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

TL;DR

This study uses infrared spectra from the Spitzer Space Telescope to analyze silicate dust properties in 13 quasar absorption systems at redshifts less than 1.4, revealing variations in dust composition and crystallinity that impact our understanding of galaxy evolution.

Contribution

First comprehensive analysis of silicate dust in multiple quasar absorption systems at intermediate redshifts, highlighting diversity in dust grain properties and potential implications for galaxy evolution.

Findings

Detected 10 and 18 micron silicate features in multiple QASs.

Observed variations in silicate feature profiles indicating diverse dust properties.

Evidence suggesting some distant galaxy dust may be more crystalline than Milky Way dust.

Abstract

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.