Interstellar extinction in the infrared

Extinction by interstellar dust at infrared wavelengths is reviewed. For 0.7 < λ < 7 μm the observed extinction, both in the Galaxy and in the Magellanic Clouds, appears to be consistent with a simple power law, A_λ ≡ λ^-1.75, although the observational uncertainties remain appreciable. In the 8 - 30 μm region interstellar extinction is dominated by the 9.7 μm and 18 μm silicate features; the absolute strength (relative to the continuum extinction at shorter wavelengths), the detailed wavelength-dependence of these features, and the possible variation of the profile shape from diffuse clouds to dense clouds, all remain somewhat controversial. In the far-infrared λ ⪆ 30 μm grain emissivity estimates by different authors vary considerably; future observations of thermal emission from diffuse clouds in the 300 μm region offer the prospect of substantially reducing uncertainties in far-infrared emissivities.