Fast radio burst (FRB) dispersion measures (DMs) are powerful tracers of the low density universe, but interpretation is made difficult by the integrated nature of DM. In this paper, we analyze the new ENGAWA cosmological zoom-in simulation suite to constrain the DM contribution from the Milky Way (MW) halo. The ENGAWA simulations consist of four Milky Way-like galaxies with enhanced resolution in the circumgalactic medium, making them an excellent tool to probe the properties of the gaseous halo. The median all-sky DM from the galactic halos in ENGAWA span 19-39 pc cm^-3, varying even at fixed feedback strength, halo mass, and f_CGM. These simulations show that, with enhanced circumgalactic resolution, the mean halo DM values are in general lower with more anisotropy across the sky than previous models, reaching < 10 pc cm^-3 towards the poles. Furthermore, by varying the solar position within the simulated galaxies, we have obtained an estimate of the uncertainty in DM given our location in the MW. We provide a Python package with MW halo DM values from the simulation accessible via API functions of Galactic longitude and latitude. These new, simulation-based MW halo DM estimates will provide a critical baseline for interpretations of future FRB observations and our understanding of the global gas distribution in the Universe.

Situated on opposite sides of the Large Magellanic Cloud (LMC), the star-forming regions N11 and 30 Doradus trace different evolutionary stages. Approximately 2 Myr more evolved, N11 serves as a proxy for later evolutionary stages of 30 Doradus. We study the (1) kinematics, (2) column density distribution, (3) ionization conditions, and (4) mass, mass outflow rate, and mass loading factor of the outflows from N11 and compare them with the wind from 30 Doradus. We probe N11 along 24 “down-the-barrel” sight lines using UV absorption-line observations from the Hubble Space Telescope Ultraviolet Legacy Library of Young Stars as Essential Standards survey and H i 21 cm emission-line observations from the Galactic All Sky Survey and Galactic Australian Square Kilometre Array Pathfinder. N11 predominantly generates a slow-moving wind that reaches speeds of ∼90 km s ^−1 beyond the LMC’s disk. At v _LSR ≤ +175 km s ^−1 , the origin of the gas clouds becomes ambiguous; they could be associated with the LMC and/or the Milky Way. The fast- and slow-moving absorbers have medium (4 km s ^–1 ≲ b ≲ 10 km s ^−1 ) to broad ( b ≳ 15.0 km s ^−1 ) widths, indicating the influence of nonthermal processes. We estimate that the outflows are ∼25%–69% ionized. Assuming that these winds are symmetric about both sides of the LMC, N11 has ejected M _baryon,sym = (3.7 ± 1.5) × 10 ^5 M _⊙ of gas at a rate of 0.02–0.07 M _⊙ yr ^−1 . The N11 stellar activity drives this material out at an efficiency—described by the mass-loading factor—of ${\eta }_{\mathrm{sym}}=2.{2}_{-1.3}^{+0.9}$ . These winds correspond to 2%–4% of the global LMC outflow for the low-ionization species.

We use constrained idealized simulations of the Large Magellanic Cloud (LMC)/Milky Way interaction to determine if the size of the LMC's gaseous halo (Corona) can be used to distinguish between first and second passage models-an orbital trajectory for the LMC in which it has just recently approached the Milky Way for the first time (first passage) or one in which it has had a previous pericenter (second passage). Using live circumgalactic gas particles combined with analytic dark matter potentials evolved to follow previously published orbital trajectories, we find that the first passage model is able to reproduce the observed velocity profile and column density profile of the present day LMC Corona. On the other hand, in a second passage scenario, the longer interaction time leads to the velocities and column densities around the LMC at the present day being significantly lower than observations. Based on this observed velocity profile, recent works have found that the LMC's Corona has been truncated to 17-20 kpc, and we find truncation radii of 16.6 +/- 0.5 kpc and 5.7-2.2+1.8 kpc for the first and second passage models, respectively. Thus, based on the gas properties of the LMC's circumgalactic medium at the present day, a second passage trajectory is strongly disfavored.

