Interstellar Silicate Dust in the z=0.685 Absorber Toward TXS 0218+357
  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 micron silicate absorption feature with a detection significance of
10.7-sigma. We fit laboratory-derived silicate dust profile templates obtained
from literature to the observed 10 micron absorption feature, and find that the
best single-mineral fit is obtained using an amorphous olivine template with a
measured peak optical depth of tau_10=0.49+/-0.02, which rises to
tau_10~0.67+/-0.04 if the covering factor is taken into account. We also
detected the 18 micron silicate absorption feature in our data with a >3-sigma
significance. Due to the proximity of the 18 micron absorption feature to the
edge of our covered spectral range, and associated uncertainty about the shape
of the quasar continuum normalization near 18 micron, we do not independently
fit this feature. We find, however, that the shape and depth of the 18 micron
silicate absorption are well-matched to the amorphous olivine template
prediction, given the optical depth inferred for the 10 micron feature. The
measured 10 micron peak optical depth in this absorber is significantly higher
than those found in previously studied quasar absorption systems. The
reddening, 21-cm absorption, and velocity spread of Mg II are not outliers
relative to other studied absorption systems, however. This high optical depth
may be evidence for variations in dust grain properties in the ISM between this
and the previously studied high redshift galaxies.
