Dusty galaxies in the Epoch of Reionization: simulations

Abstract

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 $\mathrm{SFR}=78{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$, a factor $\approx 4$ higher than deduced from simple Spectral Energy Distribution fitting. In this case we find: (a) dust attenuation (corresponding to $\ddot{I}{„}_V=1.4$) is consistent with a Milky Way extinction curve; (b) the dust-to-metal ratio is low, $f_{\mathrm{d}}\sim 0.08$, implying that early dust formation is rather inefficient; (c) the luminosity-weighted dust temperature is high, $T_d=91\pm 23\mathrm{K}$, as a result of the intense ($\approx 100\times$ MW) interstellar radiation field; (d) due to the high $T_d$, the ALMA Band 7 detection can be explained by a limited dust mass, $M_d=1.6\times 10^6$M${}_{\odot }$. Finally, the high dust temperatures might solve the puzzling low infrared excess recently deduced for high-$z$ galaxies from the IRX-${\hat{I}}^2$ relation.

15 pages, accepted for publication in MNRAS