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 ($\sim 1$ kpc) spatially resolved measurements of CO (1-0) and CO (2-1) intensity, $I_{\mathrm{CO}}$, and mid-IR intensity, $I_{\mathrm{MIR}}$, at 8, 12, 22, and 24$\hat{I}¼$m. The $I_{\mathrm{CO}}$ vs. $I_{\mathrm{MIR}}$ relationship is reasonably described by a power law with slopes $0.7-1.2$ and normalization $I_{\mathrm{CO}}\sim 1$ K km s${}^{-1}$ at $I_{\mathrm{MIR}}\sim 1$ 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 $8\hat{I}¼$m and $12\hat{I}¼$m bands, with strong PAH features, show steeper CO vs. mid-IR slopes than the $22\hat{I}¼$m and $24\hat{I}¼$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 $\sim 1$ 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.

29 pages, 13 figures, key quantitative results in Table 3, Accepted as part of a PHANGS-JWST Focus Issue to appear in ApJ