PHANGS–JWST First Results: Tracing the Diffuse Interstellar Medium with JWST Imaging of Polycyclic Aromatic Hydrocarbon Emission in Nearby Galaxies

Abstract

JWST observations of polycyclic aromatic hydrocarbon (PAH) emission provide some of the deepest and highest resolution views of the cold interstellar medium (ISM) in nearby galaxies. If PAHs are well mixed with the atomic and molecular gas and illuminated by the average diffuse interstellar radiation field, PAH emission may provide an approximately linear, high-resolution, high-sensitivity tracer of diffuse gas surface density. We present a pilot study that explores using PAH emission in this way based on Mid-Infrared Instrument observations of IC 5332, NGC 628, NGC 1365, and NGC 7496 from the Physics at High Angular resolution in Nearby GalaxieS-JWST Treasury. Using scaling relationships calibrated in Leroy et al., scaled F1130W provides 10–40 pc resolution and 3σ sensitivity of Σgas ∼ 2 M ⊙ pc−2. We characterize the surface densities of structures seen at <7 M ⊙ pc−2 in our targets, where we expect the gas to be H i-dominated. We highlight the existence of filaments, interarm emission, and holes in the diffuse ISM at these low surface densities. Below ∼10 M ⊙ pc−2 for NGC 628, NGC 1365, and NGC 7496 the gas distribution shows a “Swiss cheese”-like topology due to holes and bubbles pervading the relatively smooth distribution of the diffuse ISM. Comparing to recent galaxy simulations, we observe similar topology for the low-surface-density gas, though with notable variations between simulations with different setups and resolution. Such a comparison of high-resolution, low-surface-density gas with simulations is not possible with existing atomic and molecular gas maps, highlighting the unique power of JWST maps of PAH emission.

PHANGS–JWST First Results: The Influence of Stellar Clusters on Polycyclic Aromatic Hydrocarbons in Nearby Galaxies

“…Chastenet et al (2023) probe PAH band ratios across the disks of the four galaxies and find evidence of hotter, highly ionized PAHs in the vicinity of H II regions that are defined by the radiation from young stars. Finally, Sandstrom et al (2023) create maps of PAH band ratios in the same three galaxies considered in this work and find fairly flat F335M PAH /F1130W radial profiles, with amplitudes similar to what we find (0.07-0.24 in νf ν units). A follow-up control study using a larger sample of PHANGS galaxies will compare the PAH ionization levels inferred in this work near stellar clusters and associations with those of PAHs that are spread throughout the diffuse ISM.…”

“…Since our analysis relies on a combination of the F300M, 179 F335M, F360M, F770W, and F1130W images, all of the imageswere convolved with smoothing kernels following Aniano et al (2011) to achieve the effective 0 37 angular resolution of the F1130W data. Continuum-subtracted 3.3 μm PAH maps (F335M PAH ) are constructed using the method outlined in Sandstrom et al (2023) that leverages the F300M and F360M imaging to infer the underlying continua. No continuum subtraction is carried out for the F770W or F1130W imaging since the stellar contributions at these wavelengths are minor and the dust emission appearing in these two bands is dominated by PAH features (Smith et al 2007;Egorov et al 2023;Hassani et al 2023).…”

“…The other parameters in CIGALE are fixed since varying them does not significantly change our results or interpretation, e.g., opting forsolar metallicity, a Lyman continuum photon escape fraction of 0, etc. Finally, we have applied to our synthetic data the prescription outlined in Sandstrom et al (2023) for removing the underlying continuum to the 3.3 μm PAH feature emission, to be consistent with our treatment of the observations.…”

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

“…The strong similarity between observations and simulations suggests that the physical processes included in the simulations (gas self-gravity, SN feedback, and differential rotation) are at the origin of the observed morphology. Further analysis, which is out of the scope of this paper, will be necessary to quantify in more detail the statistical properties of simulations versus observations (see, e.g., Sandstrom et al 2023).…”

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).…”