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We use deep Spitzer mid-infrared spectroscopic maps of radial strips across three nearby galaxies with well-studied metallicity gradients (M101, NGC 628, and NGC 2403) to explore the physical origins of the observed deficit of polycyclic aromatic hydrocarbons (PAHs) at subsolar metallicity (i.e., the PAH–metallicity relation or PZR). These maps allow us to trace the evolution of all PAH features from 5–18 μm as metallicity decreases continuously from solar (Z ⊙) to 0.2 Z ⊙. The total PAH-to-dust luminosity ratio remains relatively constant until reaching a threshold of ∼ 2/3 Z ⊙, below which it declines smoothly but rapidly. The PZR has been attributed to preferential destruction of the smallest grains in the hard radiation environments found at low metallicity. In this scenario, a decrease in emission from the shortest-wavelength PAH features is expected. In contrast, we find a steep decline in long-wavelength power below Z ⊙, especially in the 17 μm feature, with the shorter-wavelength PAH bands carrying an increasingly large fraction of power at low metallicity. We use newly developed grain models to reproduce the observed PZR trends, including these variations in fractional PAH feature strengths. The model that best reproduces the data employs an evolving grain size distribution that shifts to smaller sizes as metallicity declines. We interpret this as a result of inhibited grain growth at low metallicity, suggesting continuous replenishment in the interstellar medium is the dominant process shaping the PAH grain population in galaxies.

We compare mid-infrared (mid-IR), extinction-corrected Hα, and CO (2–1) emission at 70–160 pc resolution in the first four PHANGS–JWST targets. We report correlation strengths, intensity ratios, and power-law fits relating emission in JWST’s F770W, F1000W, F1130W, and F2100W bands to CO and Hα. At these scales, CO and Hα each correlate strongly with mid-IR emission, and these correlations are each stronger than the one relating CO to Hα emission. This reflects that mid-IR emission simultaneously acts as a dust column density tracer, leading to a good match with the molecular-gas-tracing CO, and as a heating tracer, leading to a good match with the Hα. By combining mid-IR, CO, and Hα at scales where the overall correlation between cold gas and star formation begins to break down, we are able to separate these two effects. We model the mid-IR above I ν = 0.5 MJy sr−1 at F770W, a cut designed to select regions where the molecular gas dominates the interstellar medium (ISM) mass. This bright emission can be described to first order by a model that combines a CO-tracing component and an Hα-tracing component. The best-fitting models imply that ∼50% of the mid-IR flux arises from molecular gas heated by the diffuse interstellar radiation field, with the remaining ∼50% associated with bright, dusty star-forming regions. We discuss differences between the F770W, F1000W, and F1130W bands and the continuum-dominated F2100W band and suggest next steps for using the mid-IR as an ISM tracer.

Ratios of polycyclic aromatic hydrocarbon (PAH) vibrational bands are a promising tool for measuring the properties of the PAH population and their effect on star formation. The photometric bands of the MIRI and NIRCam instruments on JWST provide the opportunity to measure PAH emission features across entire galaxy disks at unprecedented resolution and sensitivity. Here we present the first results of this analysis in a sample of three nearby galaxies: NGC 628, NGC 1365, and NGC 7496. Based on the variations observed in the 3.3, 7.7, and 11.3 μm features, we infer changes to the average PAH size and ionization state across the different galaxy environments. High values of F335MPAH/F1130W and low values of F1130W/F770W are measured in H ii regions in all three galaxies. This suggests that these regions are populated by hotter PAHs, and/or that the PAH ionization fraction is larger. We see additional evidence of heating and/or changes in PAH size in regions with higher molecular gas content as well as increased ionization in regions with higher Hα intensity.

