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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., Comment: Accepted for publication in ApJL

We present the Deciphering the Interplay between the Interstellar medium, Stars, and the Circumgalactic medium (DIISC) Survey. This survey is designed to investigate the correlations in properties between the circumgalactic medium (CGM), the interstellar medium (ISM), stellar distributions, and young star-forming regions. The galaxies were chosen to have a QSO sightline within 3.5 times the HI radii probing the disk-CGM interface. The sample contains 34 low-redshift galaxies with a median stellar mass of 10$^{10.45}~\rm M_{\odot}$ probed at a median impact parameter of $\rho=55~kpc$. The survey combines ultraviolet spectroscopic data from the Cosmic Origins Spectrograph aboard the Hubble Space Telescope with HI 21 cm hyperfine transition imaging with the Very Large Array (VLA), ultraviolet imaging from Galaxy Evolution Explorer (GALEX), and optical imaging and spectroscopy with the MMT and Vatican Advanced Technology Telescope. We describe the specific goals of the survey, data reduction, high-level data products, and some early results. We present the discovery of a strong inverse correlation, at a confidence level of 99.99%, between Lyman $\alpha$ equivalent width, $\rm W_{Ly\alpha}$, and impact parameter normalized by the HI radius ($\rho/R_{HI}$). We find $\rho/R_{HI}$ to be a better empirical predictor of Lyman $\alpha$ equivalent width than virial radius normalized impact parameter ($\rho/R_{vir}$) or parameterizations combining $\rho,~R_{vir}$, stellar mass, and star formation rate. We conclude that the strong anticorrelation between the Lyman $\alpha$ equivalent width and $\rho/R_{HI}$ indicates that the neutral gas distribution of the CGM is more closely connected to the galaxy's gas disk rather than its stellar and dark matter content., Comment: 29 pages, 12 figures, 4 tables Submitted to AAS Journals

The Koopman operator enables the analysis of nonlinear dynamical systems through a linear perspective by describing time evolution in the infinite-dimensional space of observables. Here this formalism is applied to shear flows, specifically the minimal flow unit plane Couette flow and a nine-dimensional model of turbulent shear flow between infinite parallel free-slip walls under a sinusoidal body force (Moehlis et al., New J. Phys. 6, 56 (2004)). We identify a finite set of observables and approximate the Koopman operator using neural networks, following the method developed by Constante-Amores et al., Chaos, 34(4), 043119 (2024). Then the time evolution is determined with a method here denoted as "Projected Koopman Dynamics". Using a high-dimensional approximate Koopman operator directly for the plane Couette system is computationally infeasible due to the high state space dimension of direct numerical simulations (DNS). However, the long-term dynamics of the dissipative Navier-Stokes equations are expected to live on a lower-dimensional invariant manifold. Motivated by this fact, we perform dimension reduction on the DNS data with a variant of the IRMAE-WD (implicit rank-minimizing autoencoder-weight decay) autoencoder (Zeng et al., MLST 5(2), 025053 (2024)) before applying the Koopman approach. We compare the performance of data-driven state space (neural ordinary differential equation) and Projected Koopman Dynamics approaches in manifold coordinates for plane Couette flow. Projected Koopman Dynamics effectively captures the chaotic behavior of Couette flow and the nine-dimensional model, excelling in both short-term tracking and long-term statistical analysis, outperforming other frameworks applied to these shear flows.

The dynamics of flexible filaments entrained in flow, important for understanding many biological and industrial processes, are computationally expensive to model with full-physics simulations. This work describes a data-driven technique to create high-fidelity low-dimensional models of flexible fiber dynamics using machine learning; the technique is applied to sedimentation in a quiescent, viscous Newtonian fluid, using results from detailed simulations as the data set. The approach combines an autoencoder neural network architecture to learn a low-dimensional latent representation of the filament shape, with a neural ODE that learns the evolution of the particle in the latent state. The model was designed to model filaments of varying flexibility, characterized by an elasto-gravitational number $\mathcal{B}$, and was trained on a data set containing the evolution of fibers beginning at set angles of inclination. For the range of $\mathcal{B}$ considered here (100-10000), the filament shape dynamics can be represented with high accuracy with only four degrees of freedom, in contrast to the 93 present in the original bead-spring model used to generate the dynamic trajectories. We predict the evolution of fibers set at arbitrary angles and demonstrate that our data-driven model can accurately forecast the evolution of a fiber at both trained and untrained elasto-gravitational numbers.

