2014ApJ...785...36A

We report the detection of interstellar silicate dust in the z_abs= 0.685 absorber along the sightline toward the gravitationally lensed blazar TXS 0218+357. Using Spitzer Space Telescope Infrared Spectrograph data, we detect the 10 µm silicate absorption feature with a detection significance of 10.7σ. We fit laboratory-derived silicate dust profile templates obtained from the literature to the observed 10 µm absorption feature and find that the best single-mineral fit is obtained using an amorphous olivine template with a measured peak optical depth of τ₁₀= 0.49±0.02, which rises to τ₁₀∼ 0.67±0.04 if the covering factor is taken into account. We also detected the 18 µm silicate absorption feature in our data with a >3σ significance. Due to the proximity of the 18 µm absorption feature to the edge of our covered spectral range, and associated uncertainty about the shape of the quasar continuum normalization near 18 µm, we do not independently fit this feature. We find, however, that the shape and depth of the 18 µm silicate absorption are well matched to the amorphous olivine template prediction, given the optical depth inferred for the 10 µm feature. The measured 10 µm peak optical depth in this absorber is significantly higher than those found in previously studied quasar absorption systems. However, the reddening, 21 cm absorption, and velocity spread of Mg II are not outliers relative to other studied absorption systems. This high optical depth may be evidence for variations in dust grain properties in the interstellar medium between this and the previously studied high redshift galaxies.