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1. 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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2. The Cosmic Ultraviolet Baryon Survey (CUBS). IX. The Enriched Circumgalactic and Intergalactic Medium Around Star-forming Field Dwarf Galaxies Traced by O vi Absorption

   https://doi.org/10.3847/1538-4357/ad7b0a  

   Mishra, Nishant; Johnson, Sean D; Rudie, Gwen C; Chen, Hsiao-Wen; Schaye, Joop; Qu, Zhijie; Zahedy, Fakhri S; Boettcher, Erin T; Cantalupo, Sebastiano; Chen, Mandy C; et al (November 2024, The Astrophysical Journal)

   Abstract The shallow potential wells of star-forming dwarf galaxies make their surrounding circumgalactic and intergalactic medium (CGM/IGM) sensitive laboratories for studying the inflows and outflows thought to regulate galaxy evolution. We present new absorption-line measurements in quasar sight lines, probing within projected distances of <300 kpc from 91 star-forming field dwarf galaxies with a median stellar mass of $\log M_{\star }/M_{\odot }\approx 8.3$ at 0.077 <z< 0.73, from the Cosmic Ultraviolet Baryon Survey (CUBS). In this redshift range, the CUBS quasar spectra cover a suite of transitions including Hi, low, and intermediate metal ions (e.g., Cii, Siii, Ciii, and Siiii), and highly ionized Ovi. This CUBS-Dwarfs survey enables constraints with samples nine times larger than past dwarf CGM/IGM studies with similar ionic coverage. We find that low and intermediate ionization metal absorption is rare around dwarf galaxies, consistent with previous surveys of local dwarfs. In contrast, highly ionized Oviis commonly observed in sight lines that pass within the virial radius of a dwarf, and Ovidetection rates are nonnegligible at projected distances of 1−2× the virial radius. Based on these measurements, we estimate that the Ovi-bearing phase of the CGM/IGM accounts for a dominant share of the metal budget of dwarf galaxies. The absorption kinematics suggest that a relatively modest fraction of the Ovi-bearing gas is formally unbound. Together, these results imply that low-mass systems atz≲ 1 effectively retain a substantial fraction of their metals within the nearby CGM and IGM. 

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3. FOGGIE. X. Characterizing the Small-scale Structure of the Circumgalactic Medium and Its Imprint on Observables

   https://doi.org/10.3847/1538-4357/ae0462  

   Augustin, Ramona; Tumlinson, Jason; Peeples, Molly S; O’Shea, Brian W; Smith, Britton D; Lochhaas, Cassandra; Wright, Anna C; Acharyya, Ayan; Werk, Jessica K; Lehner, Nicolas; et al (October 2025, The Astrophysical Journal)

   Abstract One of the main unknowns in galaxy evolution is how gas flows into and out of galaxies in the circumgalactic medium (CGM). Studies observing the CGM in absorption using multiple or extended background objects suggest a high degree of variation on relatively small (≲1 kpc) spatial scales. Similarly, high-resolution simulations generally exhibit small-scale substructure in the gas around galaxies. We examine the small-scale structure of thez= 1 CGM using simulations from the Figuring Out Gas and Galaxies in Enzo (FOGGIE) project. We select gaseous substructures (“clumps”) by their local overdensity and investigate their physical properties, including temperature, metallicity, and kinematics with respect to the galaxy and the nearby surroundings. FOGGIE resolves clumps down to sphericalized radiiR ∼ 0.25 kpc atz= 1. The distribution of clumps peaks at ∼10⁵M_⊙and 10⁴K, consistent with relatively condensed, cool gas with a slight preference for inflow-like velocities. Many clumps show internal temperature and density variations, and thus internally varying ionization levels for key diagnostic ions such as Hi, Mgii, and Ovi. The average metallicity in clumps is about a factor 1.5–2× lower in metallicity than nearby gas, suggesting that the metals are not well mixed between structured and diffuse CGMs, which may have implications for observational metallicity estimations of dense CGM clouds. We estimate the survivability of CGM clumps and find that structures larger than 0.5 kpc are generally long-lived. Finally, we qualitatively compare the simulated cloud properties to Milky Way high-velocity clouds. 

