Viacheslav Klimenko (University of South Carolina) - Constraints on Dust Grain Composition and Structure in Distant Normal Galaxies with JWST Quasar Spectroscopy

We present results from a mini-survey of dust spectral features in galaxies at redshifts z=0.5-1.2 using mid-IR spectroscopy of physically unrelated background quasars obtained with the JWST MIRI MRS. We analyze observations of five quasars with small angular separations from foreground galaxies to probe the 10 micron silicate dust absorption feature. The targets were previously selected based on the presence of strong gas absorption lines and prominent 2175 A UV extinction bump.

Template fitting of the 10 micron absorption feature suggests the possible presence of crystalline silicates in three of the galaxies, in contrast to the predominantly amorphous silicate grains observed in the diffuse ISM of the Milky Way. We also derive extinction curves using existing multiwavelength data. We find that the peak optical depth of the 10 micron silicate feature is approximately three times stronger than expected for the local diffuse ISM. The relative strength of silicate to carbonaceous dust features is larger and appears to decrease with increasing total dust extinction, approaching the Galactic ISM value at AV~2. We find no significant correlation between the silicate optical depth and RV. In contrast, the silicate optical depth is strongly correlated with the gas-phase Mg II absorption strength.  Possible interpretations include that some quasar sightlines probe dust in the circumgalactic medium (CGM).