Simulating the small-scale features and dynamics of the circumgalactic medium (CGM) is computationally challenging due to its large volume, low densities, multiphase structure, and chaotic environmental effects. Traditional mass-based refinement schemes focus computational power on the high-density regions, thus alternative techniques are required to study the details of the CGM. In this paper, we introduce a new suite of four cosmological zoom-in simulations of Milky Way-like galaxies in which we include fixed-volume refinement throughout the CGM combined with the IllustrisTNG stellar and AGN feedback model down to redshift zero. Reaching spatial resolutions of 200 pc, we see enhancements in low ion column densities (H I and Mg II) and the number of cold clouds around galaxies, relieving some of the longstanding tensions between simulations and observations of the CGM. We additionally apply the COLT radiative transfer code in post-processing to account for stellar radiation, providing a more realistic gauge of ion populations. We find a reduction in the H I with minimal impact to the Mg II and O VI, tempering the impact of resolution while still providing results consistent with observations. In addition to the increase in the number of cold clouds in the CGM, we find that their intermediate temperature boundary regions are reduced in size as the resolution is increased, leading to smoother transitions to the ambient CGM temperature. This paper outlines initial results from this fixed-volume simulation suite which will serve as a basis for future explorations of CGM dynamics, gas accretion, and galaxy evolution.

We identify and investigate a preinfall analog of the Large and Small Magellanic Clouds (LMCs, SMCs) in the High-Resolution Environmental Simulations of the Immediate Area suite of constrained cosmological simulations. The system, dynamically isolated from the Local Group, evolves over ∼6 Gyr and forms a multiphase warm coronal halo and a neutral gas stream via repeated tidal interactions, ∼150 kpc in length. The LMC analog’s corona forms self-consistently through virial accretion and inhibits the survival of clumpy neutral structures beyond ∼600 Myr. The SMC analog remains bound through to z = 0, and the pair also exhibits bridge-like and leading-arm features. These results suggest that while most of the ionized stream is formed by the LMC coronal gas, the neutral gas stream, bridge, and leading arm components of the Magellanic System can arise from dwarf–dwarf interactions prior to infall, while the survival and ionization of these features likely require additional environmental processing. Furthermore, we identify a stellar component out of phase to the neutral component of the stream, implying that if the Magellanic stellar stream exists, it may not be spatially coexistent to the dominant H i stream. This system offers a valuable preinfall reference point for interpreting the Magellanic System and identifying analogs beyond the Local Group.

Recent discoveries have shown that a population of hypervelocity stars (HVSs) originates from the Large Magellanic Cloud (LMC). We use three such HVSs as dynamical tracers to constrain the past orbit of the LMC. Since each star was ejected at a finite time in the past, it must intersect the past position of the LMC’s central black hole at its ejection time. We model the LMC’s orbit under the influence of dynamical friction and extended mass distributions for both the LMC and the Milky Way, generating a large ensemble of orbital realizations. By evaluating which orbits intersect the back-integrated HVS trajectories, we compute posterior distributions over the LMC’s orbital history. This approach provides significantly tighter constraints on the past motion of the LMC than previously possible. We find two previously published orbital models that are consistent with these new constraints: a first-passage trajectory from a self-consistent hydrodynamic simulation, and a second-passage trajectory from a collisionless N -body simulation. In parallel, we infer the present-day ejection site of the HVSs—likely tracing the LMC’s dynamical center and supermassive black hole—independent of conventional methods.

