GOALS-JWST: Tracing AGN Feedback on the Star-forming Interstellar Medium in NGC 7469

et al. 2023

ApJL

Self Cite

We present maps tracing the fraction of dust in the form of polycyclic aromatic hydrocarbons (PAHs) in IC 5332, NGC 628, NGC 1365, and NGC 7496 from JWST/MIRI observations. We trace the PAH fraction by combining the F770W (7.7 μm) and F1130W (11.3 μm) filters to track ionized and neutral PAH emission, respectively, and comparing the PAH emission to F2100W, which traces small, hot dust grains. We find the average R PAH = (F770W + F1130W)/F2100W values of 3.3, 4.7, 5.1, and 3.6 in IC 5332, NGC 628, NGC 1365, and NGC 7496, respectively. We find that H ii regions traced by MUSE Hα show a systematically low PAH fraction. The PAH fraction remains relatively constant across other galactic environments, with slight variations. We use CO+Hi +Hα to trace the interstellar gas phase and find that the PAH fraction decreases above a value of I H α / Σ H I + H 2 ∼ 10 37.5 erg s − 1 kpc − 2 ( M ⊙ pc − 2 ) − 1 in all four galaxies. Radial profiles also show a decreasing PAH fraction with increasing radius, correlated with lower metallicity, in line with previous results showing a strong metallicity dependence to the PAH fraction. Our results suggest that the process of PAH destruction in ionized gas operates similarly across the four targets.

Section: Qualitative Descriptionmentioning

confidence: 99%

PHANGS–JWST First Results: Variations in PAH Fraction as a Function of ISM Phase and Metallicity

Chastenet

Sutter

et al. 2023

ApJL

Self Cite

“…We consider mid-IR emission from the four MIRI bands imaged by PHANGS-JWST, F770W (7.7μm), F1000W (10μm), F1130W (11.3μm), and F2100W (21μm). To first order, the F770W and F1130W bands are expected to be dominated by emission from strong PAH bands while the F2100W filter covers mainly continuum emission from dust (e.g., see spectra in Draine & Li 2007;Smith et al 2007;Galliano et al 2018;Hensley & Draine 2022;Lai & Armus 2022). As discussed in Section 4.4, the situation with F1000W appears more ambiguous.…”

Section: Mid-infrared Bands and Band Ratiosmentioning

confidence: 99%

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

Leroy

Rosolowsky

et al. 2023

ApJL

Self Cite

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.

“…To do that, we reproject these masks to the JWST images. Since two of our galaxies contain an AGN, which produces hard ionizing radiation that can significantly affect the PAH fraction (e.g., Jensen et al 2017;García-Bernete et al 2022;Lai et al 2022), we exclude the centers of the galaxies from our analysis. Masks that isolate these particular environments are taken from Querejeta et al (2021).…”

Section: Selection Of the Nebulae And Associated Pah Emissionmentioning

confidence: 99%

PHANGS–JWST First Results: Destruction of the PAH Molecules in H ii Regions Probed by JWST and MUSE

Egorov¹,

Kreckel²,

et al. 2023

ApJL

Self Cite

Polycyclic aromatic hydrocarbons (PAHs) play a critical role in the reprocessing of stellar radiation and balancing the heating and cooling processes in the interstellar medium but appear to be destroyed in H ii regions. However, the mechanisms driving their destruction are still not completely understood. Using PHANGS–JWST and PHANGS–MUSE observations, we investigate how the PAH fraction changes in about 1500 H ii regions across four nearby star-forming galaxies (NGC 628, NGC 1365, NGC 7496, and IC 5332). We find a strong anticorrelation between the PAH fraction and the ionization parameter (the ratio between the ionizing photon flux and the hydrogen density) of H ii regions. This relation becomes steeper for more luminous H ii regions. The metallicity of H ii regions has only a minor impact on these results in our galaxy sample. We find that the PAH fraction decreases with the Hα equivalent width—a proxy for the age of the H ii regions—although this trend is much weaker than the one identified using the ionization parameter. Our results are consistent with a scenario where hydrogen-ionizing UV radiation is the dominant source of PAH destruction in star-forming regions.