We analyze the \ovi\ content and kinematics for 126 {\hi}-selected absorbers at $0.14 \lesssim z \lesssim 0.73$ for which the metallicities of their cool photoionized phase have been determined. We separate the absorbers into 100 strong {\lya} forest systems (SLFSs with $15 \la$\,{\colden}\,$< 16.2$) and 26 partial Lyman Limit systems (pLLSs with $16.2\le$\,{\colden}\,$\le 17.2$). The sample is drawn from the COS CGM Compendium (CCC) and has \ovi\ coverage in $\sn \geq 8$ {\it HST}/COS G130M/G160M QSO spectra, yielding a $2\sigma$ completeness level of {\coldenovi}$\,\geq 13.6$. The \ovi\ detection rates differ substantially between low-metallicity (LM; {\xh} $\leq -1.4$) and high-metallicity (HM; {\xh} $> -1.4$) SLFSs, with 20\% and 60\% detection rates, respectively. The \ovi\ detection frequency for the HM and LM pLLSs is, however, similar at $\sim$60\%. The SLFSs and pLLSs without detected \ovi\ are consistent with the absorbing gas being in a single phase, while those with \ovi\ trace multiphase gas. We show that the \ovi\ velocity widths and column densities have different distributions in LM and HM gas. We find a strong correlation between \ovi\ column density and metallicity. The strongest (\coldenovi$\,\ga 14$) and broadest {\ovi} absorbers are nearly always associated with HM absorbers, while weaker \ovi\ absorbers are found in both LM and HM absorbers. From comparisons with galaxy-selected and blind \ovi\ surveys, we conclude absorbers with \coldenovi$\,\ga 14$ most likely arise in the circumgalactic medium (CGM) of star-forming galaxies. Absorbers with weak \ovi\ likely trace the extended CGM or intergalactic medium (IGM), while those without \ovi\ likely originate in the IGM., Comment: Accepted for publication in ApJ

We analyze the far-ultraviolet spectroscopy of 20 young and massive star clusters (YSCs) in 11 nearby star-forming galaxies. We probe the interstellar gas intervening along the line of sight, detecting several metal absorption lines of a wide range of ionization potentials, from 6.0 eV to 77.5 eV. Multiple-component Voigt fits to the absorption lines are used to study the kinematics of the gas. We find that nearly all targets in the sample feature gas outflowing from 30 up to 190 km per second, often both in the neutral and ionized phase. The outflow velocities correlate with the underlying stellar population properties directly linked to the feedback: the mass of the YSCs, the photon production rate and the instantaneous mechanical luminosity produced by stellar winds and SNe. We detect a neutral inflow in 4 targets, which we interpret as likely not associated with the star cluster but tracing larger scale gas kinematics. A comparison between the outflows energy and that produced by the associated young stellar populations suggests an average coupling efficiency of 10 per cent with a broad scatter. Our results extend the relation found in previous works between galactic outflows and the host galaxy star-formation rate to smaller scales, pointing towards the key role that clustered star formation and feedback play in regulating galaxy growth., Comment: Accepted for publication on the Astronomical Journal on 14th February 2024. 32 pages, 13 figures

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 (Lucchini et al. 2020, 2021) 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 $>2\times10^9$ M$_\odot$ at $3\times10^5$ K, our simulations can reproduce the observed total mass of the neutral and ionized components of the Trailing Stream, size of the LMC disk, ionization fractions along the Stream, morphology of the neutral gas, and 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., Comment: 12 pages, 7 figures, 2 tables. Accepted to ApJ

