Gaseous satellites orbiting massive host galaxies experience gas stripping and may contribute additional gas to the hosts' circumgalactic medium (CGM). We identify a sample of 21 local spiral galaxies (<15 Mpc) and characterize their gaseous satellite populations from existing HI surveys to investigate the connection between host galaxy CGM and satellite gas content. Most of our spiral hosts have ≤3 gaseous satellites in their halos with M_ HI≳10^7 M_⊙. Using 26 HST/COS QSO sightlines at impact parameters of 0.1–0.9 R_ 200c, we find that the CGM of z∼0 spiral galaxies show large intrinsic scatters (∼1-2 dex) in ion column densities and harbor a total cool gas mass of 2.5^+7.5_ -1.9×10^9 (0.3Z_⊙/Z') M_⊙, largely consistent with their z∼0.2 counterparts (e.g., COS-Halos). Splitting our sample by the presence of gaseous satellites, we find that galaxy hosts with gaseous satellites have higher detection rates (up to 50%) in metal ion absorbers, including OI, AlII, CII, SiII, SiIII, and SiIV, possibly because their CGM is more metal enriched or has more ionized gas in the cool phase. However, the CGM column densities show no significant correlation with either the number of gaseous satellites or their total HI masses, suggesting that the contribution from the gaseous satellites to the host CGM is likely small compared to the intrinsic scatters in the CGM profiles, and the profile trends are mainly influenced by sightlines' proximity to the hosts. The CGM detection rate becomes elevated when a sightline is within half the virial radius of a massive gaseous satellite (LMC-like or higher mass), likely due to stripped debris.

Galaxy environment plays a crucial role in quenching star formation in dwarf galaxies. In Milky Way (MW)-like environments, dwarf satellite quenching is primarily driven by ram pressure stripping (RPS), i.e., the direct removal of satellite gas by the host halo gas. Using a suite of 20 pc resolution hydrodynamical wind tunnel simulations, we constrain the satellite mass scale at which the stripping of a dwarf galaxy’s interstellar medium (ISM) becomes inefficient in MW-like halos. The simulations include radiative cooling in a multiphase satellite ISM, star formation, and stellar feedback, and vary both satellite masses ( M _⋆ = 10 ^6.2 , 10 ^6.8 , and 10 ^7.2 M _⊙ ) and host halo gas densities along a first-infall and postpericentric orbit. We find that the degree of ISM stripping in our dwarf galaxies is consistent with the analytical prediction by McCarthy et al. Star formation is rapidly quenched when RPS is effective, but can be mildly enhanced or temporarily quenched and subsequently reignited when RPS is incomplete. ISM stripping is efficient for satellites with M _⋆ ≲ 10 ^7 M _⊙ (or M _200 ≲ 10 ^10 M _⊙ ) but highly inefficient above this scale. This transitional mass ( M _⋆ ≈ 10 ^7 M _⊙ ) is 0.5–1 dex lower than that found in observations and cosmological simulations, suggesting that additional mechanisms are needed to quench more massive satellites, such as tidal stripping of the satellite dark matter or RPS from a clumpy gaseous halo.

Galaxy environment plays a crucial role in quenching star formation in dwarf galaxies. In Milky Way (MW)-like environments, dwarf satellite quenching is primarily driven by ram pressure stripping (RPS), the direct removal of satellite gas by the host halo gas. Using a suite of 20-pc resolution hydrodynamical wind tunnel simulations, we constrain the satellite mass scale at which the stripping of a dwarf galaxy's interstellar medium (ISM) becomes inefficient in MW-like halos. The simulations include radiative cooling in a multiphase satellite ISM, star formation, and stellar feedback, and vary both satellite masses (M_⋆=10^6.2, 10^6.8, 10^7.2 M_⊙) and host halo gas densities along a first-infall and post-pericentric orbit. We find that the degree of ISM stripping in our dwarf galaxies is consistent with the analytical prediction by McCarthy et al. (2008). Star formation is rapidly quenched when RPS is effective, but can be mildly enhanced or temporarily quenched and subsequently reignited when RPS is incomplete. ISM stripping is efficient for satellites with M_⋆≲ 10^7 M_⊙ (or M_200≲ 10^10 M_⊙) but highly inefficient above this scale. This transitional mass (M_⋆≈ 10^7 M_⊙) is 0.5-1 dex lower than that found in observations and cosmological simulations, suggesting that additional mechanisms are needed to quench more massive satellites, such as tidal stripping of the satellite dark matter or RPS from a clumpy gaseous halo.

