PHANGS-JWST First Results: Mid-infrared Emission Traces Both Gas Column Density and Heating at 100 pc Scales
Bolatto, Alberto D.; Henshaw, Jonathan D.; Kruijssen, J. M. Diederik; Leroy, Adam K.; Rosolowsky, Erik; Chevance, Mélanie; Meidt, Sharon E.; Glover, Simon C. O.; Hughes, Annie; Klessen, Ralf S.; Schinnerer, Eva; Blanc, Guillermo A.; Emsellem, Eric; Koch, Eric W.; Pety, Jérôme; Sun, Jiayi; Liu, Daizhong; Chastenet, Jérémy; Williams, Thomas G.; Egorov, Oleg V.; Kreckel, Kathryn; Grasha, Kathryn; Neumann, Justus; Lee, Janice C.; Thilker, David A.; Dale, Daniel A.; Larson, Kirsten L.; Watkins, Elizabeth J.; Barnes, Ashley. T.; Kim, Jaeyeon; Bigiel, F.; Hassani, Hamid; Faesi, Christopher M.; Cao, Yixian; Belfiore, Francesco; Pessa, Ismael; Groves, Brent; Saito, Toshiki; Levy, Rebecca C.; Congiu, Enrico; Whitcomb, Cory M.; Sandstrom, Karin; Murphy, Eric J.; Lopez, Laura A.; Jiménez-Donaire, María J.; Usero, Antonio; Chown, Ryan; Sardone, Amy; Bešlić, Ivana; Eibensteiner, Cosima; Chen, Ness Mayker
United States, Canada, Italy, Germany, Chile, Belgium, France, Australia, United Kingdom, Spain, Japan
Abstract
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.