  Early galaxies were the radiation source for reionization, with the
photoheating feedback from the reionization process expected to reduce the
efficiency of star formation in low mass haloes. Hence, to fully understand
reionization and galaxy formation, we must study their impact on each other.
The THESAN project has so far aimed to study the impact of galaxy formation
physics on reionization, but here we present the new THESAN simulations with a
factor 50 higher resolution ($m_{\rm b} \approx 10^4$~M$_\odot$) that aim to
self-consistently study the back-reaction of reionization on galaxies. By
resolving haloes with virial temperatures $T_{\rm vir} < 10^4$~K, we are able
to demonstrate that simplistic, spatially-uniform, reionization models are not
sufficient to study early galaxy evolution. Comparing the self-consistent
THESAN model (employing fully coupled radiation hydrodynamics) to a uniform UV
background, we are able to show that galaxies in THESAN are predicted to be
larger in physical extent (by a factor $\sim 2$), less metal enriched (by $\sim
0.2$~dex), and less abundant (by a factor $\sim 10$ at $M_{\rm 1500}~=~-10$) by
$z=5$. We show that differences in star formation and enrichment patterns lead
to significantly different predictions for star formation in low mass haloes,
low-metallicity star formation, and even the occupation fraction of haloes. We
posit that cosmological galaxy formation simulations aiming to study early
galaxy formation $z \gtrsim 3$ must employ a spatially inhomogeneous UV
background to accurately reproduce galaxy properties.

Astrophysics of Galaxies;Cosmology and Nongalactic Astrophysics