The Large Magellanic Cloud (LMC) has experienced disruption from tidal and ram-pressure forces as it travels through the halo of the Milky Way. In this project, we combine radio emission-line observations from the Galactic All Sky Survey and Galactic Australian Square Kilometre Array Pathfinder surveys with UV absorption-line observations from the Hubble Space Telescope Ultraviolet Legacy Library of Young Stars as Essential Standards (ULLYSES) program to trace the material in front of the LMC. Along our eight stellar sight lines near 30 Doradus, we observe gaseous structures likely associated with two arm-like features flowing in and around the LMC's disk. We detect the nearside gas in neutral, low-, and medium-ionization species. The lower-ionization species likely undergo both thermal and nonthermal broadening while the moderately ionized phase is influenced by more nonthermal processes. The total integrated column density of Al iii decreases with increasing angular offset from 30 Doradus, with sight lines within Delta theta less than or similar to 0 .degrees 25 containing more moderately ionized gas. We demonstrate from a Gaussian decomposition technique on the H i emission that both arms likely trace an additional similar to 1.degrees 0 in Galactic longitude toward the 30 Doradus region than previously predicted. We constrain the orientation of the arms by suggesting that they likely converge around (l,b)=(280.degrees 5,-31.degrees 2) and at least partially cross in front of the LMC. Our observations are consistent with two competing origins of the arms: (1) outflowing material is swept back by tidal and ram-pressure forces or (2) tidally stripped inflows fuel the ongoing stellar activity inside the LMC. Future studies are needed to distinguish between these scenarios.

Classical bulges and stellar bars are common features in disk galaxies and serve as key tracers of galactic evolution. Angular momentum exchange at bar resonances drives secular morphological changes throughout the disk, including bar slowing and lengthening, and affects the structure of accompanying bulges. In this study, using a suite of N-body simulations, we quantify the secular reconfiguration of classical bulges through resonant trapping by evolving stellar bars. We use orbital frequency analysis to identify bar-resonant populations and find that up to 50 in 2:1 resonant orbits and adopt disk-like kinematics. This transformation renders much of the classical bulge observationally indistinguishable from the disk. We compare these results with a sample of 210 MaNGA disk galaxies, finding that slow bars–indicative of older systems–are preferentially associated with weaker bulges. These results suggest that long-lived bars can significantly reshape classical bulges, potentially explaining their scarcity in the local universe and the low classical bulge fraction found in the Milky Way.

Galactic high-velocity clouds (HVCs) are known to be complex, multiphase systems consisting of neutral and/or ionized gas moving at high velocities relative to the rotation of the disk. In this work, we investigate Milky Way-like galaxies from the TNG50 simulation to characterize the properties, morphology, and accretion rates of the warm and hot ionized material comoving with neutral HVCs visible in H i. We find that the ionized gas forms an envelope around the neutral material, and in most cases (73% of the HVCs), it is prolate in morphology. We also find that the ionized mass is similar to 6 times greater than the neutral mass, which leads to significantly more accretion being possible from the ionized gas ( Mion ) than the neutral gas ( Mneut ), consistent with estimates made from observations of our own Galaxy. We investigate the accretion rates from both phases of HVCs around 47 Milky Way-like galaxies, finding that Mion scales with Mneut and both scale with the star formation rate of the galaxy. Finally, we find that, on average, Mion could account for 81% of the galactic star formation rate (assuming the material can sufficiently cool and condense), while Mneut can only balance 11%. Thus, the diffuse, ionized, high-velocity circumgalactic medium plays a defining role in the evolution and growth of galaxies at low redshift.