We explore the physical origins of the observed deficit of polycyclic aromatic hydrocarbons (PAHs) at subsolar metallicity using JWST/NIRCam imaging of the nearby galaxy M101, covering regions from solar metallicity (Z⊙) down to 0.4 Z⊙. These maps are used to trace the radial evolution of the shortest-wavelength PAH feature at 3.3 μm, which is emitted preferentially by the smallest PAHs (<100 carbon atoms). The fractional contribution of PAH 3.3 μm to the total PAH luminosity (ΣPAH) increases by 3× as metallicity declines, rising from ∼1% to ∼3% over the observed range, consistent with prior predictions from the inhibited grain growth model based on Spitzer spectroscopy. We explore model refinements including photon effects and alternative size evolution prescriptions and find that a modest amount of small grain photodestruction remains possible, provided the grain size cutoff does not exceed ∼55 carbon atoms. The best-fit models predict 3.3 μm/ΣPAH will rise to ∼5.6%–7.7% at 10% Z⊙. Surprisingly, even as ΣPAH drops significantly relative to the total infrared luminosity (TIR) as metallicity declines, 3.3 μm/TIR alone rises, potentially indicating the mass fraction of the smallest PAH grains increases as the total dust content in galaxies drops. The current model cannot fully reproduce this trend even if the unusually strong effects of changing radiation field hardness on 3.3 μm/TIR are included. This may be evidence that the smallest PAHs are uniquely robust against destruction and inhibited growth effects. These results highlight the pivotal role that short-wavelength PAH emission can play in studies of low-metallicity and high-redshift galaxies.

We explore the relationship between mid-infrared (mid-IR) and CO rotational line emission from massive star-forming galaxies, which is one of the tightest scalings in the local universe. We assemble a large set of unresolved and moderately (∼1 kpc) spatially resolved measurements of CO (1–0) and CO (2–1) intensity, I CO, and mid-IR intensity, I MIR, at 8, 12, 22, and 24 μm. The I CO versus I MIR relationship is reasonably described by a power law with slopes 0.7–1.2 and normalization I CO ∼ 1 K km s−1 at I MIR ∼ 1 MJy sr−1. Both the slopes and intercepts vary systematically with choice of line and band. The comparison between the relations measured for CO (1–0) and CO (2–1) allow us to infer that R21∝IMIR0.2 , in good agreement with other work. The 8 μm and 12 μm bands, with strong polycyclic aromatic hydrocarbon (PAH) features, show steeper CO versus mid-IR slopes than the 22 and 24 μm, consistent with PAH emission arising not just from CO-bright gas but also from atomic or CO-dark gas. The CO-to-mid-IR ratio correlates with global galaxy stellar mass (M ⋆) and anticorrelates with star formation rate/M ⋆. At ∼1 kpc resolution, the first four PHANGS–JWST targets show CO-to-mid-IR relationships that are quantitatively similar to our larger literature sample, including showing the steep CO-to-mid-IR slopes for the JWST PAH-tracing bands, although we caution that these initial data have a small sample size and span a limited range of intensities.

We present the Spitzer/IRS Mapping Legacy Archive (SIMLA); a complete set of mid-infrared spectral cubes built from low-resolution mapping-mode fixed-target observations from the Spitzer Infrared Spectrograph (IRS; 5.2–38 μm, R ∼ 60–130). Contained in this dataset are spectral maps for several hundred spatially resolved and unresolved objects, including galaxies, molecular clouds, supernova remnants, H ii regions, and more. Each cube has been carefully treated to remove astronomical foregrounds and backgrounds and detector effects using a novel pipeline. Cube assembly was facilitated by the CUBISM code, which included automatic detection and removal of bad pixels. We describe the SIMLA pipeline for reducing and validating the cubes, and we show that synthetic photometry derived from SIMLA spectra and corresponding Wide-field Infrared Survey Explorer photometry typically agree within a few percent. SIMLA products and documentation related to their use are available at the NASA/IPAC Infrared Science Archive (doi:10.26131/IRSA655).

We present James Webb Space Telescope (JWST) Near Infrared Spectrograph (NIRSpec) and Mid-infrared Instrument integral field spectroscopy of the nearby blue compact dwarf II Zw 40, which has a low metallicity of 25% of solar. Leveraging the high spatial/spectral resolution and wavelength coverage of JWST/NIRSpec, we present robust detections of the 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission on 20 pc scales. The strength of the Pfδ emission relative to the 3.3 PAH feature is significantly stronger than typical higher-metallicity star-forming galaxies. We find that 3.3 μm PAH emission is concentrated near the northern super star cluster and is cospatial with CO gas. A strong correlation exists between the 3.3/11.3 PAH ratio and radiation hardness probed by [Ne iii]/[Ne ii], providing evidence of photodestruction of PAH molecules in intense radiation environments. Our analysis shows that while the overall PAH fraction is lower in II Zw 40 than in higher-metallicity galaxies, the contribution of the 3.3 μm PAH feature to the total PAH emission is higher. We propose that the PAH size distribution is fundamentally shaped by two competing mechanisms in low-metallicity environments: photodestruction and inhibited growth. Additionally, the high radiation field intensity in II Zw 40 suggests that multiphoton heating of PAHs may be an important effect. As one of the first spatially resolved studies of aromatic emission in a low-metallicity environment, our spectroscopic results offer practical guidance for future observations of the 3.3 μm PAH feature in low-metallicity galaxies using JWST.

