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

TL;DR

This study leverages JWST/MIRI MRS to probe dust composition in five quasar foreground absorbers at $z\sim0.5-1.2$ by measuring the redshifted $10~\mu$m silicate absorption and searching for weak $3.0$ and $3.4~\mu$m features. It details a robust observational and data-reduction framework, including targeted background subtraction, artifact masking, and a careful, multi-faceted approach to reconstructing intrinsic quasar continua tailored to each source’s AGN morphology. The results reveal a diverse set of silicate profiles—peaks from $9.7$ to $11.2~\mu$m, widths from $1.3$ to $3~\mu$m, and varying asymmetries—indicating dust grains in distant galaxies differ in composition/structure from the Milky Way, with tentative detections of additional carbonaceous and ice features at $3-4~\mu$m that warrant follow-up. The work demonstrates the power of high-resolution infrared absorption spectroscopy with JWST to study dust evolution across cosmic time and sets the stage for deeper mineralogical analyses (Paper II) and broader surveys.

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 MIRI 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 AGN 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 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 (Klimenko et al. 2026b). 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 to confirm these detections.

Figures (8)

• Figure 1: Top: Background-subtracted MIRI MRS images for our quasars. The images are shown for channel 3A, and presented in the IFU-aligned coordinate system. The color gradient represents the logarithm of the surface brightness (SB), averaged along the dispersion axis and normalized to unity at the brightest spaxel. The residual emission surrounding the quasars originates from the wings of the MIRI PSF, and has a characteristic hexagonal shape. The variation in the background around quasars is usually less than a few percent. For AO0235+164, we detect emission from two nearby galaxies, visible as bright spots to the east and south of the quasar. Bottom: MIRI MRS spectra (rebinned by a factor of 4, corresponding to a spectral resolution of $\sim600$) for the quasars, shown in the absorption system rest-frame. Spectra are normalized to unity near 4 $\mu$m and vertically offset by 2 units. The increasing uncertainties in the spectra at large wavelengths are due to the rising thermal noise of the detector in channel 4C. The vertical shaded regions indicate the wavelength ranges of 3.0 $\mu$m (H$_2$O), 3.4 $\mu$m (C–H), 6.2 $\mu$m (C-C) and 10 $\mu$m (silicate) absorptions.
• Figure 2: Fit to the continuum and normalized spectrum for the blazar AO 0235$+$164 (upper two panels) and the radio-loud AGN J1007+2853 (lower two panels). The black curve represents MIRI MRS data (binned by a factor of 2 for visual clarity). The red shaded regions indicate the portions of the spectrum used for continuum fitting. The orange shaded area marks the region with the foreground galaxy 10 $\mu$m silicate absorption feature. The orange vertical lines shows the position of 9.7 $\mu$m (the peak wavelength in Milky-Way-type silicate profiles) at the galaxy rest-frame. The green and red curves represent two fits for the continuum (see text for details). For AO 0235$+$164, the normalized spectrum from archival Spitzer IRS observations is shown in blue, closely matching the MIRI MRS data. For J1007+2853, the identified AGN emission lines are labeled in blue.
• Figure 3: Fit to the AGN continuum for J0900$+$0214, J0901$+$2044, and J1017$+$4749. Top panels for each AGN show the MIRI MRS spectrum (in black, rebinned by a factor of 2) overlaid with the constructed composite AGN continuum (red solid line), along with a version that excludes emission features and is corrected for flux variations below 3-5 $\mu$m at the AGN rest frame (red dashed line). The red and orange shaded areas are the same as in Fig.\ref{['fig:j0235-j1007-continum']}. The bottom panels for each AGN show the Spitzer/IRS spectra of AGN with similar properties used to construct the composite AGN continuum. The IRS spectra and the AGN composite are shown in different colors and in black, respectively. All spectra are normalized to unity at the wavelength ($\lambda_n$) marked by the gray-shaded vertical line.
• Figure 4: Top panel: The 10 $\mu$m silicate absorption in the normalized MIRI MRS quasar spectra (binned by a factor of 4) is shown in the absorber rest frame. Vertical shaded areas indicate the peak wavelengths of the observed profiles. Additionally, the rightmost panel shows the profile of the silicate absorption in the local ISM Decleir2025. Bottom panels: The optical depth profiles, normalized to the peak optical depth (or a slightly bluer peak optical depth for J0900+0214 and J0901+2044 due to the Ch4C thermal noise), calculated at the peak wavelengths (within the shaded area). The red dashed line shows the best fit to the silicate feature in the local ISM Decleir2025.
• Figure 5: Portions of the normalized MIRI MRS quasar spectra (binned by a factor of 10) showing the 3 $\mu$m (top panels), 3.4 $\mu$m (middle panels) and 6.2 $\mu$m (bottom panels) ice/dust feature locations in the rest frame of the foreground, absorber galaxies. The equivalent width of features (in $10^{-3}~\mu$m) is shown in the upper left corner of the panels. For the 3 $\mu$m feature, the red, green and blue lines show examples of the profile of the feature, centered at 3.0 $\mu$m, and normalized at the peak optical depth of $\tau_3=0.02$, 0.04 and 0.06, respectively. For the 3.4 $\mu$m and 6.2 $\mu$m features, the red line shows the profile for the Galactic center Quintuplet Cluster as fit following Chiar2013, assuming an optical depth value of $\tau=0.1\tau_{10}$. The red shaded area shows the uncertainty of the profile due to the range of possible values of $\tau/\tau_{10}=0.06-0.14$, as it suggested by Gao2010. The gray shaded area indicates regions overlapping with quasar emission lines.