PHANGS-JWST First Results: Mid-infrared Emission Traces Both Gas Column Density and Heating at 100 pc Scales
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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- February 2023
- DOI:
- arXiv:
- arXiv:2212.10574
- Bibcode:
- 2023ApJ...944L...9L
- Keywords:
-
- Disk galaxies;
- Galaxy physics;
- Dust continuum emission;
- Molecular gas;
- Infrared astronomy;
- Millimeter astronomy;
- Interstellar emissions;
- Star formation;
- 391;
- 612;
- 412;
- 1073;
- 786;
- 1061;
- 840;
- 1569;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 49 pages, 17 figures, Section 8 provides a detailed summary, accepted for publication in ApJ Letters, part of a PHANGS-JWST Focus Issue to appear in ApJ