High-velocity clouds (HVCs) are multi-phase gas structures whose velocities (|v_LSR|>100 km/s) are too high to be explained by Galactic disk rotation. While large HVCs are well characterized, compact and small HVCs (with HI angular sizes of a few degrees) are poorly understood. Possible origins for such small clouds include Milky Way halo gas or fragments of the Magellanic System, but neither their origin nor their connection to the Milky Way halo has been confirmed. We use new Hubble Space Telescope/Cosmic Origins Spectrograph UV spectra and Green Bank Telescope HI spectra to measure the metallicities of five small HVCs in the southern Galactic sky projected near the Magellanic System. We build a set of distance-dependent Cloudy photoionization models for each cloud and calculate their ionization-corrected metallicities. All five small HVCs have oxygen metallicities <0.17 Z_sun, indicating they do not originate in the disk of the Milky Way. Two of the five have metallicities of 0.16-0.17 Z_sun, similar to the Magellanic Stream, suggesting these clouds are fragments of the Magellanic System. The remaining three clouds have much lower metallicities of 0.02-0.04 Z_sun. While the origin of these low-metallicity clouds is unclear, they could be gaseous mini-halos or gas stripped from dwarf galaxies by ram pressure or tidal interactions. These results suggest that small HVCs do not all reside in the inner Milky Way halo or the Magellanic System, but instead can trace more distant structures., Comment: 25 pages, accepted for publication in ApJ

We explore the growth of the stellar disks in 14 nearby spiral galaxies as part of the Deciphering the Interplay between the Interstellar medium, Stars, and the Circumgalactic medium (DIISC) survey. We study the radial distribution of specific star formation rates (sSFR) and investigate the ratio of the difference in the outer and inner sSFR ($\Delta_{sSFR}~={\rm sSFR}_{out}-{\rm sSFR}_{in}$) of the disk and the total sSFR, $\Delta_{sSFR}$/sSFR to quantify disk growth. We find $\Delta_{sSFR}$/sSFR and the HI gas fraction to show a mild correlation of Spearman's $\rho=0.30$, indicating that star formation and disk growth are likely to proceed outward in galactic disks with high HI gas fractions. The HI gas fractions and $\Delta_{sSFR}$/sSFR of the galaxies also increase with the distance to the nearest L$_\star$ neighbor, suggesting that galaxies are likely to sustain their ISM cold gas and exhibit inside-out growth in isolated environments. However, the HI content in their circumgalactic medium, probed by the Ly$\alpha$ equivalent width (W$_{Ly\alpha}$) excess, is observed to be suppressed in isolated environments, apparent from the strong anti-correlation between the W$_{Ly\alpha}$ excess and the distance to the 5$^{\rm th}$ nearest L$_\star$ neighbor (Spearman's $\rho=-0.62$). As expected, W$_{Ly\alpha}$ is also found to be suppressed in cluster galaxies. We find no relation between the W$_{Ly\alpha}$ excess of the detected CGM absorber and $\Delta_{sSFR}$/sSFR implying that the enhancement and suppression of the circumgalactic HI gas does not affect the direction in which star formation proceeds in a galactic disk or vice-versa., Comment: Accepted for publication in ApJ

Understanding how galaxies interact with the circumgalactic medium (CGM) requires determining how galaxies morphological and stellar properties correlate with their CGM properties. We report an analysis of 66 well-imaged galaxies detected in HST and VLT MUSE observations and determined to be within $\pm$500 km s$^{-1}$ of the redshifts of strong intervening quasar absorbers at $0.2 \lesssim z \lesssim 1.4$ with H I column densities $N_{\rm H I}$ $>$ $10^{18}$ $\rm cm^{-2}$. We present the geometrical properties (S\'ersic indices, effective radii, axis ratios, and position angles) of these galaxies determined using GALFIT. Using these properties along with star formation rates (SFRs, estimated using the H$\alpha$ or [O II] luminosity) and stellar masses ($M_{*}$ estimated from spectral energy distribution fits), we examine correlations among various stellar and CGM properties. Our main findings are as follows: (1) SFR correlates well with $M_{*}$, and most absorption-selected galaxies are consistent with the star formation main sequence (SFMS) of the global population. (2) More massive absorber counterparts are more centrally concentrated and are larger in size. (3) Galaxy sizes and normalized impact parameters correlate negatively with $N_{\rm H I}$, consistent with higher $N_{\rm H I}$ absorption arising in smaller galaxies, and closer to galaxy centers. (4) Absorption and emission metallicities correlate with $M_{*}$ and sSFR, implying metal-poor absorbers arise in galaxies with low past star formation and faster current gas consumption rates. (5) SFR surface densities of absorption-selected galaxies are higher than predicted by the Kennicutt-Schmidt relation for local galaxies, suggesting a higher star formation efficiency in the absorption-selected galaxies., Comment: Accepted for publication in MNRAS, 25 pages, 19 figures

