9.7 μm Silicate Absorption in a Damped Lyα Absorber at z = 0.52

Abstract

We report a detection of the 9.7 μm silicate absorption feature in a damped Lyα absorber (DLA) at zabs = 0.524 toward AO 0235+164, using the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope. The feature shows a broad shallow profile over ≈8-12 μm in the absorber rest frame and appears to be >15 σ significant in equivalent width. The feature is fit reasonably well by the silicate absorption profiles for laboratory amorphous olivine or diffuse Galactic interstellar clouds. To our knowledge, this is the first indication of 9.7 μm silicate absorption in a DLA. We discuss potential implications of this finding for the nature of the dust in quasar absorbers. Although the feature is relatively shallow (τ9.7 ≈ 0.08-0.09), it is ≈2 times deeper than expected from extrapolation of the τ9.7 versus E(B - V) relation known for diffuse Galactic interstellar clouds. Further studies of the 9.7 μm silicate feature in quasar absorbers will open a new window on the dust in distant galaxies.