Dusty galaxies in the Epoch of Reionization: simulations

Abstract

The recent discovery of dusty galaxies well into the Epoch of Reionization\n(redshift $z>6$) poses challenging questions about the properties of the\ninterstellar medium in these pristine systems. By combining state-of-the-art\nhydrodynamic and dust radiative transfer simulations, we address these\nquestions focusing on the recently discovered dusty galaxy A2744_YD4 ($z=8.38$,\nLaporte et al. 2017}). We show that we can reproduce the observed spectral\nenergy distribution (SED) only using different physical values with respect to\nthe inferred ones by Laporte et al(2017), i.e. a star formation rate of\n$\\mathrm{SFR} = 78\\;\\rm M_\\odot \\rm yr^{-1}$, a factor $\\approx 4$ higher than\ndeduced from simple Spectral Energy Distribution fitting. In this case we find:\n(a) dust attenuation (corresponding to $\\tau_V=1.4$) is consistent with a Milky\nWay extinction curve; (b) the dust-to-metal ratio is low, $f_\\mathrm{d} \\sim\n0.08$, implying that early dust formation is rather inefficient; (c) the\nluminosity-weighted dust temperature is high, $T_d=91\\pm 23\\, \\rm K$, as a\nresult of the intense ($\\approx 100\\times$ MW) interstellar radiation field;\n(d) due to the high $T_d$, the ALMA Band 7 detection can be explained by a\nlimited dust mass, $M_d=1.6\\times 10^6 $M$_\\odot$. Finally, the high dust\ntemperatures might solve the puzzling low infrared excess recently deduced for\nhigh-$z$ galaxies from the IRX-$\\beta$ relation.\n