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Absorption lines in the spectra of distant quasars whose sightlines passthrough foreground galaxies provide a valuable tool to probe the dust and gascompositions of the interstellar medium (ISM) in galaxies. The first evidenceof silicate dust in a quasar absorption system (QAS) was provided through ourdetection of the 10 micron silicate feature in the z=0.52 absorber toward thequasar AO 0235+164. We present results from 2 follow-up programs using archivalSpitzer Space Telescope infrared spectra to study the interstellar silicatedust grain properties in a total of 13 QASs at 0.1<z<1.4. We find cleardetections of the 10 micron silicate feature in the QASs studied. We alsodetect the 18 micron silicate feature in the sources with adequate spectralcoverage. We find variations in the breadth, peak wavelength, and substructureof the 10 micron interstellar silicate absorption features among the absorbers.This suggests that the silicate dust grain properties in these distant galaxiesmay differ relative to one another, and relative to those in the Milky Way. Wealso find suggestions in several sources, based on comparisons withlaboratory-derived profiles from the literature, that the silicate dust grainsmay be significantly more crystalline than those in the amorphous Milky WayISM. This is particularly evident in the z=0.89 absorber toward the quasar PKS1830-211, where substructure near 10 micron is consistent with a crystallineolivine composition. If confirmed, these grain property variations may haveimplications for both dust and galaxy evolution over the past 9 Gyrs, and forthe commonly-made assumption that high-redshift dust is similar to local dust.We also discuss indications of trends between silicate dust absorption strengthand both carbonaceous dust properties and gas-phase metal properties, such asthe gas velocity spread, determined from UV/optical spectra.

Probing Interstellar Silicate Dust Grain Properties in Quasar Absorption Systems at Redshifts z<1.4