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1. DIISC-VI (COS-DIISC): Ultraviolet Metal Absorption Relative to the H i Disk of Galaxies

   https://doi.org/10.3847/1538-4357/adb8ca  

   Koplitz, Brad; Borthakur, Sanchayeeta; Heckman, Timothy; Padave, Mansi; McCabe, Tyler; Tumlinson, Jason; Fox, Andrew J; Kauffmann, Guinevere (March 2025, The Astrophysical Journal)

   Abstract As part of the Deciphering the Interplay between the Interstellar medium, Stars, and the Circumgalactic medium (DIISC) survey, we present the UV metal absorption features in the circumgalactic medium (CGM) near the Higas disk (<4.5R_HI) of 31 nearby galaxies through quasar absorption-line spectroscopy. Of the ions under study, Siiiiλ1206 was most frequently detected (18 of 31 sight lines), while Ciiλ1334 and Siiiλ1260 were detected in 17 and 15 of 31 sight lines, respectively. Many components were consistent with photoionization equilibrium models; most of the cold and cool gas phase clouds were found to have lengths smaller than 2 kpc. Sight lines with smaller impact parameters (ρ) normalized by the galaxy’s virial radius (R_vir) and Hiradius (R_HI) tend to have more components and larger rest-frame equivalent widths (W_r) than those that probe the CGM at larger radii. In particular, we find that the location of metals are better traced byρ/R_HIrather than the traditionalρ/R_vir. Larger covering fractions are found closer to galaxies, with a radial decline that depends on theW_rlimit used. Our results provide new insights into the spatial distribution of metals around the Hidisks of low-redshift galaxies. 

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2. On the Metallicities and Kinematics of the Circumgalactic Media of Damped Lyα Systems at z ∼ 2.5*

   https://doi.org/10.3847/1538-4357/acd34a  

   Urbano Stawinski, Stephanie M.; Rubin, Kate H.; Prochaska, J. Xavier; Hennawi, Joseph F.; Tejos, Nicolas; Fumagalli, Michele; Rafelski, Marc; Kirby, Evan N.; Lusso, Elisabeta; Hafen, Zachary (July 2023, The Astrophysical Journal)

   Abstract We use medium- and high-resolution spectroscopy of close pairs of quasars to analyze the circumgalactic medium (CGM) surrounding 32 damped Ly α absorption systems (DLAs). The primary quasar sightline in each pair probes an intervening DLA in the redshift range 1.6 < z abs < 3.5, such that the secondary sightline probes absorption from Ly α and a large suite of metal-line transitions (including O i , C ii , C iv , Si ii , and Si iv ) in the DLA host galaxy’s CGM at transverse distances 24 kpc ≤ R ⊥ ≤ 284 kpc. Analysis of Ly α in the CGM sightlines shows an anticorrelation between R ⊥ and H i column density ( N HI ) with 99.8% confidence, similar to that observed around luminous galaxies. The incidences of C ii and Si ii with N > 10 13 cm −2 within 100 kpc of DLAs are larger by 2 σ than those measured in the CGM of Lyman break galaxies (C f ( N C II ) > 0.89 and C f ( N Si II ) = 0.75 − 0.17 + 0.12 ). Metallicity constraints derived from ionic ratios for nine CGM systems with negligible ionization corrections and N HI > 10 18.5 cm −2 show a significant degree of scatter (with metallicities/limits across the range − 2.06 ≲ log Z / Z ⊙ ≲ − 0.75 ), suggesting inhomogeneity in the metal distribution in these environments. Velocity widths of C iv λ 1548 and low-ionization metal species in the DLA versus CGM sightlines are strongly (>2 σ ) correlated, suggesting that they trace the potential well of the host halo over R ⊥ ≲ 300 kpc scales. At the same time, velocity centroids for C iv λ 1548 differ in DLA versus CGM sightlines by >100 km s −1 for ∼50% of velocity components, but few components have velocities that would exceed the escape velocity assuming dark matter host halos of ≥10 12 M ⊙ . 