This paper explores the extent to which the circumgalactic medium (CGM) of Milky Way (MW)-like galaxies is located in an extended, ionized, disklike structure. To test this hypothesis, we analyze the spatial and kinematic distributions of different ion species within a sample of MW-like systems in IllustrisTNG. We model commonly observed ions (H I, Mg II, Si IV, C IV, and O VI) and calculate (1) their angular momentum misalignment from the star-forming disk (theta) and (2) the fraction of absorption consistent with galaxy rotation (fEWcorot). We find that 63% of Mg ii, 45% of Si iv, 38% of C iv, and 35% of O vi mass along the major axis have kinematics aligned with the galaxy angular momentum axis. We extend this to a mock absorption line survey and quantify fEWcorot. We find that fEWcorot(Mg ii) similar to 80% and fEWcorot(O vi) similar to 60% at similar to 0.5R200c, in agreement with recent observational work. We find that in the typical MW analog, there is evidence of cool-warm material in an extended, corotating structure, regardless of whether the angular momentum or observational definition is used. Hence, we expect that the typical MW CGM, especially in the low ions, should be mainly on the plane.

We present findings of 3D filamentary structures in the Smith Cloud, a high-velocity cloud (HVC) located at $l=38^{\circ}$, $b=-13^{\circ}$. We use data from the Galactic Arecibo L-Band Feed Array \ion{H}{i} (GALFA-\ion{H}{i}) along with our new filament detection algorithm, \texttt{fil3d}, to characterize these structures. In this paper, we also discuss how different input parameters affect the output of \texttt{fil3d}. We study filaments in the local ISM and compare them to those found in the Smith Cloud. Based on thermal linewidth estimations we find supporting evidence that the Smith Cloud filaments are part of its warm neutral medium. We also find a relationship between thermal linewidth and the $v_{LSR}$ of the filaments. We study the plane-of-sky magnetic field as traced by Planck 353 GHz polarized dust emission along the line of sight and find the HI filaments in this region are not aligned with the magnetic field. This is likely related to their location close to dynamic processes in the Galactic Plane and/or the low column density of the filaments relative to emission in the Plane. The results show the HI filaments are found in a wide range of Galactic environments and form through multiple processes.

The Magellanic Stream (MS) is a vast gaseous structure in the Milky Way halo, containing most of its mass in ionized form and tracing the interaction between the Large and Small Magellanic Clouds and the Galaxy. Using Hubble Space Telescope/Cosmic Origin Spectrograph G160M spectra from the Plane Quasar Survey, we detect C iv absorbers likely associated with the MS, extending to the northern side of the Galactic plane, approximately 60 ^∘ beyond its previously known ionized extent. These absorbers exhibit position and kinematic alignment and show consistent ionization trends with previously studied MS sight lines. The nondetection of low ions such as Al ii and Si ii , and the detection of C iv (and Si iv in some sight lines), indicates a highly ionized gas phase. The observed Si iv /C iv column density ratios suggest a gas temperature of T ∼ 10 ^5.3 K and favor collisional ionization over photoionization. We estimate the newly detected extension increases the previous ionized gas mass of the MS, and its coherent kinematics suggest that it was stripped within the past few hundred Myr and has not yet mixed with the Milky Way halo. The existence of highly ionized MS gas at a location above the Galactic plane may constrain the orbital direction of the Magellanic Clouds.

We present a new technique to identify associations of H i emission in the Magellanic Stream (MS) and ultraviolet (UV) absorbers from 92 QSO sight lines near the MS. We quantify the level of associations of individual H i elements to the main H i body of the Stream using Wasserstein distance-based models, and derive characteristic spatial and kinematic distances of the H i emission in the MS. With the emission-based model, we further develop a comparison metric, which identifies the dominant associations of individual UV absorbers with respect to the MS and nearby galaxies. For ionized gas associated with the MS probed by C ii , C iv , Si ii , Si iii , Si iv , we find that the ion column densities are generally ∼0.5 dex higher than those that are not associated, and that the gas is more ionized toward the tail of the MS as indicated by the spatial trend of the C ii /C iv ratios. For nearby galaxies, we identify potential new absorbers associated with the circumgalactic medium of M33 and NGC 300, and affirm the associations of absorbers with IC 1613 and the Wolf–Lundmark–Mellote galaxy. For M31, we find the previously identified gradient in column densities as a function of the impact parameter, and that absorbers with higher column densities beyond M31's virial radius are more likely to be associated with the MS. Our analysis of absorbers associated with the Magellanic Clouds reveals the presence of continuous and blended diffuse ionized gas between the Stream and the Clouds. Our technique can be applied to future applications of identifying associations within physically complex gaseous structures.

