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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. Resolved Measurements of the CO-to-H 2 Conversion Factor in 37 Nearby Galaxies

   https://doi.org/10.3847/1538-4357/ad23ed  

   Chiang 江, I-Da 宜達 ; Sandstrom, Karin M. ; Chastenet, Jérémy ; Bolatto, Alberto D. ; Koch, Eric W. ; Leroy, Adam K. ; Sun 孙, Jiayi 嘉懿 ; Teng, Yu-Hsuan ; Williams, Thomas G. ( March 2024 , The Astrophysical Journal)

   We measure the CO-to-H₂conversion factor (α_CO) in 37 galaxies at 2 kpc resolution, using the dust surface density inferred from far-infrared emission as a tracer of the gas surface density and assuming a constant dust-to-metal ratio. In total, we have ∼790 and ∼610 independent measurements ofα_COfor CO (2–1) and (1–0), respectively. The mean values forα_{CO (2–1)}andα_{CO (1–0)}are${9.3}_{-5.4}^{+4.6}$and${4.2}_{-2.0}^{+1.9}{M}_{\odot }{\mathrm{pc}}^{-2}{(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1})}^{-1}$, respectively. The CO-intensity-weighted mean is 5.69 forα_{CO (2–1)}and 3.33 forα_{CO (1–0)}. We examine howα_COscales with several physical quantities, e.g., the star formation rate (SFR), stellar mass, and dust-mass-weighted average interstellar radiation field strength ($\overline{U}$). Among them,$\overline{U}$, Σ_SFR, and the integrated CO intensity (W_CO) have the strongest anticorrelation with spatially resolvedα_CO. We provide linear regression results toα_COfor all quantities tested. At galaxy-integrated scales, we observe significant correlations betweenα_COandW_CO, metallicity,$\overline{U}$, and Σ_SFR. We also find thatα_COin each galaxy decreases with the stellar mass surface density (Σ_⋆) in high-surface-density regions (Σ_⋆≥ 100M_⊙pc⁻²), following the power-law relations${\alpha }_{\mathrm{CO}(2–1)}\propto {\mathrm{\Sigma }}_{\star }^{-0.5}$and${\alpha }_{\mathrm{CO}(1–0)}\propto {\mathrm{\Sigma }}_{\star }^{-0.2}$. The power-law index is insensitive to the assumed dust-to-metal ratio. We interpret the decrease inα_COwith increasing Σ_⋆as a result of higher velocity dispersion compared to isolated, self-gravitating clouds due to the additional gravitational force from stellar sources, which leads to the reduction inα_CO. The decrease inα_COat high Σ_⋆is important for accurately assessing molecular gas content and star formation efficiency in the centers of galaxies, which bridge “Milky Way–like” to “starburst-like” conversion factors.

    

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3. The Direct-method Oxygen Abundance of Typical Dwarf Galaxies at Cosmic High Noon∗

   https://doi.org/10.3847/1538-4357/acb153  

   Gburek, Timothy ; Siana, Brian ; Alavi, Anahita ; Emami, Najmeh ; Richard, Johan ; Freeman, William R. ; Stark, Daniel P. ; Snapp-Kolas, Christopher ( May 2023 , The Astrophysical Journal)

   We present a Keck/MOSFIRE rest-optical composite spectrum of 16 typical gravitationally lensed star-forming dwarf galaxies at 1.7 ≲z≲ 2.6 (z_mean= 2.30), all chosen independent of emission-line strength. These galaxies have a median stellar mass of$\log {(M_{*}/M_{\odot })}_{\mathrm{med}}={8.29}_{-0.43}^{+0.51}$and a median star formation rate of${\mathrm{S}\mathrm{F}\mathrm{R}}_{\mathrm{H}\alpha }^{\mathrm{m}\mathrm{e}\mathrm{d}}={2.25}_{-1.26}^{+2.15}{M}_{\odot }{\mathrm{y}\mathrm{r}}^{-1}$. We measure the faint electron-temperature-sensitive [Oiii]λ4363 emission line at 2.5σ(4.1σ) significance when considering a bootstrapped (statistical-only) uncertainty spectrum. This yields a direct-method oxygen abundance of$12+\log {(\mathrm{O}/\mathrm{H})}_{\mathrm{direct}}={7.88}_{-0.22}^{+0.25}$(${0.15}_{-0.06}^{+0.12}{Z}_{\odot }$). We investigate the applicability at highzof locally calibrated oxygen-based strong-line metallicity relations, finding that the local reference calibrations of Bian et al. best reproduce (≲0.12 dex) our composite metallicity at fixed strong-line ratio. At fixedM_*, our composite is well represented by thez∼ 2.3 direct-method stellar mass—gas-phase metallicity relation (MZR) of Sanders et al. When comparing to predicted MZRs from the IllustrisTNG and FIRE simulations, having recalculated our stellar masses with more realistic nonparametric star formation histories$(\log {(M_{*}/M_{\odot })}_{\mathrm{med}}={8.92}_{-0.22}^{+0.31})$, we find excellent agreement with the FIRE MZR. Our composite is consistent with no metallicity evolution, at fixedM_*and SFR, of the locally defined fundamental metallicity relation. We measure the doublet ratio [Oii]λ3729/[Oii]λ3726 = 1.56 ± 0.32 (1.51 ± 0.12) and a corresponding electron density of$n_e=1_{-0}^{+215}{\mathrm{cm}}^{-3}$($n_e=1_{-0}^{+74}{\mathrm{cm}}^{-3}$) when considering the bootstrapped (statistical-only) error spectrum. This result suggests that lower-mass galaxies have lower densities than higher-mass galaxies atz∼ 2.