The Galactic Center (GC) is surrounded by plasma lobes that extend up to ~14 kpc above and below the plane. Until now, UV absorption studies of these lobes have only focused on high-velocity components (|v_LSR| > 100 km/s) because low- and intermediate-velocity (LIV) components (|v_LSR| <100 km/s) are blended with foreground interstellar medium. To overcome this difficulty, we present a differential experiment to compare the LIV absorption between different structures within the GC region, including the Fermi Bubbles (FBs; seen in gamma-rays), the eROSITA Bubbles (eBs; seen in X-rays), and the Loop I North Polar Spur (LNPS) association, an X-ray and radio feature within the northern eB. We use far-UV spectra from Hubble Space Telescope to measure LIV Si IV absorption in 61 AGN sight lines, of which 21 pass through the FBs, 53 pass through the eBs, and 18 pass through the LNPS. We also compare our measurements to those in the literature from sight lines covering the disk-halo interface and CGM. We find that the FBs and eBs have enhancements in measured columns of 0.22-0.29 dex in log. We also remove the contribution of a modeled disk and CGM component from the measured Si IV columns and find that the northern eB still retains a Si IV enhancement of 0.62 dex in log. A similar enhancement is not seen in the southern eB. Since the LNPS model-subtracted residuals show an enhancement compared to the rest of the northern eB of 0.69 dex, the northern eB enhancement may be caused by the LNPS., Comment: Accepted for publication in ApJ

The flow of gas into and out of galaxies leaves traces in the circumgalactic medium which can then be studied using absorption lines towards background quasars. We analyse 27 log(N_HI) > 18.0 HI absorbers at z = 0.2 to 1.4 from the MUSE-ALMA Halos survey with at least one galaxy counterpart within a line of sight velocity of +/-500 km s^{-1}. We perform 3D kinematic forward modelling of these associated galaxies to examine the flow of dense, neutral gas in the circumgalactic medium. From the VLT/MUSE, HST broadband imaging and VLT/UVES and Keck/HIRES high-resolution UV quasar spectroscopy observations, we compare the impact parameters, star-formation rates and stellar masses of the associated galaxies with the absorber properties. We find marginal evidence for a bimodal distribution in azimuthal angles for strong HI absorbers, similar to previous studies of the MgII and OVI absorption lines. There is no clear metallicity dependence on azimuthal angle and we suggest a larger sample of absorbers are required to fully test the relationship predicted by cosmological hydrodynamical simulations. A case-by-case study of the absorbers reveals that ten per cent of absorbers are consistent with gas accretion, up to 30 per cent trace outflows while the remainder trace gas in the galaxy disk, the intragroup medium and low-mass galaxies below the MUSE detection limit. Our results highlight that the baryon cycle directly affects the dense neutral gas required for star-formation and plays a critical role in galaxy evolution., Comment: 13 pages, 6 figures, 12 pages of appendix. Accepted for publication in MNRAS

While many phenomena in physics and engineering are formally high-dimensional, their long-time dynamics often live on a lower-dimensional manifold. The present work introduces an autoencoder framework that combines implicit regularization with internal linear layers and $L_2$ regularization (weight decay) to automatically estimate the underlying dimensionality of a data set, produce an orthogonal manifold coordinate system, and provide the mapping functions between the ambient space and manifold space, allowing for out-of-sample projections. We validate our framework's ability to estimate the manifold dimension for a series of datasets from dynamical systems of varying complexities and compare to other state-of-the-art estimators. We analyze the training dynamics of the network to glean insight into the mechanism of low-rank learning and find that collectively each of the implicit regularizing layers compound the low-rank representation and even self-correct during training. Analysis of gradient descent dynamics for this architecture in the linear case reveals the role of the internal linear layers in leading to faster decay of a "collective weight variable" incorporating all layers, and the role of weight decay in breaking degeneracies and thus driving convergence along directions in which no decay would occur in its absence. We show that this framework can be naturally extended for applications of state-space modeling and forecasting by generating a data-driven dynamic model of a spatiotemporally chaotic partial differential equation using only the manifold coordinates. Finally, we demonstrate that our framework is robust to hyperparameter choices.