The Large Magellanic Cloud (LMC) is home to many H ii regions, which may lead to significant outflows. We examine the LMC's multiphase gas (T similar to 104-5 K) in H i, S ii, Si iv, and C iv using 110 stellar sight lines from the Hubble Space Telescope's Ultraviolet Legacy Library of Young Stars as Essential Standards program. We develop a continuum fitting algorithm based on the concept of Gaussian process regression and identify reliable LMC interstellar absorption over v helio = 175-375 km s-1. Our analyses show disk-wide ionized outflows in Si iv and C iv across the LMC with bulk velocities of divided by v out, bulk divided by similar to 20-60 km s-1, which indicates that most of the outflowing mass is gravitationally bound. The outflows' column densities correlate with the LMC's star formation rate surface densities (Sigma SFR), and the outflows with higher Sigma SFR tend to be more ionized. Considering outflows from both sides of the LMC as traced by C iv, we conservatively estimate a total outflow rate of Mout greater than or similar to 0.03M circle dot yr-1 and a mass-loading factor of eta greater than or similar to 0.15. We compare the LMC's outflows with those detected in starburst galaxies and simulation predictions, and find a universal scaling relation of divided by vout,bulk divided by proportional to Sigma SFR0.23 over a wide range of star-forming conditions (Sigma SFR similar to 10-4.5-102 M circle dot yr-1 kpc-2). Lastly, we find that the outflows are corotating with the LMC's young stellar disk and the velocity field does not seem to be significantly impacted by external forces; we thus speculate on the existence of a bow shock leading the LMC, which may have shielded the outflows from ram pressure as the LMC orbits the Milky Way.

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 with M _* = 10 ^6.5–9.5 M _⊙ ( M _200m = 10 ^10.0–11.5 M _⊙ ) using the Hubble Space Telescope/Cosmic Origins Spectrograph. While H i (Ly α ) is ubiquitously detected (89%) within the CGM, we find low detection rates (≈5%–22%) in C ii , C iv , Si ii , Si iii , and Si iv , largely consistent with literature values. Assuming these ions form in the cool ( T ≈ 10 ^4 K) CGM with photoionization equilibrium, the observed H i and 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 with M _200m = 10 ^10.9 M _⊙ (median of the sample), our model predicts a cool gas mass of M _CGM,cool ∼ 10 ^8.4 M _⊙ , 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 H i and low ions may arise from a dense cool medium, while C iv arises 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.

We present a sample of 305 QSO candidates having ∣ b ∣ < 30°, the majority with GALEX magnitudes near-UV < 18.75. To generate this sample, we apply UV–IR color selection criteria to photometric data from the Ultraviolet Galactic Plane Survey as part of GALEX-CAUSE, the Million Quasars Catalog, Gaia DR2, and Pan-STARRS DR1. 165 of these 305 candidate UV-bright active galactic nuclei (AGN; 54%) have published spectroscopic redshifts from 45 different surveys, confirming them as AGN. We further obtained low-dispersion, optical, long-slit spectra with the Apache Point Observatory 3.5 m, MDM 2.4 m, and MDM 1.3 m telescopes for 84 of the candidates, and confirm 86% ( N = 72) as AGN, generally with z < 0.6. Of these 72 confirmed AGN, 25 are newly discovered low-latitude QSOs without any previous spectroscopy. These sources fill a gap in the Galactic latitude coverage of the available samples of known UV-bright QSO background probes. Along with a description of the confirmed QSO properties, we provide the fully reduced, flux- and wavelength-calibrated spectra of 72 low-latitude QSOs through the Mikulski Archive for Space Telescopes. Future Hubble Space Telescope/Cosmic Origins Spectrograph spectroscopy of these low-Galactic-latitude QSOs has the potential to transform our view of the Milky Way and Local Group circumgalactic medium.