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4. The Cool Circumgalactic Medium of Low-redshift Star-forming Galaxies. I. Empirical Model and Mean Properties

   https://doi.org/10.3847/1538-4357/acf217  

   Faerman, Yakov; Werk, Jessica K (October 2023, The Astrophysical Journal)

   Abstract We present an analytic model for the cool,T∼ 10⁴K, circumgalactic medium (CGM), describing the gas distribution, and thermal and ionization states. Our model assumes (total) pressure equilibrium with the ambient warm/hot CGM, photoionization by the metagalactic radiation, and allows for nonthermal pressure support, parameterized by the ratio of thermal pressures,η=P_hot,th/P_cool,th. We apply the model to the COS-Halos measurements and find that a nominal model withη= 3, gas distribution out tor≈ 0.6R_vir, andM_cool= 3 × 10⁹M_⊙, corresponding to a volume filling fraction off_V,cool≈ 1%, reproduces the Hiand low/intermediate metal ions (Cii, Ciii, Siii, Siiii, and Mgii) mean column densities. Variation of ±0.5 dex inηorM_coolencompasses ∼2/3 of the scatter between objects. Our nominal model underproduces the measured Civand Siivcolumns, and these can be reproduced with (i) a cool phase withM_cool∼ 10¹⁰M_⊙andη≈ 5, or (ii) cooling or mixing gas at intermediate temperatures, withM∼ 1.5 × 10¹⁰M_⊙and occupying ∼1/2 of the total CGM volume. For cool gas withf_V,cool≈ 1%, we estimate an upper limit on the cloud sizes,R_cl≲ 0.5 kpc. Our results suggest that for the average galaxy CGM, the mass and nonthermal support in the cool phase are lower than previously estimated, and extreme scenarios are not necessary. We estimate the rates of cool gas depletion and replenishment, and find accretion onto the galaxy can be offset, allowing ${\dot{M}}_{\mathrm{cool}}\approx 0$ over long timescales. 

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5. A Comprehensive Investigation of Metals in the Circumgalactic Medium of Nearby Dwarf Galaxies

   https://doi.org/10.3847/1538-4357/acfe6b  

   Zheng_郑, Yong 永; Faerman, Yakov; Oppenheimer, Benjamin D; Putman, Mary E; McQuinn, Kristen_B W; Kirby, Evan N; Burchett, Joseph N; Telford, O Grace; Werk, Jessica K; Kim, Doyeon A (December 2023, The Astrophysical Journal)

   Abstract Dwarf galaxies are found to have lost most of their metals via feedback processes; however, there still lacks consistent assessment on the retention rate of metals in their circumgalactic medium (CGM). Here we investigate the metal content in the CGM of 45 isolated dwarf galaxies withM_*= 10^6.5–9.5M_⊙(M_200m= 10^10.0–11.5M_⊙) using the Hubble Space Telescope/Cosmic Origins Spectrograph. While Hi(Lyα) is ubiquitously detected (89%) within the CGM, we find low detection rates (≈5%–22%) in Cii, Civ, Siii, Siiii, and Siiv, largely consistent with literature values. Assuming these ions form in the cool (T≈ 10⁴K) CGM with photoionization equilibrium, the observed Hiand metal column density profiles can be best explained by an empirical model with low gas density and high volume filling factor. For a typical galaxy withM_200m= 10^10.9M_⊙(median of the sample), our model predicts a cool gas mass ofM_CGM,cool∼ 10^8.4M_⊙, corresponding to ∼2% of the galaxy’s baryonic budget. Assuming a metallicity of 0.3 Z_⊙, we estimate that the dwarf galaxy’s cool CGM likely harbors ∼10% of the metals ever produced, with the rest either in more ionized states in the CGM or transported to the intergalactic medium. We further examine the EAGLE simulation and show that Hiand low ions may arise from a dense cool medium, while Civarises from a diffuse warmer medium. Our work provides the community with a uniform data set on dwarf galaxies’ CGM that combines our recent observations, additional archival data and literature compilation, which can be used to test various theoretical models of dwarf galaxies. 

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