We present the detection of dust depletion in Complex C, a massive, infalling, low-metallicity high-velocity cloud in the northern Galactic hemisphere that traces the ongoing accretion of gas onto the Milky Way. We analyze a very high signal-to-noise HST/COS spectrum of AGN Mrk 817 formed by coadding 165 individual exposures taken under the AGN STORM 2 program, allowing us to determine dust-depletion patterns in Complex C at unprecedented precision. By fitting Voigt components to the O I, S II, N I, Si II, Fe II, and Al II absorption and applying ionization corrections from customized Cloudy photoionization models, we find sub-solar elemental abundance ratios of [Fe/S]=-0.42+/-0.08, [Si/S]=-0.29+/-0.05, and [Al/S]=-0.53+/-0.08. These ratios indicate the depletion of Fe, Si, and Al into dust grains, since S is mostly undepleted. The detection of dust provides an important constraint on the origin of Complex C, as dust grains indicate the gas has been processed through galaxies, rather than being purely extragalactic. We also derive a low metallicity of Complex C of [S/H]=-0.51+/-0.16 (31% solar), confirming earlier results from this sightline. We discuss origin models that could explain the presence of dust in Complex C, including Galactic fountain models, tidal stripping from the Magellanic Clouds or other satellite galaxies, and precipitation of coronal gas onto dust-bearing ``seed" clouds., Comment: 8 pages, 3 figures, accepted for publication in ApJ Letters. This version has been updated with proof corrections

Dynamical systems with extreme events are difficult to capture with data-driven modeling, due to the relative scarcity of data within extreme events compared to the typical dynamics of the system, and the strong dependence of the long-time occurrence of extreme events on short-time conditions.A recently developed technique [Floryan, D. & Graham, M. D. Data-driven discovery of intrinsic dynamics. Nat Mach Intell $\textbf{4}$, 1113-1120 (2022)], here denoted as $\textit{Charts and Atlases for Nonlinear Data-Driven Dynamics on Manifolds}$, or CANDyMan, overcomes these difficulties by decomposing the time series into separate charts based on data similarity, learning dynamical models on each chart via individual time-mapping neural networks, then stitching the charts together to create a single atlas to yield a global dynamical model. We apply CANDyMan to a nine-dimensional model of turbulent shear flow between infinite parallel free-slip walls under a sinusoidal body force [Moehlis, J., Faisst, H. & Eckhardt, B. A low-dimensional model for turbulent shear flows. New J Phys $\textbf{6}$, 56 (2004)], which undergoes extreme events in the form of intermittent quasi-laminarization and long-time full laminarization. We demonstrate that the CANDyMan method allows the trained dynamical models to more accurately forecast the evolution of the model coefficients, reducing the error in the predictions as the model evolves forward in time. The technique exhibits more accurate predictions of extreme events, capturing the frequency of quasi-laminarization events and predicting the time until full laminarization more accurately than a single neural network., Comment: 9 pages, 7 figures

