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
Ellison, Sara L.; Wilson, Christine D.; Bolatto, Alberto D.; Henshaw, Jonathan D.; Kruijssen, J. M. Diederik; Leroy, Adam K.; Rosolowsky, Erik; Chevance, Mélanie; Meidt, Sharon E.; Hughes, Annie; Klessen, Ralf S.; Schinnerer, Eva; Emsellem, Eric; Koch, Eric W.; Pety, Jérôme; Sun, Jiayi; Boquien, Médéric; Salim, Samir; Liu, Daizhong; Chiang, I-Da; Chastenet, Jérémy; Williams, Thomas G.; Kreckel, Kathryn; Grasha, Kathryn; Lee, Janice C.; Thilker, David A.; Larson, Kirsten L.; Watkins, Elizabeth J.; Barnes, Ashley. T.; Sormani, Mattia C.; Kim, Jaeyeon; Bigiel, F.; Cao, Yixian; Saito, Toshiki; Levy, Rebecca C.; Querejeta, Miguel; Whitcomb, Cory M.; Sandstrom, Karin; Lin, Lihwai; Rubio, Mónica; Usero, Antonio; Chown, Ryan; den Brok, Jakob S.; Vogel, Stuart N.; Colombo, Dario
United States, Canada, Germany, Chile, Belgium, France, Taiwan, Australia, United Kingdom, Spain, Japan
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 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 ${R}_{21}\propto {I}_{\mathrm{MIR}}^{0.2}$ , 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.