A JWST MIRI LRS Survey of 37 Massive Star-Forming Galaxies and AGN at Cosmic Noon -- Overview and First Results

Jed McKinney, Miriam Eleazer, Alexandra Pope, Anna Sajina, Stacey Alberts, Meredith Stone, Leonid Sajkov, Virginia Vanicek, Allison Kirkpatrick, Thomas Lai, Caitlin M. Casey, Lee Armus, Tanio Diaz-Santos, Andrew Korkus, Olivia Cooper, Lindsay R. House, Hollis Akins, Erini Lambrides, Arianna Long, Lin Yan

First listed 2025-10-08 | Last updated 2025-10-08

Abstract

We present a large spectroscopic survey with \textit{JWST}'s Mid-Infrared Instrument (MIRI) Low Resolution Spectrometer (LRS) targeting $37$ infrared-bright galaxies between $z=0.65-2.46$ with infrared luminosities $\log L_{\rm IR}/L_\odot>11.5$ and $\log M_*/M_\odot=10-11.5$. Targets were taken from a \textit{Spitzer} $24\,μ$m-selected sample with archival spectroscopy from the Infrared Spectrograph (IRS) and include a mix of star-forming galaxies and dust-obscured AGN. By combining IRS with the increased sensitivity of LRS, we expand the range of spectral features observed between $5-30\,μ$m for every galaxy in our sample. In this paper, we outline the sample selection, \textit{JWST} data reduction, 1D spectral extraction, and polycyclic aromatic hydrocarbon (PAH) feature measurements from $λ_{rest}=3.3-11.2\,μ$m. In the \textit{JWST} spectra, we detect PAH emission features at $3.3-5.3\,μ$m, as well as Paschen and Brackett lines. The $3.3\,μ$m feature can be as bright as $1\%$ of the $8-1000\,μ$m infrared luminosity and exhibits a tight correlation with the dust-obscured star-formation rate. We detect absorption features from CO gas, CO$_2$ ice, H$_2$O ice, and aliphatic dust. From the joint \textit{JWST} and \textit{Spitzer} analysis we find that the $11.3/3.3\,μ$m PAH ratios are on-average three times higher than that of local luminous, infrared galaxies. This is interpreted as evidence that the PAH grains are larger at $z\sim1-2$. The size distribution may be affected by coagulation of grains due to high gas densities and low temperatures. These conditions are supported by the observation of strong water ice absorption at $3.05\,μ$m, and can lower stellar radiative feedback as large PAHs transmit less energy per photon into the interstellar medium.

Short digest

Overview paper for a JWST/MIRI LRS survey stitches new 5–14 μm spectra to archival Spitzer/IRS to cover 5–30 μm for 37 infrared-bright galaxies and obscured AGN at z=0.65–2.46. They detect bright 3.3–5.3 μm PAH emission plus Paschen/Brackett lines; the 3.3 μm PAH reaches up to 1% of L_IR and tightly traces the dust-obscured SFR. CO gas, CO2 and H2O ices, and aliphatic dust are common in absorption, and the joint spectra show 11.3/3.3 μm PAH ratios about three times higher than local LIRGs, pointing to systematically larger PAH grains at z~1–2. The implied grain growth in dense, cold ISM at cosmic noon suggests reduced photoelectric heating and weaker stellar feedback.

Key figures to inspect

• Stitched MIRI LRS + Spitzer/IRS spectra (5–30 μm) for representative sources: verify 3.3, 6.2, 7.7, 8.6, and 11.2 μm PAHs alongside Paschen/Brackett lines, and note where absorption troughs (CO, CO2, H2O ice, aliphatics) imprint on the continuum.
• Correlation plot of L(3.3 μm PAH) or L(3.3 μm)/L_IR versus dust-obscured SFR: check scatter, outliers, and any AGN/SF separation to assess the tightness and utility of 3.3 μm as an SFR tracer.
• Distribution or comparison figure for PAH 11.3/3.3 μm ratios: examine the factor ~3 elevation over local LIRGs and how it trends with redshift, L_IR, or M_* as evidence for larger PAH sizes.
• Zoom-in spectral panels around 3.0–3.5 μm: inspect the 3.05 μm H2O ice absorption profile and nearby aliphatic features to gauge ISM coldness and column densities.
• Feature maps or stacked spectra highlighting CO gas and CO2 ice (e.g., ~4.27 μm): confirm detections and relative depths versus continuum to link dense, cold conditions with the PAH-size inference.