    

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4. Evidence of a Cloud–Cloud Collision from Overshooting Gas in the Galactic Center

   https://doi.org/10.3847/1538-4357/ad01be  

   Gramze, Savannah R. ; Ginsburg, Adam ; Meier, David S. ; Ott, Juergen ; Shirley, Yancy ; Sormani, Mattia C. ; Svoboda, Brian E. ( December 2023 , The Astrophysical Journal)

   The Milky Way is a barred spiral galaxy withbar lanesthat bring gas toward the Galactic center. Gas flowing along these bar lanes often overshoots, and instead of accreting onto the Central Molecular Zone (CMZ), it collides with the bar lane on the opposite side of the Galaxy. We observed G5, a cloud that we believe is the site of one such collision, near the Galactic center at (ℓ,b) = ( +5.4, −0.4) with the Atacama Large Millimeter/submillimeter Array/Atacama Compact Array. We took measurements of the spectral lines¹²COJ= 2 → 1,¹³COJ= 2 → 1, C¹⁸OJ= 2 → 1, H₂COJ= 3₀₃→ 2₀₂, H₂COJ= 3₂₂→ 2₂₁, CH₃OHJ= 4₂₂→ 3₁₂, OCSJ= 18 → 17, and SiOJ= 5 → 4. We observed a velocity bridge between two clouds at ∼50 and ∼150 km s⁻¹in our position–velocity diagram, which is direct evidence of a cloud–cloud collision. We measured an average gas temperature of ∼60 K in G5 using H₂CO integrated-intensity line ratios. We observed that the¹²C/¹³C ratio in G5 is consistent with optically thin, or at most marginally optically thick¹²CO. We measured$1.5\times {10}^{19}{\mathrm{cm}}^{-2}{(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1})}^{-1}$for the local X_CO, 10–20× less than the average Galactic value. G5 is strong direct observational evidence of gas overshooting the CMZ and colliding with a bar lane on the opposite side of the Galactic center.

    

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5. The FRB 20190520B Sight Line Intersects Foreground Galaxy Clusters

   https://doi.org/10.3847/2041-8213/acefb5  

   Lee, Khee-Gan ; Khrykin, Ilya S. ; Simha, Sunil ; Ata, Metin ; Huang, Yuxin ; Prochaska, J. Xavier ; Tejos, Nicolas ; Cooke, Jeff ; Nagamine, Kentaro ; Zhang, Jielai ( August 2023 , The Astrophysical Journal Letters)

   The repeating fast radio burst FRB 20190520B is an anomaly of the FRB population thanks to its high dispersion measure (DM = 1205 pc cm⁻³) despite its low redshift ofz_frb= 0.241. This excess has been attributed to a large host contribution of DM_host≈ 900 pc cm⁻³, far larger than any other known FRB. In this paper, we describe spectroscopic observations of the FRB 20190520B field obtained as part of the FLIMFLAM survey, which yielded 701 galaxy redshifts in the field. We find multiple foreground galaxy groups and clusters, for which we then estimated halo masses by comparing their richness with numerical simulations. We discover two separateM_halo> 10¹⁴M_⊙galaxy clusters atz= 0.1867 and 0.2170 that are directly intersected by the FRB sight line within their characteristic halo radiusr₂₀₀. Subtracting off their estimated DM contributions, as well that of the diffuse intergalactic medium, we estimate a host contribution of${\mathrm{D}\mathrm{M}}_{\mathrm{h}\mathrm{o}\mathrm{s}\mathrm{t}}={430}_{-220}^{+140}$or${280}_{-170}^{+140}\mathrm{p}\mathrm{c}{\mathrm{c}\mathrm{m}}^{-3}$(observed frame), depending on whether we assume that the halo gas extends tor₂₀₀or 2 ×r₂₀₀. This significantly smaller DM_host—no longer the largest known value—is now consistent with Hαemission measures of the host galaxy without invoking unusually high gas temperatures. Combined with the observed FRB scattering timescale, we estimate the turbulent fluctuation and geometric amplification factor of the scattering layer to be$\tilde{F}G\approx 4.5–11{({\mathrm{pc}}^2\mathrm{km})}^{-1/3}$, suggesting that most of the gas is close to the FRB host. This result illustrates the importance of incorporating foreground data for FRB analyses both for understanding the nature of FRBs and to realize their potential as a cosmological probe.

    

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