Widespread galactic winds emanate from the Large Magellanic Cloud (LMC), with the 30 Doradus starburst region generating the fastest and most concentrated gas flows. We report on the gas distribution, kinematics, and ionization conditions of the near-side outflow along 8 down-the-barrel sightlines using UV absorption-line observations from the HST's ULLYSES program for this region along with H I 21-cm observations from the GASS and GASKAP surveys. We find that within 1.7 degrees from the center of 30 Doradus, the wind reaches maximum speeds of $100-150\,\text{km}\,\text{s}^{-1}$ from the LMC's disk. The total integrated column densities of low-ions (O I, Si II, and Fe II) in the blueshifted wind, up to $v_{\rm LSR}=150\,\text{km}\,\text{s}^{-1}$, are highest near the center and decline radially outward. We estimate an outflow mass of $M_{\rm outflow,\,Si II}\approx(5.7-8.6)\,\times 10^{5} M_{\odot}$, outflow rate of $\dot{M}_{\rm outflow}\gtrsim0.02 M_{\odot}\,\text{yr}^{-1}$, and mass loading factor of $\eta\gtrsim0.10$ within 0.52 degrees from the center of 30 Doradus. The observed ion ratios$-$together with photoionization modeling$-$reveal that this wind is roughly $40-97\%$ photoionized. The metallicities and dust depletion patterns of the high-velocity absorbers at $v_{\rm LSR}\approx+120\,\text{km}\,\text{s}^{-1}$ can be explained by either a foreground Milky Way (MW) halo cloud or an outflow from the LMC. For the high-ions, Si IV and C IV are broader and kinematically offset from the low-ions, suggesting turbulent mixing layers (TMLs) existing in the wind. Finally, our hydrodynamical simulations of the Magellanic Clouds (MCs) and MW system suggest that the Magellanic Corona can protect the LMC winds from the ram-pressure forces exerted by the MW's halo.

Hypervelocity stars (HVSs) are produced by the Hills mechanism when a stellar binary is disrupted by a supermassive black hole (SMBH). The HVS Survey detected 21 unbound B-type main-sequence stars in the Milky Way’s outer halo that are consistent with ejection via the Hills mechanism. We revisit the trajectories of these stars in light of proper motions from Gaia DR3 and modern constraints on the Milky Way–Large Magellanic Cloud (LMC) orbit. We find that half of the unbound HVSs discovered by the HVS Survey trace back not to the Galactic Center but to the LMC. Motivated by this finding, we construct a forward model for HVSs ejected from an SMBH in the LMC and observed through the selection function of the HVS Survey. The predicted spatial and kinematic distributions of the simulated HVSs are remarkably similar to the observed distributions. In particular, we reproduce the conspicuous angular clustering of HVSs around the constellation Leo. This clustering occurs because HVSs from the LMC are boosted by ∼300 km s −1 by the orbital motion of the LMC, and stars launched parallel to this motion are preferentially selected as HVS candidates. We find that the birth rate and clustering of LMC HVSs cannot be explained by supernova runaways or dynamical ejection scenarios not involving an SMBH. From the ejection velocities and relative number of Magellanic versus Galactic HVSs, we constrain the mass of the LMC SMBH to be 1 0 5 . 8 − 0.4 + 0.2 M ⊙ (≃6 × 10 5 M ⊙ ).

The Magellanic Stream is a large tail of neutral and ionized gas originating from tidal and hydrodynamical interactions between the Magellanic Clouds as they orbit the Milky Way (MW). It carries a significant gas reservoir that could impact the future evolution of the MW. Despite its importance, no direct observational constraints on the Stream’s distance have been previously published. In this study, we analyze Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph spectra of five blue horizontal branch stars in the MW halo located at distances ranging from 13 to 56 kpc near two regions of the Stream (near Stream longitudes of –79 ^∘ and –98 ^∘ ), with the aim of detecting Ca ii and Na i absorption. No Ca ii or Na i absorption is detected at Stream velocities in any of the individual spectra or in higher signal-to-noise stacks of the spectra. The resulting limits on the Ca ii absorption are significantly lower than the Ca ii columns measured in the Stream along extragalactic sight lines. These nondetections establish a firm lower distance limit of 42 kpc for one region of the Stream. For the other region, we set a firm lower limit of 20 kpc and a tentative lower limit of 55 kpc from the most distant star, but deeper spectra are needed to confirm this. Our results provide the first observational constraints on the gaseous Stream’s distance.

