Probing Interstellar Silicate Dust Grain Properties in Quasar Absorption
  Systems at Redshifts z<1.4
  Absorption lines in the spectra of distant quasars whose sightlines pass
through foreground galaxies provide a valuable tool to probe the dust and gas
compositions of the interstellar medium (ISM) in galaxies. The first evidence
of silicate dust in a quasar absorption system (QAS) was provided through our
detection of the 10 micron silicate feature in the z=0.52 absorber toward the
quasar AO 0235+164. We present results from 2 follow-up programs using archival
Spitzer Space Telescope infrared spectra to study the interstellar silicate
dust grain properties in a total of 13 QASs at 0.1<z<1.4. We find clear
detections of the 10 micron silicate feature in the QASs studied. We also
detect the 18 micron silicate feature in the sources with adequate spectral
coverage. We find variations in the breadth, peak wavelength, and substructure
of the 10 micron interstellar silicate absorption features among the absorbers.
This suggests that the silicate dust grain properties in these distant galaxies
may differ relative to one another, and relative to those in the Milky Way. We
also find suggestions in several sources, based on comparisons with
laboratory-derived profiles from the literature, that the silicate dust grains
may be significantly more crystalline than those in the amorphous Milky Way
ISM. This is particularly evident in the z=0.89 absorber toward the quasar PKS
1830-211, where substructure near 10 micron is consistent with a crystalline
olivine composition. If confirmed, these grain property variations may have
implications for both dust and galaxy evolution over the past 9 Gyrs, and for
the commonly-made assumption that high-redshift dust is similar to local dust.
We also discuss indications of trends between silicate dust absorption strength
and both carbonaceous dust properties and gas-phase metal properties, such as
the gas velocity spread, determined from UV/optical spectra.
