PHANGS–JWST First Results: Mid-infrared Emission Traces Both Gas Column Density and Heating at 100 pc Scales

Journal article

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

Authors: Leroy, AK; Sandstrom, K; Rosolowsky, E; Belfiore, F; Bolatto, AD

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: American Astronomical Society
• Journal: The Astrophysical Journal Letters
• Volume: 944
• Issue: 2
• Pages: L9-L9
• Publication date: 2023-02-16
• DOI: 10.3847/2041-8213/acaf85
• EISSN: 2041-8213
• ISSN: 2041-8205
• Language: English
• Keywords: Optics; Physics; Galaxy; Tracing; Star formation; Extinction (optical mineralogy); Infrared; Nuclear physics; Stars; Interstellar medium; Astrophysics; Astronomy; TRACER