| Summary: | © 2020 The Author(s). We study the relation between the UV slope, β, and the ratio between the infrared-and UV luminosities (IRX) of galaxies from TNG50, the latest installment of the IllustrisTNG galaxy formation simulations. We select 7280 star-forming main-sequence (SFMS) galaxies with stellar mass ≥109 M⊙ at redshifts 0 ≤ z ≤ 4 and perform radiative transfer with skirt to model effects of interstellar medium dust on the emitted stellar light. Assuming a Milky Way dust type and a dust-to-metal ratio of 0.3, we find that TNG50 SFMS galaxies generally agree with observationally derived IRX-β relations at z ≳ 1. However, we find a redshift-dependent systematic offset with respect to empirically derived local relations, with the TNG50 IRX-β relation shifting towards lower β and steepening at higher redshifts. This is partially driven by variations in the dust-uncorrected UV slope of galaxies, due to different star formation histories of galaxies selected at different cosmic epochs; we suggest the remainder of the effect is caused by differences in the effective dust attenuation curves of galaxies as a function of redshift. We find a typical galaxy-to-galaxy variation of 0.3 dex in infrared excess (IRX) at fixed β, correlated with intrinsic galaxy properties: galaxies with higher star formation rates, star formation efficiencies, gas metallicities and stellar masses exhibit larger IRX values. We demonstrate a degeneracy between stellar age, dust geometry, and dust composition: z = 4 galaxies with a Small Magellanic Cloud dust type follow the same IRX-β relation as low-redshift galaxies with MW dust. We provide a redshift-dependent fitting function for the IRX-β relation for MW dust based on our models.
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