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1. 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)

   Abstract 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 $1.5\times {10}^{19}{\mathrm{cm}}^{-2}{\left(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}\right)}^{-1}$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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2. The RESOLVE and ECO G3 Initiative: Drivers of H i Content and X-Ray Emission in Galaxy Groups

   https://doi.org/10.3847/1538-4357/adc395  

   Hutchens, Zackary L; Kannappan, Sheila J; Hess, Kelley M; Baker, Andrew J; Sun, Ming; Carr, Derrick S; Eckert, Kathleen D; Stark, David V (May 2025, The Astrophysical Journal)

   Abstract Adding to the RESOLVE and ECO Gas in Galaxy Groups (G3) initiative, we examine possible drivers of group-integrated Hi-to-halo mass ratios (M_HI,grp/M_halo) and group X-ray emission, including group halo mass (M_halo), virialization as probed by crossing time (t_cross), presence of active galactic nuclei (AGN), and group-integrated fractional stellar mass growth rate (FSMGR_grp). G3 groups spanM_halo= 10¹¹−10^14.5M_⊙with comprehensive Higas and AGN information, which we combine with X-ray stacking of ROSAT All-Sky data. We detect hot gas emission exceeding AGN and X-ray binary backgrounds confidently forM_halo= 10^12.6−10¹⁴M_⊙and unambiguously forM_halo> 10¹⁴M_⊙, reflecting an inverse dependence ofM_HI,grp/M_haloand hot gas emission on halo mass. At fixed halo mass,M_HI,grp/M_halotransitions to greater spread belowt_cross∼ 2 Gyr. Dividing groups across this transition, lower-t_crossgroups show elevated X-ray emission compared to higher-t_crossgroups forM_halo> 10^13.3M_⊙, but this trend reverses forM_halo= 10^12.6−10^13.3M_⊙. Additionally, AGN-hosting halos belowM_halo∼ 10^12.1M_⊙exhibit a broad, ∼0.25 dex deep valley inM_HI,grp/M_halocompared to non-AGN-hosting halos with correspondingly reduced FSMGR_grp. When diluted by non-AGN-hosting halos, this valley becomes shallower and narrower, falling roughly between $M_{\mathrm{halo}}={10}^{11.5}{M}_{\odot }$and $M_{\mathrm{halo}}={10}^{12.1}{M}_{\odot }$in the overallM_HI,grp/M_halovs.M_halorelation. We may also detect a second, less easily interpreted valley atM_halo∼ 10¹³M_⊙. Neither valley matches theoretical predictions of a deeper valley located at or above $M_{\mathrm{halo}}={10}^{12.1}{M}_{\odot }$. 

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3. 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)

   Abstract 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 ${4.2}_{-2.0}^{+1.9}{M}_{\odot }{\mathrm{pc}}^{-2}{\left(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}\right)}^{-1}$, 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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4. Inside the Stagnation Radius of the Nearest Billion-solar-mass Black Hole

   https://doi.org/10.3847/1538-4357/adfa93  

   Wrobel, J M; Pesce, D W; Nyland, K E (September 2025, The Astrophysical Journal)

   Abstract We used the NSF Jansky Very Large Array at a frequencyν = 22 GHz to study the nearest billion-solar-mass black hole (BH), in the early-type galaxy NGC 3115 at a distance of 9.7 Mpc. We localize a faint continuum nucleus, with flux densityS_{22 GHz} = 48.2 ± 6.4μJy, to a FWHM diameterd_{22 GHz} < 59 mas (2.8 pc). We find no evidence for adjacent emission within a stagnation region of radiusR_sta ∼ 360 mas (17 pc) identified in a recent hydrodynamic simulation tailored to NGC 3115. Within that region, the simulated gas flow developed into an advection-dominated accretion flow (ADAF). The nucleus’ luminosity densityL_{22 GHz} = 5.4 × 10¹⁷W Hz⁻¹is about 60 times that of Sagittarius A^⋆. The nucleus’ spectral index ${\alpha }_{10\mathrm{GHz}}^{22\mathrm{GHz}}=-1.85\pm 0.18$(S_ν ∝ ν^α) indicates optically thin synchrotron emission. The spectral energy distribution of the nucleus peaks nearν_peak = 9 GHz. Modeling this radio peak as an ADAF implies a BH massM_ADAF = (1.2 ± 0.2) × 10⁹M_⊙, consistent with previous estimates of (1–2) × 10⁹M_⊙from stellar or hot-gas dynamics. Also, the Eddington-scaled accretion rate for NGC 3115, ${\dot{M}}_{\mathrm{ADAF}}/{\dot{M}}_{\mathrm{Edd}}=1.2_{-0.6}^{+1.0}\times 10^{-8}$, is about 4–8 times lower than recent estimates for Sagittarius A^⋆. 

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5. The Pan-STARRS1 z > 5.6 Quasar Survey. III. The z ≈ 6 Quasar Luminosity Function

   https://doi.org/10.3847/1538-4357/aca7ca  

   Schindler, Jan-Torge; Bañados, Eduardo; Connor, Thomas; Decarli, Roberto; Fan, Xiaohui; Farina, Emanuele Paolo; Mazzucchelli, Chiara; Nanni, Riccardo; Rix, Hans-Walter; Stern, Daniel; et al (January 2023, The Astrophysical Journal)

   Abstract We present thez≈ 6 type-1 quasar luminosity function (QLF), based on the Pan-STARRS1 (PS1) quasar survey. The PS1 sample includes 125 quasars atz≈ 5.7–6.2, with −28 ≲M₁₄₅₀≲ −25. With the addition of 48 fainter quasars from the SHELLQs survey, we evaluate thez≈ 6 QLF over −28 ≲M₁₄₅₀≲ −22. Adopting a double power law with an exponential evolution of the quasar density (Φ(z) ∝ 10^k(z−6);k= −0.7), we use a maximum likelihood method to model our data. We find a break magnitude of $M^{*}=-{26.38}_{-0.60}^{+0.79}\mathrm{mag}$, a faint-end slope of $\alpha =-{1.70}_{-0.19}^{+0.29}$, and a steep bright-end slope of $\beta =-{3.84}_{-1.21}^{+0.63}$. Based on our new QLF model, we determine the quasar comoving spatial density atz≈ 6 to be $n(M_{1450}<-26)={1.16}_{-0.12}^{+0.13}{\mathrm{cGpc}}^{-3}$. In comparison with the literature, we find the quasar density to evolve with a constant value ofk≈ −0.7, fromz≈ 7 toz≈ 4. Additionally, we derive an ionizing emissivity of ${ϵ}_{912}(z=6)={7.23}_{-1.02}^{+1.65}\times {10}^{22}\mathrm{erg}{\mathrm{s}}^{-1}{\mathrm{Hz}}^{-1}{\mathrm{cMpc}}^{-3}$, based on the QLF measurement. Given standard assumptions, and the recent measurement of the mean free path by Becker et al. atz≈ 6, we calculate an Hiphotoionizing rate of Γ_{H I}(z= 6) ≈ 6 × 10⁻¹⁶s⁻¹, strongly disfavoring a dominant role of quasars in hydrogen reionization. 

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