The gas cycling in the circumgalactic regions of galaxies is known to be multi-phase. The MUSE-ALMA Haloes survey gathers a large multi-wavelength observational sample of absorption and emission data with the goal to significantly advance our understanding of the physical properties of such CGM gas. A key component of the MUSE-ALMA Haloes survey is the multi-facility observational campaign conducted with VLT/MUSE, ALMA and HST. MUSE-ALMA Haloes targets comprise 19 VLT/MUSE IFS quasar fields, including 32 $z_{\rm abs}<$0.85 strong absorbers with measured N$_{HI}$ $\geq 10^{18}$ cm$^{\rm -2}$ from UV-spectroscopy. We additionally use a new complementary HST medium program to characterise the stellar content of the galaxies through a 40-orbit three-band UVIS and IR WFC3 imaging. Beyond the absorber-selected targets, we detect 3658 sources all fields combined, including 703 objects with spectroscopic redshifts. This galaxy-selected sample constitutes the main focus of the current paper. We have secured millimeter ALMA observations of some of the fields to probe the molecular gas properties of these objects. Here, we present the overall survey science goals, target selection, observational strategy, data processing and source identification of the full sample. Furthermore, we provide catalogues of magnitude measurements for all objects detected in VLT/MUSE, ALMA and HST broad-band images and associated spectroscopic redshifts derived from VLT/MUSE observations. Together, this data set provides robust characterisation of the neutral atomic gas, molecular gas and stars in the same objects resulting in the baryon census of condensed matter in complex galaxy structures., Comment: 19 pages, 4 figures. This is the final (proof-corrected) version, published in MNRAS. Galaxy catalogues are available online

We characterize the chemical and physical conditions in an outflowing high-velocity cloud in the inner Galaxy. We report a super-solar metallicity of [O/H] = $+0.36\pm0.12$ for the high-velocity cloud at $v_\mathrm{LSR}$ = 125.6 km s$^{-1}$ toward the star HD 156359 ($l$ = 328.$^{\circ}$7, $b$ = $-$14.$^{\circ}$5, $d$ = 9 kpc, $z$ = $-$2.3 kpc). Using archival observations from FUSE, HST STIS, and ESO FEROS we measure high-velocity absorption in H I, O I, C II, N II, Si II, Ca II, Si III, Fe III, C IV, Si IV, N V, and O VI. We measure a low H I column density of log $N$(H I) = $15.54\pm0.05$ in the HVC from multiple unsaturated H I Lyman series lines in the FUSE data. We determine a low dust depletion level in the HVC from the relative strength of silicon, iron, and calcium absorption relative to oxygen, with [Si/O]=$-0.33\pm0.14$, [Fe/O]=$-0.30\pm0.20$, and [Ca/O] =$-0.56\pm0.16$. Analysis of the high-ion absorption using collisional ionization models indicates that the hot plasma is multi-phase, with the C IV and Si IV tracing 10$^{4.9}$ K gas and N V and O VI tracing 10$^{5.4}$ K gas. The cloud's metallicity, dust content, kinematics, and close proximity to the disk are all consistent with a Galactic wind origin. As the HD 156359 line of sight probes the inner Galaxy, the HVC appears to be a young cloud caught in the act of being entrained in a multi-phase Galactic outflow and driven out into the halo., Comment: Submitted to ApJ, 18 pages, 13 figures, comments welcome

The Large and Small Magellanic Clouds (LMC/SMC) are the closest major satellite galaxies of the Milky Way. They are likely on their first passage on an infalling orbit towards our Galaxy (Besla et al. 2007) and trace the ongoing dynamics of the Local Group (D'Onghia & Fox 2016). Recent measurements of a high mass for the LMC (M_halo = 10^(11.1-11.4) solar masses; Penarrubia et al. 2016, Erkal et al. 2018, 2019, Kallivayalil et al. 2018) imply the LMC should host a Magellanic Corona: a collisionally ionized, warm-hot gaseous halo at the virial temperature (10^(5.3-5.5) K) initially extending out to the virial radius (100-130 kpc). Such a Corona would have shaped the formation of the Magellanic Stream (Lucchini et al. 2020), a tidal gas structure extending over 200 degrees across the sky (D'Onghia & Fox 2016, Besla et al. 2012, Nidever et al. 2010) that is bringing in metal poor gas to the Milky Way (Fox et al. 2014). No observational evidence for such an extended Corona has been published previously, with detections of highly ionized gas only reported in directions directly toward the LMC, where winds from the LMC disk may dominate (deBoer & Savage 1980, Wakker et al. 1998). Here we show evidence for this Magellanic Corona with a potential direct detection in highly ionized oxygen (O^+5), and indirectly via triply-ionized carbon and silicon, seen in ultraviolet absorption toward background quasars. We find that the Magellanic Corona is part of a pervasive multiphase Magellanic circumgalactic medium (CGM) seen in many ionization states with a declining projected radial profile out to at least 35 kpc from the LMC and a total ionized CGM mass of log_10(M_HII;CGM/solar masses) = 9.1 +/- 0.2. The evidence for the Magellanic Corona is a crucial step forward in characterizing the Magellanic Group and its nested evolution with the Local Group., Comment: View published, open access version here: https://www.nature.com/articles/s41586-022-05090-5 Main Text: 7 Pages, 4 Figures Methods: 19 Pages, 7 Extended Data Figures, 3 Extended Data Tables

