Dust attenuation, dust emission, and dust temperature in galaxies at z ≥ 5: a view from the FIRE-2 simulations

We present a suite of 34 high-resolution cosmological zoom-in simulations consisting of thousands of haloes up to M_halo∼ 10^{12} M_{⊙ } (M_{\ast }∼ 10^{10.5} M_{⊙ }) at z ≥ 5 from the Feedback in Realistic Environments project. We post-process our simulations with a three-dimensional Monte Carlo dust radiative transfer code to study dust attenuation, dust emission, and dust temperature within these simulated z ≥ 5 galaxies. Our sample forms a tight correlation between infrared excess (IRX ≡ F_IR/F_UV) and ultraviolet (UV)-continuum slope (β_UV), despite the patchy, clumpy dust geometry shown in our simulations. We find that the IRX-β_UV relation is mainly determined by the shape of the attenuation law and is independent of its normalization (set by the dust-to-gas ratio). The bolometric IR luminosity (L_IR) correlates with the intrinsic UV luminosity and the star formation rate (SFR) averaged over the past 10 Myr. We predict that at a given L_IR, the peak wavelength of the dust spectral energy distributions for z ≥ 5 galaxies is smaller by a factor of 2 (due to higher dust temperatures on average) than at z = 0. The higher dust temperatures are driven by higher specific SFRs and SFR surface densities with increasing redshift. We derive the galaxy UV luminosity functions (UVLFs) at z = 5-10 from our simulations and confirm that a heavy attenuation is required to reproduce the observed bright-end UVLFs. We also predict the IR luminosity functions (IRLFs) and UV luminosity densities at z = 5-10. We discuss the implications of our results on current and future observations probing dust attenuation and emission in z ≥ 5 galaxies.