Probing Dust Composition in Distant Galaxies with JWST Mid-IR Spectroscopy of Quasars with Foreground 2175 Å Absorbers. I. Methodology

Abstract Interstellar dust plays a crucial role in gas cooling and molecule formation, influencing galaxy evolution. However, the composition and structure of dust in distant galaxies are still poorly understood. We have started a JWST Mid-Infrared Instrument (MIRI) medium-resolution spectrograph (MRS) program investigating the dust features in gas-rich and dusty galaxies at redshifts z < 1.2, with strong 2175 Å bumps detected in absorption along the lines of sight to distant background quasars. Here, we describe our program strategy, and present MIRI MRS observations of IR dust features at z = 0.5–1.2 in five quasar spectra that form the first part of our full sample. We identify artifacts in MIRI MRS data that affect the background in IFU cubes, and propose methods to reduce their effects. We pay special attention to modeling the quasar mid-IR continuum, which shows significant variation depending on active galactic nucleus morphology, redshift, and black hole mass. Dust in foreground galaxies produces significant absorption from the 10 μ m silicate feature in all five quasar spectra. Compared with the average 10 μ m silicate feature in the diffuse interstellar medium (ISM) of the Milky Way, we find differences in the absorption peak position, width of the features, and asymmetry of the profiles. A detailed study of these silicate features is presented in our next paper. In two quasar spectra, we tentatively detect weak IR features near 3.0 and 3.4 μ m. Their strengths are comparable to those seen in the Milky Way ISM, but follow-up observations are required in order to confirm these detections.