Dark dust II: Properties in the general field of the diffuse ISM

Abstract

Distance estimates derived from spectroscopy or parallax have been unified by considering extinction by large grains. The addition of such a population of what is called Dark Dust to models of the diffuse interstellar medium is tested against a contemporary set of observational constraints. The dark dust model explains, by respecting representative solid-phase element abundances, simultaneously the typical wavelength-dependent reddening, extinction, and emission of polarized and unpolarized light by interstellar dust particles between far UV and millimetre wavelengths. The physical properties of dark dust are derived. Dark dust consists of micrometre-sized particles, which have been recently detected in-situ. It provides significant wavelength-independent reddening from the far UV to the near-infrared. Light absorbed by dark dust is re-emitted in the submillimeter region by grains at dust temperatures of 8-12K. Such very cold dust has been frequently observed in external galaxies. Dark dust contributes to the polarisation at greater than about 1 mm to ~35% and at shorter wavelengths marginally. Optical constants for silicate dust analogous are investigated. By mixing 3% in mass of Mg$_{0.8}$Fe$^{2+}_{0.2}$ SiO$_3$ to MgO$-$0.5 SiO$_2$ a good fit to the data is derived that still can accommodate up to 5 - 10% of mass in dark dust. The extra diming of light by dark dust is unexplored when discussing SN~Ia light curves and in other research. Previous models that ignore dark dust do not account for the unification of the distance scales.