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1. Differential Accretion of Ionized Low-velocity Gas at the Milky Way’s Disk–Halo Interface

   https://doi.org/10.3847/1538-4357/ae2741  

   Bish, Hannah V; Werk, Jessica K; Peek, JEG; Di_Teodoro, Enrico; Zheng, Yong; Putman, Mary (January 2026, The Astrophysical Journal)

   Abstract We measure line-of-sight velocities of metal absorption and HIemission along 132 QSO sight lines in order to study gas accretion and outflow at the disk–halo interface of the Milky Way. While previous studies have focused on high- and intermediate-velocity clouds and complexes, we examine material at predominantly low velocities relative to the local standard of rest (i.e., all absorbers at ∣v_LSR∣ < 90 km s⁻¹, and absorbers at 90 < ∣v_LSR∣ < 150 km s⁻¹not associated with any well-defined cloud complexes). We find that gas accretion velocities in the northern Galactic hemisphere are correlated with the ionization potential energy of the multiphase metal ions we include in our analysis; more highly ionized material traced by Civ, Siiv, and Nvis moving toward the disk 10−15 km s⁻¹faster than low ionization state material traced by Sii, Cii* , and Niii. We interpret this dependence as potential evidence of warm accreting gas cooling as it reaches the disk–halo interface, causing a pileup of slower-moving cool gas. We find that with the number of available sight lines, kinematic modeling cannot rule out exponential density distributions or layers of gas that sandwich the Galactic disk. Our results paint a picture of a complex, dynamic disk–halo interface in which low-velocity material likely plays an important role in fueling star formation in the Milky Way. 

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2. Physical Conditions of the Ionized Superwind in NGC 253 with VLT/MUSE

   https://doi.org/10.3847/1538-4357/add738  

   Cronin, Serena A; Bolatto, Alberto D; Congiu, Enrico; Donaghue, Keaton; Kreckel, Kathryn; Leroy, Adam K; Levy, Rebecca C; Veilleux, Sylvain; Walter, Fabian; Nolasco, Lenin (July 2025, The Astrophysical Journal)

   Abstract We present an analysis of the Hα-emitting ionized gas in the warm phase of the NGC 253 outflow using integral field spectroscopy from the Multi Unit Spectroscopic Explorer. In each spaxel, we decompose Hα, [Nii], and [Sii] emission lines into a system of up to three Gaussian components, accounting for the velocity contributions due to the disk and both intercepted walls of an outflow cone. In the approaching southern lobe of the outflow, we find maximum deprojected outflow velocities down to ∼−500 km s⁻¹. Velocity gradients of this outflowing gas range from ∼−350 to −550 km s⁻¹kpc⁻¹with increasing distance from the nucleus. Additionally, [Nii]/Hαand [Sii]/Hαintegrated line ratios are suggestive of shocks as the dominant ionization source throughout the wind. Electron densities, inferred from the [Sii] doublet, peak at 2100 cm⁻³near the nucleus and reach ≲50 cm⁻³in the wind. Finally, at an uncertainty of 0.3 dex on the inferred mass of 4 × 10⁵M_⊙, the mass-outflow rate of the Hα-emitting gas in the southern outflow lobe is ∼0.4M_⊙yr⁻¹. This yields a mass-loading factor ofη ∼ 0.1 and a ∼2% starburst energy efficiency. 

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3. Resolved Stellar and Nebular Kinematics of a Star-forming Galaxy at z  ∼ 2

   https://doi.org/10.3847/1538-4357/adfa22  

   Rhoades, Sunny; Jones, Tucker; Vasan_G_C, Keerthi; Chen, Yuguang; Leethochawalit, Nicha; Ellis, Richard; Shajib, Anowar J; Glazebrook, Karl; Mortensen, Kris; Sanders, Ryan L (September 2025, The Astrophysical Journal)

   The kinematics of star-forming galaxy populations at high redshifts are integral to our understanding of disk properties, merger rates, and other defining characteristics. Nebular gas emission is a common tracer of galaxies’ gravitational potential and angular momenta, but is sensitive to nongravitational forces as well as galactic outflows, and thus might not accurately trace the host galaxy dynamics. We present kinematic maps of young stars from rest-ultraviolet photospheric absorption in the star-forming galaxy CASSOWARY 13 (a.k.a. SDSS J1237+5533) atz= 1.87 using the Keck Cosmic Web Imager, alongside nebular emission measurements from the same observations. Gravitational lensing magnification of the galaxy enables good spatial sampling of multiple independent lensed images. We find close agreement between the stellar and nebular velocity fields. We measure a mean local velocity dispersion ofσ = 64 ± 12 km s⁻¹for the young stars, consistent with that of the Hiiregions traced by nebular Ciii] emission (52  ±  9 km s⁻¹). The ∼20 km s⁻¹average difference in line-of-sight velocity is much smaller than the local velocity width and the velocity gradient (≳100 km s⁻¹). We find no evidence of asymmetric drift nor evidence that outflows bias the nebular kinematics, and thus we conclude that nebular emission appears to be a reasonable dynamical tracer of young stars in the galaxy. These results support the picture of star formation in thick disks with high velocity dispersion atz ∼ 2, and they represent an important step toward establishing robust kinematics of early galaxies using collisionless tracers. 