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3. 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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4. SDSS-IV MaNGA: Refining Strong Line Diagnostic Classifications Using Spatially Resolved Gas Dynamics

   https://doi.org/10.3847/1538-4357/abfe0a  

   Law, David R.; Ji, Xihan; Belfiore, Francesco; Bershady, Matthew A.; Cappellari, Michele; Westfall, Kyle B.; Yan, Renbin; Bizyaev, Dmitry; Brownstein, Joel R.; Drory, Niv; et al (July 2021, The Astrophysical Journal)

   We use the statistical power of the MaNGA integral-field spectroscopic galaxy survey to improve the definition of strong line diagnostic boundaries used to classify gas ionization properties in galaxies. We detect line emission from 3.6 million spaxels distributed across 7400 individual galaxies spanning a wide range of stellar masses, star formation rates, and morphological types, and find that the gas-phase velocity dispersion σHα correlates strongly with traditional optical emission-line ratios such as [S II]/Hα, [N II]/Hα, [O I]/Hα, and [O III]/Hβ. Spaxels whose line ratios are most consistent with ionization by galactic H II regions exhibit a narrow range of dynamically cold line-of-sight velocity distributions (LOSVDs) peaked around 25 km s-1 corresponding to a galactic thin disk, while those consistent with ionization by active galactic nuclei (AGNs) and low-ionization emission-line regions (LI(N)ERs) have significantly broader LOSVDs extending to 200 km s-1. Star-forming, AGN, and LI(N)ER regions are additionally well separated from each other in terms of their stellar velocity dispersion, stellar population age, Hα equivalent width, and typical radius within a given galaxy. We use our observations to revise the traditional emission-line diagnostic classifications so that they reliably identify distinct dynamical samples both in two-dimensional representations of the diagnostic line ratio space and in a multidimensional space that accounts for the complex folding of the star-forming model surface. By comparing the MaNGA observations to the SDSS single-fiber galaxy sample, we note that the latter is systematically biased against young, low-metallicity star-forming regions that lie outside of the 3″ fiber footprint. 

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5. Project AMIGA: The Inner Circumgalactic Medium of Andromeda from Thick Disk to Halo*

   https://doi.org/10.3847/1538-4357/ae1f13  

   Lehner, Nicolas; Howk, J Christopher; Collins, Lucy; Sameer; Wakker, Bart P; Augustin, Ramona; Barger, Kathleen A; Berg, Michelle A; Bordoloi, Rongmon; Brown, Thomas M; et al (January 2026, The Astrophysical Journal)

   Abstract The inner circumgalactic medium (CGM) of galaxies, where disk and halo processes intersect, remains poorly characterized despite its critical role in regulating galaxy evolution. We present results from Project AMIGA Insider, mapping Andromeda’s (M31) inner CGM within 0.25R_vir(∼75 kpc) using 11 QSO sightlines, bringing our total sample to 54 sightlines from the disk to 2R_vir. We detect a clear transition between M31’s thick disk and CGM atR ≲ 30 kpc, where low/intermediate ions show thick-disk corotating components with higher column densities than the CGM ones, while high ions exhibit similar column densities in both the CGM and thick disk. Beyond this region, all ion column densities decrease with impact parameter, with steeper gradients for low ions than high ions. The inner CGM (R ≲ 100 kpc) shows more complex gas phases and multicomponent absorption compared to the predominantly single-component outer CGM. We find no significant azimuthal dependence for any observed ions, suggesting M31’s CGM is shaped by radial processes (e.g., cooling flows, precipitation) rather than disk-aligned outflows. We estimate the total metal mass in M31’s cool (Siii, Siiii, Siiv) CGM withinR_virto be (1.9 ± 0. 3_stat ± 0. 7_sys) × 10⁷M_⊙, leading to a cool gas mass of $\approx 6\times 10^9{(Z/0.3Z_{\odot })}^{-1}$ M_⊙. The warmer Ovigas may contain at least 10 times more metal and gas mass. Compared to the COS-HalosL*galaxies, M31’s cool CGM shows lower Si column densities atR ≲ 0.4R₂₀₀and overall lower cool CGM masses, likely reflecting differences in galaxy mass and environment. 

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