We characterize the Magellanic Corona, the warm gaseous halo around the Large Magellanic Cloud (LMC). The Corona is a key ingredient in the formation of the Magellanic Stream and has recently been observed in high-ion absorption around the LMC. In this work, we present a suite of high-resolution hydrodynamical simulations to constrain its total mass and temperature prior to the infall of the Magellanic Clouds to our Galaxy. We find that the LMC is able to host a stable Corona before and during its approach to the MW through to the present day. With a Magellanic Corona of >2 × 10 ^9 M _⊙ at 3 × 10 ^5 K, our simulations can reproduce the observed total mass of the neutral and ionized components of the Trailing Stream, the size of the LMC disk, the ionization fractions along the Stream, the morphology of the neutral gas, and the on-sky extent of the ionized gas. The Corona plays an integral role in the survival, morphology, and composition of the Magellanic Clouds and the Trailing Stream.

The formation of the Magellanic Stream has puzzled astronomers for decades. In this review, we outline the history of our understanding of the Magellanic System highlighting key observations that have revolutionized thinking of its evolution. We also walk through the major models and theoretical advances that have led to our current paradigm - (1) the LMC and SMC have just had their first pericentric passage around the Milky Way, having approached recently as a bound pair; (2) the LMC and SMC have had several tidal interactions in which material has been stripped out into the Trailing Stream and Leading Arm; (3) the LMC hosted an ionized gas circumgalactic medium which envelops the Clouds and the neutral Stream today, providing the bulk of the associated mass; and (4) the MW's circumgalactic gas provides strong ram pressure and hydrodynamic forces to shape the morphology of the Magellanic System including the formation of a bow shock due to the LMC's supersonic approach.

The distribution of moving groups in the solar neighborhood has been used to constrain dynamical properties of the Milky Way for decades. The kinematic bimodality between the main mode (Hyades, Pleiades, Coma Berenices, and Sirius) and Hercules can be explained by two different bar models – via the outer Lindblad resonance of a bar with a high pattern speed (∼55 km s^-1 kpc^-1), or via the corotation resonance of a bar with a low pattern speed (∼40 km s^-1 kpc^-1). Recent works directly studying the kinematics of bar stars and gas flows near the center of the Galaxy have converged on the low pattern speed model. In this paper, we independently confirm this result by using Gaia DR3 to directly study the variation of Hercules across Galactic azimuth. We find that Hercules increases in V_ϕ and becomes stronger as we move towards the minor axis of the bar, and decreases in V_ϕ and becomes weaker as we move towards the major axis of the bar. This is in direct agreement with theoretical predictions of a low pattern speed bar model in which Hercules is formed by the corotation resonance with stars orbiting the bar's L4/L5 Lagrange points.

The Large Magellanic Cloud (LMC) is the nearest massive galaxy to the Milky Way. Its circumgalactic medium is complex and multi-phase, containing both stripped HI structures like the Magellanic Stream and Bridge, and a diffuse warm corona seen in high-ion absorption. We analyze 28 AGN sightlines passing within 35 kpc of the LMC with archival HST/COS spectra to characterize the cool (T\approx10^4$ K) gas in the LMC CGM, via new measurements of UV absorption in six low ions (OI, FeII, SiII, AlII, SII, and NiII) and one intermediate ion (SiIII). We show that a declining column-density profile is present in all seven ions, with the low-ion profiles having a steeper slope than the high-ion profiles in CIV and SiIV reported by Krishnarao et al. 2022. Crucially, absorption at the LMC systemic velocity is only detected (in all ions) out to 17 kpc. Beyond this distance, the gas has a lower velocity and is associated with the Magellanic Stream. These results demonstrate that the LMC's CGM is composed of two distinct components: a compact inner halo extending to 17 kpc, and a more extended stripped region associated with the Stream. The compactness and truncation of the LMC's inner CGM agree with recent simulations of ram-pressure stripping of the LMC by the Milky Way's extended corona.