The circumgalactic medium (CGM) of star-forming dwarf galaxies plays a key role in regulating the galactic baryonic cycle. We investigate how susceptible the CGM of dwarf satellite galaxies is to ram pressure stripping (RPS) in Milky Way-like environments. In a suite of hydrodynamical wind tunnel simulations, we model an intermediate-mass dwarf satellite galaxy ($M_{*} = 10^{7.2}~M_{\odot}$) with a multiphase interstellar medium (ISM; $M_{\rm ISM} = 10^{7.9}~M_{\odot}$) and CGM ($M_{\rm CGM,vir} = 10^{8.5}~M_{\odot}$) along two first-infall orbits to more than 500 Myr past pericenter of a Milky Way-like host. The spatial resolution is $\sim$79 pc in the star-forming ISM and $316-632$ pc in the CGM. Our simulations show that the dwarf satellite CGM removal is fast and effective: more than $95\%$ of the CGM mass is ram-pressure-stripped within a few hundred Myrs, even under a weak ram pressure orbit where the ISM stripping is negligible. The conditions for CGM survival are consistent with the analytical halo gas stripping predictions in McCarthy et al. (2008). We also find that including the satellite CGM does not effectively shield its galaxy, and therefore the ISM stripping rate is unaffected. Our results imply that a dwarf galaxy CGM is unlikely to be detected in satellite galaxies; and that the star formation of gaseous dwarf satellites is likely devoid of replenishment from a CGM.

The Magellanic Stream (MS), a tail of diffuse gas formed from tidal and ram pressure interactions between the Small and Large Magellanic Clouds (SMC and LMC) and the Halo of the Milky Way, is primarily composed of neutral atomic hydrogen (HI). The deficiency of dust and the diffuse nature of the present gas make molecular formation rare and difficult, but if present, could lead to regions potentially suitable for star formation, thereby allowing us to probe conditions of star formation similar to those at high redshifts. We search for HCO++, HCN, HNC, and C2H using the highest sensitivity observations of molecular absorption data from the Atacama Large Millimeter Array (ALMA) to trace these regions, comparing with HI archival data from the Galactic Arecibo L-Band Feed Array (GALFA) HI Survey and the Galactic All Sky Survey (GASS) to compare these environments in the MS to the HI column density threshold for molecular formation in the Milky Way. We also compare the line of sight locations with confirmed locations of stars, molecular hydrogen, and OI detections, though at higher sensitivities than the observations presented here. We find no detections to a 3 sigma significance, despite four sightlines having column densities surpassing the threshold for molecular formation in the diffuse regions of the Milky Way. Here we present our calculations for the upper limits of the column densities of each of these molecular absorption lines, ranging from 3x10(10)to 1x10(13)cm(-2). The non-detection of HCO+ suggests that at least one of the following is true: (i) X(HCO+,MS )is significantly lower than the Milky Way value; (ii) that the widespread diffuse molecular gas observed by Rybarczyk (2022b, ApJ, 928, 79) in the Milky Way's diffuse interstellar medium (ISM) does not have a direct analogue in the MS; (iii) the HI-to-H2H2 transition occurs in the MS at a higher surface density in the MS than in the LMC or SMC; or (iv) molecular gas exists in the MS, but only in small, dense clumps.

The Mittelman-di Cicco-Walker (MDW) H alpha Sky Survey is an autonomously operated and ongoing all-sky imaging survey in the narrowband H alpha wavelength. The survey was founded by amateur astronomers and is presented here in its first stage of refinement for rigorous scientific use. Each field is exposed through an H alpha filter with a 3 nm bandwidth for a total of 4 hr, with a pixel scale of 3.'' 2. Here, we introduce the first Data Release of the MDW H alpha Survey (Data Release 0, or DR0), spanning 238 fields in the region of Orion (similar to 3100 deg2). DR0 includes: calibrated mean fields, star-removed mean fields, a point-source catalog matched to Data Release 1 of the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1) and the Isaac Newton Telescope Galactic Plane Survey (IGAPS), and mosaics. 3 3 DR0 components are available at http://mdw.astro.columbia.edu; the DR0 catalog can also be found on the AAS Journals Zenodo repository: doi:10.5281/zenodo.12747455.

Metals in the diffuse, ionized gas at the boundary between the Milky Way's interstellar medium (ISM) and circumgalactic medium, known as the disk-halo interface (DHI), are valuable tracers of the feedback processes that drive the Galactic fountain. However, metallicity measurements in this region are challenging due to obscuration by the Milky Way ISM and uncertain ionization corrections that affect the total hydrogen column density. In this work, we constrain ionization corrections to neutral hydrogen column densities using precisely measured electron column densities from the dispersion measures of pulsars that lie in the same globular clusters as UV-bright targets with high-resolution absorption spectroscopy. We address the blending of absorption lines with the ISM by jointly fitting Voigt profiles to all absorption components. We present our metallicity estimates for the DHI of the Milky Way based on detailed photoionization modeling of the absorption from ionized metal lines and ionization-corrected total hydrogen columns. Generally, the gas clouds show a large scatter in metallicity, ranging between 0.04 and 3.2 Z(circle dot), implying that the DHI consists of a mixture of gaseous structures having multiple origins. We estimate the inflow and outflow timescales of the DHI ionized clouds to be 6-35 Myr. We report the detection of an infalling cloud with supersolar metallicity that suggests a Galactic fountain mechanism, whereas at least one low-metallicity outflowing cloud (Z < 0.1 Z(circle dot)) poses a challenge for Galactic fountain and feedback models.

