Probing Dust Composition in Distant Galaxies with JWST Mid-IR Spectroscopy of Quasars with Foreground 2175 {\AA} Absorbers II. Measurements of Grain Composition and Extinction Properties

Abstract

We present results from a mini-survey of dust spectral features arising in galaxies at redshifts $0.5 < z < 1.2$ in our James Webb Space Telescope (JWST) mid-infrared spectra of physically-unrelated background quasars. We analyze the JWST Mid-infrared Instrument (MIRI) Medium-Resolution Spectrometer (MRS) spectra of five quasars presented in Klimenko, Kulkarni, \& Aller 2025a (Paper I) to determine the best-fit silicate mineralogies. Template profile fits to the 10 $\mu$m feature suggest the possible presence of crystalline silicates in three of the galaxies. This contrasts with the predominately amorphous silicate grains in the Milky Way diffuse interstellar medium (ISM). We also measure the extinction curves using existing data from UV to mid-IR. Combining our results with past Spitzer IRS studies, we find that (i) the 10~$\mu$m silicate peak optical depth ($\tau_{10}$) is about three times stronger than expected for the local diffuse ISM over the range $A_V =0.1-2.0$, with $\tau_{10}/A_V$=$0.17\pm0.09$. (ii) The relative strength of the UV bump is similar to that in the local ISM. However, the ratio $\tau_{10}/A_{2175}$ is larger ($\sim0.1-1$), and appears to decrease with $A_V$, approaching the Galactic ISM value ($\sim 0.1$) at $A_V\sim1.5-2$. (iii) No significant correlation of $\tau_{10}/A_V$ with $R_V$. (iv) $\tau_{10}$ is strongly correlated with the gas-phase Mg~II absorption strength for the quasar sightlines. Possible interpretations include that some quasar sightlines probe dust in the circumgalactic medium (CGM), and that dust grains may have been significantly reprocessed in the ISM and CGM under conditions that may differ from those in the local ISM.