The origin of our Galaxy's high-velocity clouds (HVCs) remains a mystery after many decades of effort. In this paper, we use the TNG50 simulation of the IllustrisTNG project to identify cool, dense clouds that match observations of Galactic H i HVCs. We track these clouds back in time to determine their origin. For a TNG50 Milky Way-like galaxy, we find that only 17% of HVCs can be tracked directly to the disk and 21% to material stripped out of satellites. The majority of HVCs (62%) arise from warm and hot circumgalactic gas that cools through thermal instability. They then obtain their anomalous velocities through interactions with the turbulent circumgalactic medium. At TNG50 resolution, we do not see evidence for HVCs forming out of very low metallicity intergalactic material. Instead, low-metallicity HVCs are most likely associated with satellites. These results suggest that Galactic HVCs are highly heterogeneous in their origin and can provide insight into the physical processes that shape the circumgalactic medium, such as disk outflows, satellite accretion, and thermal instabilities.

The kinematic plane of stars near the Sun has proven an indispensable tool for untangling the complexities of the structure of our Milky Way (MW). With ever improving data, numerous kinematic "moving groups" of stars have been better characterized and new ones continue to be discovered. Here we present an improved method for detecting these groups using MGwave, a new open-source 2D wavelet transformation code that we have developed. Our code implements similar techniques to previous wavelet software; however, we include a more robust significance methodology and also allow for the investigation of underdensities which can eventually provide further information about the MW's non-axisymmetric features. Applying MGwave to the latest data release from Gaia (DR3), we detect 47 groups of stars with coherent velocities. We reproduce the majority of the previously detected moving groups in addition to identifying three additional significant candidates: one within Arcturus, and two in regions without much substructure at low V_R. Finally, we have followed these associations of stars beyond the solar neighborhood, from Galactocentric radius of 6.5 to 10 kpc. Most detected groups are extended throughout radius indicating that they are streams of stars possibly due to non-axisymmetric features of the MW.

We characterize the Magellanic Corona model of the formation of the Magellanic Stream, which we introduced in Lucchini et al. (2020, 2021). Using high-resolution hydrodynamic simulations, we constrain the properties of the primordial Magellanic Clouds, including the Magellanic Corona -- the gaseous halo around the Large Magellanic Cloud (LMC). With an LMC mass of $1.75\times10^{11}$ M$_\odot$, a Magellanic Corona of $>5\times10^9$ M$_\odot$ at $3\times10^5$ K, a total Small Magellanic Cloud mass $<10^{10}$ M$_\odot$, and a Milky Way corona of $2\times10^{10}$ M$_\odot$, we can reproduce the observed total mass of the neutral and ionized components of the Trailing Stream, ionization fractions along the Stream, morphology of the neutral gas, and on-sky extent of the ionized gas. The inclusion of advanced physical routines in the simulations allow the first direct comparison of a hydrodynamical model with UV absorption-line spectroscopic data. Our model reproduces O I, O VI, and C IV observations from HST/COS and FUSE. The stripped material is also nearby ($<50$ kpc from the Sun), as found in our prior models including a Magellanic Corona.

The distribution of moving groups in the solar neighborhood has been used to constrain dynamical properties of the Milky Way for decades. The kinematic bimodality between the main mode (Hyades, Pleiades, Coma Berenices, and Sirius) and Hercules can be explained by two different bar models -- via the outer Lindblad resonance of a bar with a high pattern speed ($\sim$55 km s$^{-1}$ kpc$^{-1}$), or via the corotation resonance of a bar with a low pattern speed ($\sim$40 km s$^{-1}$ kpc$^{-1}$). Recent works directly studying the kinematics of bar stars and gas flows near the center of the Galaxy have converged on the low pattern speed model. In this paper, we independently confirm this result by using Gaia DR3 to directly study the variation of Hercules across Galactic azimuth. We find that Hercules increases in $V_\phi$ and becomes stronger as we move towards the minor axis of the bar, and decreases in $V_\phi$ and becomes weaker as we move towards the major axis of the bar. This is in direct agreement with theoretical predictions of a low pattern speed bar model in which Hercules is formed by the corotation resonance with stars orbiting the bar's L4/L5 Lagrange points.