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4. Braving the Storm: Quantifying Disk-wide Ionized Outflows in the Large Magellanic Cloud with ULLYSES

   https://doi.org/10.3847/1538-4357/ad64d2  

   Zheng_郑, Yong 永; Tchernyshyov, Kirill; Olsen, Knut; Choi, Yumi; Bustard, Chad; Roman-Duval, Julia; Zhu, Robert; Di_Teodoro, Enrico M; Werk, Jessica; Putman, Mary; et al (October 2024, The Astrophysical Journal)

   Abstract The Large Magellanic Cloud (LMC) is home to many Hiiregions, which may lead to significant outflows. We examine the LMC’s multiphase gas (T∼10^4-5K) in Hi, Sii, Siiv, and Civusing 110 stellar sight lines from the Hubble Space Telescope’s Ultraviolet Legacy Library of Young Stars as Essential Standards program. We develop a continuum fitting algorithm based on the concept of Gaussian process regression and identify reliable LMC interstellar absorption overv_helio= 175–375 km s⁻¹. Our analyses show disk-wide ionized outflows in Siivand Civacross the LMC with bulk velocities of ∣v_{out, bulk}∣ ∼ 20–60 km s⁻¹, which indicates that most of the outflowing mass is gravitationally bound. The outflows’ column densities correlate with the LMC’s star formation rate surface densities (Σ_SFR), and the outflows with higher Σ_SFRtend to be more ionized. Considering outflows from both sides of the LMC as traced by Civ, we conservatively estimate a total outflow rate of ${\dot{M}}_{\mathrm{out}}\gtrsim 0.03M_{\odot }{\mathrm{yr}}^{-1}$and a mass-loading factor ofη≳ 0.15. We compare the LMC’s outflows with those detected in starburst galaxies and simulation predictions, and find a universal scaling relation of $\mid v_{\mathrm{out},\mathrm{bulk}}\mid \propto {\mathrm{\Sigma }}_{\mathrm{SFR}}^{0.23}$over a wide range of star-forming conditions (Σ_SFR∼ 10^−4.5–10²M_⊙yr⁻¹kpc⁻²). Lastly, we find that the outflows are corotating with the LMC’s young stellar disk and the velocity field does not seem to be significantly impacted by external forces; we thus speculate on the existence of a bow shock leading the LMC, which may have shielded the outflows from ram pressure as the LMC orbits the Milky Way. 

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5. ALMA-IMF. XIV. Free–Free Templates Derived from H41α and Ionized Gas Content in 15 Massive Protoclusters

   https://doi.org/10.3847/1538-4365/ad61e6  

   Galván-Madrid, Roberto; Díaz-González, Daniel J; Motte, Frédérique; Ginsburg, Adam; Cunningham, Nichol; Menten, Karl M; Armante, Mélanie; Bonfand, Mélisse; Braine, Jonathan; Csengeri, Timea; et al (September 2024, The Astrophysical Journal Supplement Series)

   Abstract We use the H41αrecombination line to create templates of the millimeter free–free emission in the ALMA-IMF continuum maps, which allows us to separate it from dust emission. This method complements spectral-index information and extrapolation from centimeter-wavelength maps. We use the derived maps to estimate the properties of up to 34 Hiiregions across the ALMA-IMF protoclusters. The hydrogen ionizing photon rateQ₀and spectral types follow the evolutionary trend proposed by Motte et al. The youngest protoclusters lack detectable ionized gas, followed by protoclusters with increasing numbers of OB stars. The totalQ₀increases from ∼10⁴⁵s⁻¹to >10⁴⁹s⁻¹. We used the adjacent He41αline to measure the relative number abundances of helium, finding values consistent with the Galactic interstellar medium, although a few outliers are discussed. A search for sites of maser amplification of the H41αline returned negative results. We looked for possible correlations between the electron densities, emission measures, andQ₀with Hiiregion sizeD. The latter is the best correlated, withQ₀∝D^{2.49 ± 0.18}. This favors interpretations in which smaller ultracompact Hiiregions are not necessarily the less dynamically evolved versions of larger ones but rather are ionized by less massive stars. Moderate correlations were found between the dynamical width ΔV_dynwithDandQ₀. ΔV_dynincreases from about 1 to 2 times the ionized-gas sound speed. Finally, an outlier Hiiregion south of W43-MM2 is discussed. We suggest that this source could harbor an embedded stellar or disk wind. 

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