Photometric properties of reionization-epoch galaxies in the Simba
  simulations
  We study the photometric properties and sizes of the reionization-epoch
galaxies in high-resolution Simba cosmological hydrodynamical simulations with
box sizes of $[25,50]~h^{-1}$Mpc. Assuming various attenuation laws, we compute
photometry by extincting each star particle's spectrum using the line-of-sight
gas metal column density. The predicted ultraviolet luminosity function (UVLF)
generally agrees with observations at $z=6$, owing to a partial cancellation
between the high metallicities of the simulated galaxies and lower
dust-to-metal ratios. The simulated $z=8$ UVLF is low compared to observations,
likely owing to excessive dust extinction. Simba predicts UV continuum slopes
($\beta$) in agreement with the $z=6$ observations, with the best agreement
obtained using a Calzetti extinction law. Interestingly, the gas-phase
mass-metallicity relation in Simba is higher at $z\sim 6$ than at $z\sim 2$,
suggesting that rapid early enrichment (and dust growth) might be necessary to
match the observed $\beta$. We find that $\beta$ is more sensitive to the dust
extinction law than the UVLF. By generating mock James Webb Space Telescope
(JWST) images and analysing in a manner similar to observations, we show that
Simba's galaxy size-luminosity relation well reproduces the current $z=6$
Hubble observations. Unlike observations at lower redshifts, Simba predicts
similar rest-UV and rest-optical sizes of $z=6$ galaxies, owing to weak age
gradients and dust extinction in star-forming regions counteract each other to
weaken the color gradients within galaxies. These predictions will be testable
with JWST.
