Dark dust

II. Properties in the general field of the diffuse ISM

European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany
e-mail: Ralf.Siebenmorgen@eso.org

Received: 25 April 2022
Accepted: 18 November 2022

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. By respecting representative solid-phase element abundances, the dark dust model simultaneously explains the typical wavelength-dependent reddening, extinction, and emission of polarised and unpolarised light by interstellar dust particles between far-UV and millimeter wavelengths. The physical properties of dark dust were derived. Dark dust consists of micrometer-sized particles. These particles have recently been detected in situ. Dark dust 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 8–12 K. This very cold dust has frequently been observed in external galaxies. Dark dust contributes to the polarisation at ≳1 mm to ~35% and marginally at shorter wavelengths. Optical constants for silicate dust analogous were investigated. By mixing 3% in mass of Mg_0.8Fe_0.2²⁺ SiO₃ to MgO−0.5 SiO₂, a good fit to the data was derived that can still accommodate up to 5–10% of mass in dark dust. The additional diming of light by dark dust is unexplored when supernova Ia light curves are discussed and in other research. Previous models that ignored dark dust do not account for the unification of the distance scales.