PHANGS-JWST First Results: A Global and Moderately Resolved View of Mid-infrared and CO Line Emission from Galaxies at the Start of the JWST Era

We 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 (similar to 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 mu m. The I-CO vs. 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 R-21 proportional to I-MIR(0.2), in good agreement with other work. The 8 mu m and 12 mu m bands, with strong PAH features, show steeper CO vs. mid-IR slopes than the 22 and 24 mu m, 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-star) and anti-correlates with SFR/M-star. 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.