Photometric properties of reionization-epoch galaxies in the SIMBA simulations

Abstract

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 similar to 6 than at z similar to 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 colour gradients within galaxies. These predictions will be testable with JWST.