The Galactic Center is surrounded by two giant plasma lobes known as the Fermi Bubbles, extending ~10 kpc both above and below the Galactic plane. Spectroscopic observations of Fermi Bubble directions at radio, ultraviolet, and optical wavelengths have detected multi-phase gas clouds thought to be embedded within the bubbles referred to as Fermi Bubble high-velocity clouds (FB HVCs). While these clouds have kinematics that can be modeled by a biconical nuclear wind launched from the Galactic center, their exact origin is unknown because, until now, there has been little information on their heavy-metal abundance (metallicity). Here we show that FB HVCs have a wide range of metallicities from <20% solar to ~320% solar. This result is based on the first metallicity survey of FB HVCs. These metallicities challenge the previously accepted tenet that all FB HVCs are launched from the Galactic center into the Fermi Bubbles with solar or super-solar metallicities. Instead, we suggest that FB HVCs originate in both the Milky Way's disk and halo. As such, some of these clouds may characterize circumgalactic medium that the Fermi Bubbles expand into, rather than material carried outward by the nuclear wind, changing the canonical picture of FB HVCs. More broadly, these results reveal that nuclear outflows from spiral galaxies can operate by sweeping up gas in their halos while simultaneously removing gas from their disks., Comment: This version of the article has been accepted for publication on Nature Astronomy after peer review. This version is not the Version of Record (https://doi.org/10.1038/s41550-022-01720-0) and does not reflect post-acceptance improvements, or any corrections

The bimodal absorption system imaging campaign (BASIC) aims to characterize the galaxy environments of a sample of 36 HI-selected partial Lyman limit systems (pLLSs) and Lyman limit systems (LLSs) in 23 QSO fields at $z \lesssim 1$. These pLLSs/LLSs provide a unique sample of absorbers with unbiased and well-constrained metallicities, allowing us to explore the origins of metal-rich and low-metallicity circumgalactic medium (CGM) at $z<1$. Here we present Keck/KCWI and VLT/MUSE observations of 11 of these QSO fields (19 pLLSs) that we combine with HST/ACS imaging to identify and characterize the absorber-associated galaxies. We find 23 unique absorber-associated galaxies, with an average of one associated galaxy per absorber. For seven absorbers, all with $<10\%$ solar metallicities, we find no associated galaxies with $\log M_\star \gtrsim 9.0$ within $\rho/R_{vir}$ and $|\Delta v|/v_{esc} \le$ 1.5 with respect to the absorber. We do not find any strong correlations between the metallicities or HI column densities of the gas and most of the galaxy properties, except for the stellar mass of the galaxies: the low-metallicity ([X/H] $\le -1.4$) systems have a probability of $0.39^{+0.16}_{-0.15}$ for having a host galaxy with $\log M_\star \ge 9.0$ within $\rho/R_{vir} \le 1.5$, while the higher metallicity absorbers have a probability of $0.78^{+0.10}_{-0.13}$. This implies metal-enriched pLLSs/LLSs at $z<1$ are typically associated with the CGM of galaxies with $\log M_\star > 9.0$, whereas low-metallicity pLLSs/LLSs are found in more diverse locations, with one population arising in the CGM of galaxies and another more broadly distributed in overdense regions of the universe. Using absorbers not associated with galaxies, we estimate the unweighted geometric mean metallicity of the intergalactic medium to be [X/H] $\lesssim -2.1$ at $z<1$, which is lower than previously estimated., Comment: Key figures: 13 and 18. Modified abstract. Accepted for publication in ApJ. Updated after referee comments, expanded subsection 4.4

