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1. The Diffuse Ionized Gas Halo of the Large Magellanic Cloud

   https://doi.org/10.3847/1538-4357/acc06e  

   Smart, B. M.; Haffner, L. M.; Barger, K. A.; Ciampa, D. A.; Hill, A. S.; Krishnarao, D.; Madsen, G. J. (May 2023, The Astrophysical Journal)

   Abstract The Large Magellanic Cloud (LMC) has an extensive H α emission halo that traces an extended, warm ionized component of its interstellar medium. Using the Wisconsin H α Mapper telescope, we present the first kinematic H α survey of an extensive region around the LMC, from ( ℓ , b ) = (264.°5, − 45.°5) to (295.°5, − 19.°5), covering +150 ≤ v LSR ≤ + 390 km s −1 . We find that ionized hydrogen exists throughout the galaxy and extends several degrees beyond detected neutral hydrogen emission ( log N H I / cm − 2 ≈ 18.3 ) as traced by 21 cm in current surveys. Using the column density structure of the neutral gas and stellar line-of-sight depths as a guide, we estimate the upper limit mass of the ionized component of the LMC to be roughly M ionized ≈ (0.6–1.8) × 10 9 M ☉ , which is comparable to the total neutral atomic gas mass in the same region ( M neutral ≈ 0.76–0.85 × 10 9 M ☉ ). Considering only the atomic phases, we find M ionized / M ionized+neutral , to be 46%–68% throughout the LMC and its extended halo. Additionally, we find an ionized gas cloud that extends off of the LMC at ( ℓ , b ) ≈ (285°, − 28°) into a region previously identified as the Leading Arm complex. This gas is moving at a similar line-of-sight velocity as the LMC and has M ionized / M ionized+neutral = 13%–51%. This study, combined with previous studies of the SMC and extended structures of the Magellanic Clouds, continues to suggest that warm, ionized gas is as massive and dynamically important as the neutral gas in the Magellanic System. 

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2. The Large Magellanic Cloud’s ∼30 kpc Bow Shock and Its Impact on the Circumgalactic Medium

   https://doi.org/10.3847/2041-8213/ad0da6  

   Setton, David J.; Besla, Gurtina; Patel, Ekta; Hummels, Cameron; Zheng, Yong; Schneider, Evan; Salem, Munier (December 2023, The Astrophysical Journal Letters)

   Abstract The interaction between the supersonic motion of the Large Magellanic Cloud (LMC) and the circumgalactic medium (CGM) is expected to result in a bow shock that leads the LMC’s gaseous disk. In this letter, we use hydrodynamic simulations of the LMC’s recent infall to predict the extent of this shock and its effect on the Milky Way’s (MW) CGM. The simulations clearly predict the existence of an asymmetric shock with a present-day standoff radius of ∼6.7 kpc and a transverse diameter of ∼30 kpc. Over the past 500 Myr, ∼8% of the MW’s CGM in the southern hemisphere should have interacted with the shock front. This interaction may have had the effect of smoothing over inhomogeneities and increasing mixing in the MW CGM. We find observational evidence of the existence of the bow shock in recent Hαmaps of the LMC, providing a potential explanation for the envelope of ionized gas surrounding the LMC. Furthermore, the interaction of the bow shock with the MW CGM may also explain the observations of ionized gas surrounding the Magellanic Stream. Using recent orbital histories of MW satellites, we find that many satellites have likely interacted with the LMC shock. Additionally, the dwarf galaxy Ret2 is currently sitting inside the shock, which may impact the interpretation of the reported gamma-ray excess in Ret2. This work highlights how bow shocks associated with infalling satellites are an underexplored yet potentially very important dynamical mixing process in the circumgalactic and intracluster media. 

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3. The All-sky Impact of the LMC on the Milky Way Circumgalactic Medium

   https://doi.org/10.3847/1538-4357/adbf1d  

   Carr, Christopher; Bryan, Greg L; Garavito-Camargo, Nicolás; Besla, Gurtina; Setton, David J; Johnston, Kathryn V; Su, Kung-Yi (April 2025, The Astrophysical Journal)

