PHANGS-JWST First Results: Mid-infrared emission traces both gas column density and heating at 100 pc scales

TL;DR

This study demonstrates that mid-infrared emission at 100 pc scales traces both gas column density and heating, with about half of the emission linked to molecular gas heated by diffuse radiation and half to star-forming regions.

Contribution

It introduces a model separating mid-IR emission into components associated with molecular gas and star formation, enhancing understanding of ISM properties at 100 pc scales.

Findings

Mid-IR emission correlates strongly with CO and Hα emissions.

Approximately 50% of mid-IR flux arises from molecular gas heated by diffuse radiation.

Mid-IR can be modeled as a combination of CO-tracing and Hα-tracing components.

Abstract

We compare mid-infrared (mid-IR), extinction-corrected H$\alpha$, 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$\alpha$. At these scales, CO and H$\alpha$ each correlate strongly with mid-IR emission, and these correlations are each stronger than the one relating CO to H$\alpha$ emission. This reflects that mid-IR emission simultaneously acts as a dust column density tracer, leading to the good match with the molecular gas-tracing CO, and as a heating tracer, leading to the good match with the H$\alpha$. By combining mid-IR, CO, and H$\alpha$ 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 $I_\nu = 0.5$~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$\alpha$-tracing component. The best-fitting models imply that $\sim 50\%$ of the mid-IR flux arises from molecular gas heated by the diffuse interstellar radiation field, with the remaining $\sim 50\%$ associated with bright, dusty star forming regions. We discuss differences between the F770W, F1000W, F1130W bands and the continuum dominated F2100W band and suggest next steps for using the mid-IR as an ISM tracer.