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

Abstract

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 μ m. 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 ${R}_{21}\propto {I}_{\mathrm{MIR}}^{0.2}$ , in good agreement with other work. The 8 μ m and 12 μ m bands, with strong polycyclic aromatic hydrocarbon (PAH) features, show steeper CO versus mid-IR slopes than the 22 and 24 μ 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 _⋆ ) 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.

Keywords

• Disk galaxies
• Galaxy physics
• Molecular gas
• Dust continuum emission
• Infrared astronomy
• Millimeter astronomy