PHANGS–JWST First Results: Mid-infrared Emission Traces Both Gas Column Density and Heating at 100 pc Scales
Adam K. Leroy, Karin Sandström, Erik Rosolowsky, Francesco Belfiore, Alberto D. Bolatto, Yixian Cao, Eric W. Koch, Eva Schinnerer, Ashley T. Barnes, Ivana Bešlić, Frank Bigiel, Guillermo A. Blanc, Jérémy Chastenet, Ness Mayker Chen, Mélanie Chevance, Ryan Chown, Enrico Congiu, Daniel A. Dale, Oleg V. Egorov, Éric Emsellem, Cosima Eibensteiner, Christopher M. Faesi, Simon C. O. Glover, Kathryn Grasha, Brent Groves, Hamid Hassani, Jonathan D. Henshaw, Annie Hughes, María J. Jiménez-Donaire, Jaeyeon Kim, Ralf S. Klessen, Kathryn Kreckel, J. M. Diederik Kruijssen, Kirsten L. Larson, Janice Lee, Rebecca C. Levy, Daizhong Liu, Laura A. Lopez, Sharon E. Meidt, E. J. Murphy, Justus Neumann, Ismael Pessa, J. Pety, Toshiki Saito, Amy Sardone, Jiayi Sun, David A. Thilker, A. Usero, Elizabeth J. Watkins, Cory M. Whitcomb, Thomas G. Williams
Abstract
Abstract We compare mid-infrared (mid-IR), extinction-corrected H α , 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 H α . At these scales, CO and H α each correlate strongly with mid-IR emission, and these correlations are each stronger than the one relating CO to H α 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 H α . By combining mid-IR, CO, and H α 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 H α -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.