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.

PHANGS–JWST First Results: The Dust Filament Network of NGC 628 and Its Relation to Star Formation Activity

“…The F2100W flux fractions are always significantly less than all three shorter-wavelength MIRI bands. Given the present residual ± 0.1 MJy sr −1 uncertainty in the background level of our MIRI images (see Section 2.1 and Leroy et al 2023, this Issue), we assess the impact this has on filament flux fractions by offsetting the image intensities by the uncertainty in a positive and negative sense. Dashes overplotted on the bars of Figure 7 indicate the resulting perturbed fractions.…”

PHANGS–JWST First Results: Multiwavelength View of Feedback-driven Bubbles (the Phantom Voids) across NGC 628

“…This letter demonstrates the potential for new PHANGS-JWST observations to be used to identify (morphologically) stellar-feedback-powered bubbles, and highlights the need for 14). In color scale is the total column density, which we expect (away from regions of active star formation) to correspond well to the 7.7 μm emission (Leroy et al 2023;Sandstrom et al 2023). The bottom panel shows the isolated galaxy simulation tuned for NGC 628 from E. Emsellem et al (2022, in prepararion).…”

“…The middle panel shows the isolated galaxy simulation from Tress et al (2020; the same simulation shown in the bottom panels of their Figure14). In color scale is the total column density, which we expect (away from regions of active star formation) to correspond well to the 7.7 μm emission(Leroy et al 2023;Sandstrom et al 2023). The bottom panel shows the isolated galaxy simulation tuned for NGC 628 from E.Emsellem et al (2022, in prepararion).…”

“…Specifically, the wavelength coverage of the Mid-Infrared Instrument (MIRI) is perfectly suited to this task, as it is sensitive to several polycyclic aromatic hydrocarbon (PAH) emission features (e.g., at 7.7 μm; see Draine & Li 2007;Smith et al 2007;Li 2020). Emission from PAHs is particularly useful in tracing the shells of feedback-driven bubbles (e.g., Pineda et al 2022), due to (a) the increased gas densities found in swept-up shells (PAHs are generally well mixed with the gas and illuminated by the average interstellar radiation field such that they trace the gas column very sensitively; e.g., Regan et al 2006;Leroy et al 2013;Chown et al 2021;Gao et al 2022;Leroy et al 2023), (b) the high number of ionizing photons emitted by the OB association powering the bubbles, leading to PAHs being destroyed in the photoionized interiors of the bubbles (e.g., Galliano et al 2018;Chastenet et al 2023aChastenet et al , 2023bEgorov et al 2023), and (c) the low optical depth from the shell interior to the edge, which will allow far-UV photons to easily heat the small dust grains (e.g., Draine & Li 2007;Draine 2011;Hensley & Draine 2021). Together, these cause the edges of bubbles to appear with high contrast against their interior PAH emission (e.g., Churchwell et al 2006;Watson et al 2008).…”

Dissecting the Mid-infrared Heart of M83 with JWST

“…The understanding of the MIR of starburst nuclei, and starbursting galaxies in general, will be revolutionized with the recent arrival of JWST. The medium resolution spectrometer (MRS) on the JWST Mid-Infrared Instrument (MIRI) is already allowing for studies of the detailed properties of these objects at spatial scales 7 times smaller, 50 times more sensitive, and with spectral resolutions 5 times higher than those provided by the IRS on Spitzer (Rieke et al 2015;Labiano et al 2021;Leroy et al 2023;Sandstrom et al 2023;Thilker et al 2023).…”