A JWST MIRI LRS Survey of 37 Massive Star-forming Galaxies and Active Galactic Nuclei at Cosmic Noon: Overview and First Results

Abstract We present a large spectroscopic survey with JWST’s Mid-Infrared Instrument Low Resolution Spectrometer (LRS) targeting 37 infrared-bright galaxies between z = 0.65–2.46 with infrared luminosities log L IR / L ⊙ > 11.5 and log M * / M ⊙ = 10 − 11.5 . Targets were taken from a 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 active galactic nuclei. By combining IRS with the increased sensitivity of LRS, we expand the range of spectral features observed between 5 and 30 μ m for every galaxy in our sample. In this paper, we outline the sample selection, reduction of the JWST data, extraction of the 1D spectra, and polycyclic aromatic hydrocarbon (PAH) feature measurements from λ rest = 3.3–11.3 μ m. In the 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 JWST and 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 ∼ 1–2. 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 because large PAHs transmit less energy per photon into the interstellar medium.