z ≲ 10. This inconsistency, related to the hump feature of the intrinsic size–mass relation, persists across many other cosmological simulations with different galaxy formation models and numerical resolutions, demonstrating the potential of using galaxy morphology to constrain galaxy formation models at high redshifts.