ABSTRACT High-spatial-resolution H i observations have led to the realization that the nearby (within few hundreds of parsecs) Galactic atomic filamentary structures are aligned with the ambient magnetic field. Enabled by the high-quality data from the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope for the Galactic ASKAP H i survey, we investigate the potential magnetic alignment of the $\gtrsim\!{10}\, {\rm pc}$-scale H i filaments in the Small Magellanic Cloud (SMC). Using the Rolling Hough Transform technique that automatically identifies filamentary structures, combined with our newly devised ray-tracing algorithm that compares the H i and starlight polarization data, we find that the H i filaments in the north-eastern end of the SMC main body (‘Bar’ region) and the transition area between the main body and the tidal feature (‘Wing’ region) appear preferentially aligned with the magnetic field traced by starlight polarization. Meanwhile, the remaining SMC volume lacks starlight polarization data of sufficient quality to draw any conclusions. This suggests for the first time that filamentary H i structures can be magnetically aligned across a large spatial volume ($\gtrsim\!{\rm kpc}$) outside of the Milky Way. In addition, we generate maps of the preferred orientation of H i filaments throughout the entire SMC, revealing the highly complex gaseous structures of the galaxy likely shaped by a combination of the intrinsic internal gas dynamics, tidal interactions, and star-formation feedback processes. These maps can further be compared with future measurements of the magnetic structures in other regions of the SMC.

ABSTRACT We present a search for gas-containing dwarf galaxies as satellite systems around nearby spiral galaxies using 21 cm neutral hydrogen (H i) data from the Arecibo Legacy Fast ALFA (ALFALFA) Survey. We have identified 15 spiral ‘primary’ galaxies in a local volume of 10 Mpc with a range of total masses, and have found 19 gas-containing dwarf satellite candidates within the primaries’ virial volumes (R200) and 46 candidates within 2R200. Our sensitivity using ALFALFA data converts to MH i ≈ 7.4 × 106 M⊙ at 10 Mpc, which includes 13 of the 26 gaseous dwarf galaxies in the Local Group, and the H i properties of our sample are overall similar to these 13. We found 0–3 gaseous satellites per host galaxy within R200 and 0–5 within 2R200, which agrees with the low numbers present for the Milky Way and M31. There is also agreement with the star-forming satellite numbers per host in the deep optical surveys SAGA and ELVES, and the Auriga cosmological simulations. When scaled to R200, the optical surveys do not show a trend of increasing quenched fraction with host mass; there is a slight increase in the total number of gaseous satellites with host mass for our sample. The low numbers of gaseous/star-forming satellites around spiral hosts are consistent with the idea that a universal and effective satellite quenching mechanism, such as ram pressure stripping by the host halo, is likely at play.

ABSTRACT We characterize the kinematic and magnetic properties of H i filaments located in a high Galactic latitude region (165° < α < 195° and 12° < δ < 24°). We extract three-dimensional filamentary structures using fil3d from the Galactic Arecibo L-Band Feed Array H i (GALFA-H i) survey 21-cm emission data. Our algorithm identifies coherent emission structures in neighbouring velocity channels. Based on the mean velocity, we identify a population of local and intermediate velocity cloud (IVC) filaments. We find the orientations of the local (but not the IVC) H i filaments are aligned with the magnetic field orientations inferred from Planck 353 GHz polarized dust emission. We analyse position–velocity diagrams of the velocity-coherent filaments, and find that only 15 per cent of filaments demonstrate significant major-axis velocity gradients with a median magnitude of 0.5 km s−1 pc−1, assuming a fiducial filament distance of 100 pc. We conclude that the typical diffuse H i filament does not exhibit a simple velocity gradient. The reported filament properties constrain future theoretical models of filament formation.