Deuteration of hydrocarbon material, including polycyclic aromatic hydrocarbons (PAHs), has been proposed to account for the low gas-phase abundances of D in the interstellar medium (ISM). JWST spectra of four star-forming regions in M51 show an emission feature, with central wavelength ∼4.647 μm and FWHM 0.0265 μm, corresponding to the C–D stretching mode in aliphatic hydrocarbons. The emitting aliphatic material is estimated to have (D/H)aliph. ≈0.17 ± 0.02—a factor of ∼104 enrichment relative to the overall ISM. On ∼50 pc scales, deuteration levels toward four H ii regions in M51 are 2–3 times higher than in the Orion Bar photodissociation region (PDR), with implications for the processes responsible for the formation and evolution of hydrocarbon nanoparticles, including PAHs. The deuteration of the aliphatic material is found to anticorrelate with helium ionization in the associated H ii, suggesting that harsh far-UV radiation may act to lower the deuteration of aliphatics in PDRs near massive stars. No evidence is found for deuteration of aromatic material, with (D/H)arom. ≲ 0.016: deuteration of the aliphatic material exceeds that of the aromatic material by at least a factor of 10. The observed levels of deuteration may account for the depletion of D observed in the Galactic ISM. If so, the 4.65 μm feature may be detectable in absorption.

The Antennae galaxies merger produces the brightest infrared emission of any galaxy within ≈20 Mpc, mostly from intense star formation taking place in supergiant molecular cloud complexes in the overlap region. Here, we present new, high-resolution NIRCam and MIRI images of the Antennae galaxies taken with the F150W, F187N, F335M, F360M, F410M, and F770W filters on JWST to search for the predicted but as-yet-undiscovered population of deeply embedded, optically obscured star clusters. We identify a population of 45 sources, 40 previously unknown, with high Brα/Hα and Paα/Hα flux ratios, which are likely very young clusters still embedded or just emerging from their natal cocoons, and estimate their age, extinction (AV), and mass. We find that all are extremely young (≲2.5 Myr), have AV between 2 and 10 mag, and masses between ≈104 and several ×106 M⊙. We believe we have now uncovered all clusters with M ≳ 3 × 104 M⊙ and AV ≳ 2 mag in the Antennae. While our sample represents a small fraction (≈15%) of clusters younger than 3 Myr by number, it dominates the ionizing photon luminosity across the galaxy pair (≈60%). We find elevated H2/polycyclic aromatic hydrocarbon ratios of the interstellar medium surrounding the most massive pair of embedded clusters, supporting the idea that merger-induced shock-heated gas plays an important role in the formation of extremely massive clusters.

We present the Spitzer/IRS Mapping Legacy Archive (SIMLA); a complete set of mid-infrared spectral cubes built from low-resolution mapping-mode fixed-target observations from Spitzer/IRS (5.2-38 micron, R~60-130). Contained in this dataset are spectral maps for several hundred spatially-resolved and unresolved objects, including galaxies, molecular clouds, supernova remnants, HII regions, and more. Each cube has been carefully treated to remove astronomical foregrounds and backgrounds as well as detector effects using a novel pipeline. Cube assembly was facilitated by the CUBISM code, which included automatic detection and removal of bad pixels. We describe the SIMLA pipeline for reducing and validating the cubes, and we show that synthetic photometry derived from SIMLA spectra and corresponding WISE photometry typically agree within a few percent. SIMLA products and documentation related to their use will soon be available at the NASA/IPAC Infrared Science Archive (DOI:10.26131/IRSA655).