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

   https://doi.org/10.3847/2041-8213/acadd7  

   Chastenet, Jérémy ; Sutter, Jessica ; Sandstrom, Karin ; Belfiore, Francesco ; Egorov, Oleg_V ; Larson, Kirsten_L ; Leroy, Adam_K ; Liu, Daizhong ; Rosolowsky, Erik ; Thilker, David_A ; et al ( February 2023 , The Astrophysical Journal Letters)

   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 (7.7μm) and F1130W (11.3μm) 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 averageR_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 Hiiregions traced by MUSE Hαshow a systematically low PAH fraction. The PAH fraction remains relatively constant across other galactic environments, with slight variations. We use CO+Hi+Hαto trace the interstellar gas phase and find that the PAH fraction decreases above a value of${\mathrm{I}}_{\mathrm{H}\alpha }/{\mathrm{\Sigma }}_{\mathrm{H}\mathrm{I}+{\mathrm{H}}_2}\sim {10}^{37.5}\mathrm{erg}{\mathrm{s}}^{-1}{\mathrm{kpc}}^{-2}{(M_{\odot }{\mathrm{pc}}^{-2})}^{-1}$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.

    

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2. Wide-field CO isotopologue emission and the CO-to-H 2 factor across the nearby spiral galaxy M101

   https://doi.org/10.1051/0004-6361/202245718  

   den Brok, Jakob S. ; Bigiel, Frank ; Chastenet, Jérémy ; Sandstrom, Karin ; Leroy, Adam ; Usero, Antonio ; Schinnerer, Eva ; Rosolowsky, Erik W. ; Koch, Eric W. ; Chiang, I-Da ; et al ( August 2023 , Astronomy & Astrophysics)

   Carbon monoxide (CO) emission constitutes the most widely used tracer of the bulk molecular gas in the interstellar medium (ISM) in extragalactic studies. The CO-to-H 2 conversion factor, α 12 CO(1−0) , links the observed CO emission to the total molecular gas mass. However, no single prescription perfectly describes the variation of α 12 CO(1−0) across all environments within and across galaxies as a function of metallicity, molecular gas opacity, line excitation, and other factors. Using spectral line observations of CO and its isotopologues mapped across a nearby galaxy, we can constrain the molecular gas conditions and link them to a variation in α 12 CO(1−0) . Here, we present new, wide-field (10 × 10 arcmin 2 ) IRAM 30-m telescope 1 mm and 3 mm line observations of 12 CO, 13 CO, and C 18 O across the nearby, grand-design, spiral galaxy M101. From the CO isotopologue line ratio analysis alone, we find that selective nucleosynthesis and changes in the opacity are the main drivers of the variation in the line emission across the galaxy. In a further analysis step, we estimated α 12 CO(1−0) using different approaches, including (i) via the dust mass surface density derived from far-IR emission as an independent tracer of the total gas surface density and (ii) local thermal equilibrium (LTE) based measurements using the optically thin 13 CO(1–0) intensity. We find an average value of ⟨ α 12 CO(1 − 0) ⟩ = 4.4  ±  0.9  M ⊙  pc −2  (K km s −1 ) −1 across the disk of the galaxy, with a decrease by a factor of 10 toward the 2 kpc central region. In contrast, we find LTE-based α 12 CO(1−0) values are lower by a factor of 2–3 across the disk relative to the dust-based result. Accounting for α 12 CO(1−0) variations, we found significantly reduced molecular gas depletion time by a factor 10 in the galaxy’s center. In conclusion, our result suggests implications for commonly derived scaling relations, such as an underestimation of the slope of the Kennicutt Schmidt law, if α 12 CO(1−0) variations are not accounted for. 

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

   https://doi.org/10.3847/2041-8213/aca972  

   Sandstrom, Karin_M ; Koch, Eric_W ; Leroy, Adam_K ; Rosolowsky, Erik ; Emsellem, Eric ; Smith, Rowan_J ; Egorov, Oleg_V ; Williams, Thomas_G ; Larson, Kirsten_L ; Lee, Janice_C ; et al ( February 2023 , The Astrophysical Journal Letters)

   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 3σsensitivity of Σ_gas∼ 2M_⊙pc⁻². We characterize the surface densities of structures seen at <7M_⊙pc⁻²in our targets, where we expect the gas to be Hi-dominated. We highlight the existence of filaments, interarm emission, and holes in the diffuse ISM at these low surface densities. Below ∼10M_⊙pc⁻²for 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.