   Abstract The first infall of the LMC into the Milky Way (MW) represents a large and recent disruption to the MW circumgalactic medium (CGM). In this work, we use idealized, hydrodynamical simulations of an MW-like CGM embedded in a dark matter halo with an infalling LMC-like satellite initialized with its own CGM to understand how the encounter is shaping the global physical and kinematic properties of the MW CGM. First, we find that the LMC drives order-unity enhancements in MW CGM density, temperature, and pressure due to a $\mathcal{M}\approx 2$shock from the supersonic CGM–CGM collision. The resulting shock front extends from the LMC to beyond ∼R_200,MW, amplifying column densities, X-ray brightness, thermal Sunyaev–Zeldovich distortion, and potentially synchrotron emission from cosmic rays over large angular scales across the southern hemisphere. Second, the MW’s reflex motion relative to its outer halo induces a dipole in CGM radial velocities, withv_R ± 30–50 km s⁻¹atR > 50 kpc in the northern and southern hemispheres, respectively, consistent with measurements in the stellar halo. Finally, ram pressure strips most of the LMC’s CGM, leaving ∼10⁸⁻⁹M_⊙warm ionized gas along the past orbit of the LMC, moving at high radial and/or tangential velocities ∼50–100 kpc from the MW. Massive satellites like the LMC leave their mark on the CGM structure of their host galaxies, and signatures of such interactions may be observable in key all-sky tracers of the MW CGM and those of other massive galaxies. 

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4. Estimate of Current Mass of the Large Magellanic Cloud from the Orphan–Chenab Tidal Stream

   https://doi.org/10.3847/1538-4357/ae0e6d  

   Warren, Hiroka T; Newberg, Heidi Jo; Guffey, Autumn G; Lin, JiaZhao; Mendelsohn, Eric J; Roux, Kevin (November 2025, The Astrophysical Journal)

   Abstract By fitting the tilt in the path of the Orphan–Chenab Stream (OCS), we conclude that the current mass of the Large Magellanic Cloud (LMC) within 30 kpc is (4.7–5.1) × 10¹⁰M_⊙. We note that the tidal radius of the LMC of this mass is 16.9 kpc, indicating that our measured mass approximates the current bound mass of the LMC. Previous measurements of the LMC mass based on fitting the observed path of the OCS through the Milky Way (MW) halo reported thetotalmass of the LMC. We show that because the closest approach of the LMC to the OCS, where the gravitational perturbation of the stream path is the highest, is about 20 kpc, the mass of the LMC outside 30 kpc is not constrained and depends entirely on the assumed radial profile at large radius. Our best-fit total mass varies between 4.5 × 10¹⁰and 2.2 × 10¹¹M_⊙or more, depending on the presumed radial profile of the LMC. We also show that previous measurements of the mass of the LMC that used a particle-spray method to simulate the path of the OCS suffered from systematic error because they assumed that all particles were stripped from the dwarf galaxy at the tidal radius;N-body simulations show that particles are actually released from a range of distances from the center of mass of the OCS. In contrast, the choice of MW potential has little effect on the LMC mass estimated from the OCS. 

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5. DELVE 6: An Ancient, Ultra-faint Star Cluster on the Outskirts of the Magellanic Clouds

   https://doi.org/10.3847/2041-8213/aced84  

   Cerny, W.; Drlica-Wagner, A.; Li, T. S.; Pace, A. B.; Olsen, K. A.; Noël, N. E.; van der Marel, R. P.; Carlin, J. L.; Choi, Y.; Erkal, D.; et al (August 2023, The Astrophysical Journal Letters)

   Abstract We present the discovery of DELVE 6, an ultra-faint stellar system identified in the second data release of the DECam Local Volume Exploration (DELVE) survey. Based on a maximum-likelihood fit to its structure and stellar population, we find that DELVE 6 is an old ( τ > 9.8 Gyr at 95% confidence) and metal-poor ([Fe/H] < −1.17 dex at 95% confidence) stellar system with an absolute magnitude of M V = − 1.5 − 0.6 + 0.4 mag and an azimuthally averaged half-light radius of r 1 / 2 = 10 − 3 + 4 pc. These properties are consistent with the population of ultra-faint star clusters uncovered by recent surveys. Interestingly, DELVE 6 is located at an angular separation of ∼10° from the center of the Small Magellanic Cloud (SMC), corresponding to a 3D physical separation of ∼20 kpc given the system’s observed distance ( D ⊙ = 80 kpc). This also places the system ∼35 kpc from the center of the Large Magellanic Cloud (LMC), lying within recent constraints on the size of the LMC’s dark matter halo. We tentatively measure the proper motion of DELVE 6 using data from Gaia, which we find supports a potential association between the system and the LMC/SMC. Although future kinematic measurements will be necessary to determine its origins, we highlight that DELVE 6 may represent only the second or third ancient ( τ > 9 Gyr) star cluster associated with the SMC, or one of fewer than two dozen ancient clusters associated with the LMC. Nonetheless, we cannot currently rule out the possibility that the system is a distant Milky Way halo star cluster. 

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