Metallicity estimates of circumgalactic gas based on absorption line measurements typically require photoionization modeling to account for unseen ionization states. We explore the impact of uncertainties in the extreme ultraviolet background (EUVB) radiation on such metallicity determinations for the z < 1 circumgalactic medium (CGM). In particular, we study how uncertainties in the power-law slope of the EUV radiation, $\mathrm{\alpha_{EUVB}}$, from active galactic nuclei affect metallicity estimates in a sample of 34 absorbers with HI column densities between 15.25 < log ($\mathrm{N_{HI}}$ / $\mathrm{cm^{-2}}$) < 17.25 and measured metal ion column densities. We demonstrate the sensitivity of metallicity estimates to changes in the EUV power-law slope of active galactic nuclei, $\mathrm{\alpha_{EUVB}}$, at low redshift (z < 1), showing derived absorber metallicities increase on average by approximately 0.3 dex as the EUV slope is hardened from $\mathrm{\alpha_{EUVB}}$ = -2.0 to -1.4. We use Markov Chain Monte Carlo sampling of photoionization models with $\mathrm{\alpha_{EUVB}}$ as a free parameter to derive metallicities for these absorbers. The current sample of absorbers does not provide a robust constraint on the slope, $\mathrm{\alpha_{EUVB}}$, itself; we discuss how future analyses may provide stronger constraints. Marginalizing over the uncertainty in the slope of the background, we find the average uncertainties in the metallicity determinations increase from 0.08 dex to 0.14 dex when switching from a fixed EUVB slope to one that freely varies. Thus, we demonstrate that EUVB uncertainties can be included in ionization models while still allowing for robust metallicity inferences., Comment: 42 figures (34 in appendix), 3 tables. Accepted for publication in The Astronomical Journal

We report the first direct detection of molecular hydrogen associated with the Galactic nuclear wind. The Far-Ultraviolet Spectroscopic Explorer spectrum of LS 4825, a B1 Ib-II star at $l,b$ = 1.67$^{\circ}$,$-$6.63$^{\circ}$ lying $d$ = 9.9$^{+1.4}_{-0.8}$ kpc from the Sun, $\sim$1 kpc below the Galactic plane near the Galactic Center, shows two high-velocity H$_2$ components at $v_\mathrm{LSR}$ = $-79$ and $-108$ km s$^{-1}$. In contrast, the FUSE spectrum of the nearby ($\sim$0.6$^{\circ}$ away) foreground star HD 167402 at $d$=4.9$^{+0.8}_{-0.7}$ kpc reveals no H$_2$ absorption at these velocities. Over 60 lines of H$_2$ from rotational levels $J$ = 0 to 5 are identified in the high-velocity clouds. For the $v_\mathrm{LSR}$ = $-79$ km s$^{-1}$ cloud we measure total log $N$(H$_2$) $\geq$ 16.75 cm$^{-2}$, molecular fraction $f_\mathrm{H_2}$ $\geq$ 0.8%, and $T_{01}$ $\geq$ 97 and $T_{25}$ $\leq$ 439 K for the ground- and excited-state rotational excitation temperatures. At $v_\mathrm{LSR}$ = $-108$ km s$^{-1}$, we measure log $N$(H$_2$) = 16.13 $\pm$ 0.10 cm$^{-2}$, $f_\mathrm{H_2}$ $\geq$ 0.5%, and $T_{01}$ = 77$^{+34}_{-18}$ and $T_{25}$ = 1092$^{+149}_{-117}$ K, for which the excited state ortho- to para-H$_2$ is 1.0$^{+0.3}_{-0.1}$, much less than the equilibrium value of 3 expected for gas at this temperature. This non-equilibrium ratio suggests that the $-108$ km s$^{-1}$ cloud has been recently excited and has not yet had time to equilibrate. As the LS 4825 sight line passes close by a tilted section of the Galactic disk, we propose that we are probing a boundary region where the nuclear wind is removing gas from the disk., Comment: Accepted to ApJ Letters, 11 pages, 4 figures, 1 table