Dusty galaxies in the Epoch of Reionization: simulations

The recent discovery of dusty galaxies well into the Epoch of Reionization (redshift $z>6$) poses challenging questions about the properties of the interstellar medium in these pristine systems. By combining state-of-the-art hydrodynamic and dust radiative transfer simulations, we address these questions focusing on the recently discovered dusty galaxy A2744_YD4 ($z=8.38$, Laporte et al. 2017}). We show that we can reproduce the observed spectral energy distribution (SED) only using different physical values with respect to the inferred ones by Laporte et al(2017), i.e. a star formation rate of SFR = 78\; M_ yr⁻¹, a factor ≈ 4 higher than deduced from simple Spectral Energy Distribution fitting. In this case we find: (a) dust attenuation (corresponding to τV=1.4) is consistent with a Milky Way extinction curve; (b) the dust-to-metal ratio is low, fd ~ 0.08, implying that early dust formation is rather inefficient; (c) the luminosity-weighted dust temperature is high, Td=91± 23\, K, as a result of the intense (≈ 100× MW) interstellar radiation field; (d) due to the high Td, the ALMA Band 7 detection can be explained by a limited dust mass, Md=1.6× 10⁶M_. Finally, the high dust temperatures might solve the puzzling low infrared excess recently deduced for high-z galaxies from the IRX-β relation.