The dust attenuation law in z ∼ 6 quasars

Abstract

We investigate the attenuation law in z ~ 6 quasars by combining cosmological zoom-in hydrodynamical simulations of quasar host galaxies, with multifrequency radiative transfer calculations. We consider several dust models differing in terms of grain-size distributions, dust mass, and chemical composition, and compare the resulting synthetic spectral energy distributions with data from bright, early quasars. We show that only dust models with grain-size distributions in which small grains ($a\lesssim 0.1~\mu {\rm m}$, corresponding to $\approx 60{{\ \rm per\ cent}}$ of the total dust mass) are selectively removed from the dusty medium provide a good fit to the data. Removal can occur if small grains are efficiently destroyed in quasar environments and/or early dust production preferentially results in large grains. Attenuation curves for these models are close to flat, and consistent with recent data; they correspond to an effective dust-to-metal ratio f<SUB>d</SUB> ≃ 0.38, i.e. close to the Milky Way value.