We present resolved H i synthesis maps from the Very Large Array of three interacting dwarf systems: the NGC 3664 dwarf pair, the NGC 3264 dwarf pair, and the UGC 4638 dwarf triplet. All three dwarf systems are captured at various stages of interaction and span a range of environments. We detect clear hallmarks of tidal interactions through the presence of H i bridges and diffuse H i extensions that surround the dwarfs. We overlay the H i data on Pan-STARRS r-band images and find further evidence of tidal interactions through coincident distorted H i and tidal stellar features in NGC 3264 and UGC 4638, and an unwound spiral arm pointing toward its smaller companion in NGC 3264. In UGC 4638, both the gas and diffuse stars are extended to similar radii east of the primary, which could indicate that the smaller dwarf in the system has already completed one pass through the primary. We additionally find that our three systems, and those from the Local Volume TiNy Titans survey, are not H i deficient and thus the interaction has not resulted in a loss of gas from the systems. A comparison with noninteracting dwarf galaxies shows that the interactions have a significant impact on the kinematics of the systems. Our new resolved H i kinematics, combined with detailed stellar and H i morphologies, provide crucial constraints for future dynamical modeling of hierarchical mergers and the baryon cycle at the low-mass scale.

From our position embedded within the Milky Way’s interstellar medium, we have limited ability to detect gas at low relative velocities in the extended Galactic halo because those spectral lines are blended with much stronger signals from dense foreground gas. As a result, the content of the Milky Way’s circumgalactic medium (CGM) is poorly constrained at ∣ v LSR ∣ ≲150 km s −1 . To overcome this complication, the QuaStar survey applies a spectral differencing technique using paired quasar−star sight lines to measure the obscured content of the Milky Way’s CGM for the first time. We present measurements of the C iv doublet ( λ λ 1548, 1550), a rest-frame UV metal line transition, detected in Hubble Space Telescope/Cosmic Origins Spectrograph spectra of 30 halo-star/quasar pairs evenly distributed across the sky at Galactic latitudes ∣ b ∣ > 30°. The 30 halo stars have well-constrained distances ( d ≈ 5–14 kpc) and are paired with quasars separated by <2.°8. We argue that the difference in absorption between the quasar and stellar sight lines originates primarily in the Milky Way’s extended CGM beyond ∼10 kpc. For the Milky Way’s extended, low-velocity CGM (∣ v ∣ <150 km s −1 ), we place an upper limit on the mean C iv column density of Δ log N LVCGM < 13.39 and find a covering fraction of f C IV , LVCGM ( log N > 13.65 ) = 20% [6/30], a value significantly lower than the covering fraction for star-forming galaxies at low redshift. Our results suggest either that the bulk of Milky Way’s C iv -traced CGM lies at low Galactic latitudes or that the Milky Way’s CGM is lacking in warm, ionized material compared to low-redshift ( z < 0.1) star-forming galaxy halos.

We present a systematic investigation of physical conditions and elemental abundances in four optically thick Lyman-limit systems (LLSs) at z = 0.36-0.6 discovered within the cosmic ultraviolet baryon survey (CUBS). Because intervening LLSs at z < 1 suppress far-UV (ultraviolet) light from background QSOs, an unbiased search of these absorbers requires a near-UV-selected QSO sample, as achieved by CUBS. CUBS LLSs exhibit multicomponent kinematic structure and a complex mix of multiphase gas, with associated metal transitions from multiple ionization states such as CII, CIII, NIII, MgII, SIII, SIIII, OII, OIII, OvI, and FeII absorption that span several hundred km s(-1) in line-of-sight velocity. Specifically, higher column density components (logN(HI)/cm(-2) greater than or similar to 16) in all four absorbers comprise dynamically cool gas with K and modest non-thermal broadening of km s(-1). The high quality of the QSO absorption spectra allows us to infer the physical conditions of the gas, using a detailed ionization modelling that takes into account the resolved component structures of Hi and metal transitions. The range of inferred gas densities indicates that these absorbers consist of spatially compact clouds with a median line-of-sight thickness of pc. While obtaining robust metallicity constraints for the low density, highly ionized phase remains challenging due to the uncertain , we demonstrate that the cool-phase gas in LLSs has a median metallicity of , with a 16-84 percentile range of [alpha/H] = (-1.3, -0.1). Furthermore, the wide range of inferred elemental abundance ratios ([C/alpha], [N/alpha], and [Fe/alpha]) indicate a diversity of chemical enrichment histories. Combining the absorption data with deep galaxy survey data characterizing the galaxy environment of these absorbers, we discuss the physical connection between star-forming regions in galaxies and diffuse gas associated with optically thick absorption systems in the z < 1 circumgalactic medium.