2011ApJ...726...14K

We report on a study of interstellar silicate dust in five quasar absorption systems at 0.44 < z_abs< 1.31 toward quasars Q0235+164, 3C196, Q0852+3435, Q0937+5628, and Q1203+0634, using data from the Infrared Spectrograph on board the Spitzer Space Telescope. In the absorbers toward Q0235+164, 3C196, Q0852+3435, and Q0937+5628, the 9.7 µm silicate feature is detected in absorption at ∼5σ-10σ significance, with rest-frame equivalent widths of ∼0.2-0.5 µm. For Q1203+0634, the noisy data allow us to make only a 3.6σ detection of absorption at 10 µm. Fits to the redshifted 9.7 µm features with four possible template profiles indicate that the laboratory amorphous olivine profile generally provides the best fit, with inferred peak optical depths of τ_9.7~ 0.08-0.18. For three of the quasars with optical spectra available from the Sloan Digital Sky Survey, the Large Magellanic Cloud supershell extinction curve provides the best formal fits, with E(B - V) of 0.34-0.54 mag. A 2175 Å extinction bump at the absorber redshift (known to exist in Q0235+164) appears to be present at >2.8σ level in Q0852+3435, >2.5σ level in Q1203+0634, and marginally at nearly 2σ level in Q0937+5628. We briefly explore possible correlations between τ_9.7 and the color excess, the strength of the 2175 Å bump, and the metallicity (which appears to be relatively high for these absorbers, in the range of ≳ 0.1 solar to supersolar). While our measurements are consistent with a linear relation between τ_9.7and E(B - V), the τ_9.7/E(B - V) ratios for these quasar absorbers appear to be higher than those for diffuse interstellar clouds in the Milky Way, closer to values observed for the Galactic center. Some of these quasar sightlines may trace regions in the bulges of the galaxies responsible for the absorption systems.