    

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4. GOALS-JWST: Mid-infrared Molecular Gas Excitation Probes the Local Conditions of Nuclear Star Clusters and the Active Galactic Nucleus in the LIRG VV 114

   https://doi.org/10.3847/1538-4357/ad344b  

   Buiten, Victorine A ; van_der_Werf, Paul P ; Viti, Serena ; Armus, Lee ; Barr, Andrew G ; Barcos-Muñoz, Loreto ; Evans, Aaron S ; Inami, Hanae ; Linden, Sean T ; Privon, George C ; et al ( May 2024 , The Astrophysical Journal)

   The enormous increase in mid-IR sensitivity and spatial and spectral resolution provided by the JWST spectrographs enables, for the first time, detailed extragalactic studies of molecular vibrational bands. This opens an entirely new window for the study of the molecular interstellar medium in luminous infrared galaxies (LIRGs). We present a detailed analysis of rovibrational bands of gas-phase CO, H₂O, C₂H₂, and HCN toward the heavily obscured eastern nucleus of the LIRG VV 114, as observed by NIRSpec and the medium resolution spectrograph on the Mid-InfraRed Instrument (MIRI MRS). Spectra extracted from apertures of 130 pc in radius show a clear dichotomy between the obscured active galactic nucleus (AGN) and two intense starburst regions. We detect the 2.3μm CO bandheads, characteristic of cool stellar atmospheres, in the star-forming regions, but not toward the AGN. Surprisingly, at 4.7μm, we find highly excited CO (T_ex≈ 700–800 K out to at least rotational levelJ= 27) toward the star-forming regions, but only cooler gas (T_ex≈ 200 K) toward the AGN. We conclude that only mid-infrared pumping through the rovibrational lines can account for the equilibrium conditions found for CO and H₂O in the deeply embedded starbursts. Here, the CO bands probe regions with an intense local radiation field inside dusty young massive star clusters or near the most massive young stars. The lack of high-excitation molecular gas toward the AGN is attributed to geometric dilution of the intense radiation from the bright point source. An overview of the relevant excitation and radiative transfer physics is provided in an appendix.

    

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5. PHANGS–JWST First Results: A Global and Moderately Resolved View of Mid-infrared and CO Line Emission from Galaxies at the Start of the JWST Era

   https://doi.org/10.3847/2041-8213/acab01  

   Leroy, Adam_K ; Bolatto, Alberto_D ; Sandstrom, Karin ; Rosolowsky, Erik ; Barnes, Ashley_T ; Bigiel, F. ; Boquien, Médéric ; den_Brok, Jakob_S ; Cao, Yixian ; Chastenet, Jérémy ; et al ( February 2023 , The Astrophysical Journal Letters)

   We explore the relationship between mid-infrared (mid-IR) and CO rotational line emission from massive star-forming galaxies, which is one of the tightest scalings in the local universe. We assemble a large set of unresolved and moderately (∼1 kpc) spatially resolved measurements of CO (1–0) and CO (2–1) intensity,I_CO, and mid-IR intensity,I_MIR, at 8, 12, 22, and 24μm. TheI_COversusI_MIRrelationship is reasonably described by a power law with slopes 0.7–1.2 and normalizationI_CO∼ 1 K km s⁻¹atI_MIR∼ 1 MJy sr⁻¹. Both the slopes and intercepts vary systematically with choice of line and band. The comparison between the relations measured for CO (1–0) and CO (2–1) allow us to infer that$R_{21}\propto I_{\mathrm{MIR}}^{0.2}$, in good agreement with other work. The 8μm and 12μm bands, with strong polycyclic aromatic hydrocarbon (PAH) features, show steeper CO versus mid-IR slopes than the 22 and 24μm, consistent with PAH emission arising not just from CO-bright gas but also from atomic or CO-dark gas. The CO-to-mid-IR ratio correlates with global galaxy stellar mass (M_⋆) and anticorrelates with star formation rate/M_⋆. At ∼1 kpc resolution, the first four PHANGS–JWST targets show CO-to-mid-IR relationships that are quantitatively similar to our larger literature sample, including showing the steep CO-to-mid-IR slopes for the JWST PAH-tracing bands, although we caution that these initial data have a small sample size and span a limited range of intensities.

    

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