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Quasar absorption systems encode a wealth of information about the abundances, ionization structure, and physical conditions in intergalactic and circumgalactic media. Simple (often single-phase) photoionization models are frequently used to decode such data. Using five discrete absorbers from the COS Absorption Survey of Baryon Harbors (CASBaH) that exhibit a wide range of detected ions (e.g. Mg  ii , S  ii – S  vi , O  ii – O  vi , Ne  viii), we show several examples where single-phase ionization models cannot reproduce the full set of measured column densities. To explore models that can self-consistently explain the measurements and kinematic alignment of disparate ions, we develop a Bayesian multiphase ionization modelling framework that characterizes discrete phases by their unique physical conditions and also investigates variations in the shape of the UV flux field, metallicity, and relative abundances. Our models require at least two (but favour three) distinct ionization phases ranging from T ≈ 104 K photoionized gas to warm-hot phases at T ≲ 105.8 K. For some ions, an apparently single absorption 'component' includes contributions from more than one phase, and up to 30 per cent of the H  i is not from the lowest ionization phase. If we assume that all of the phases are photoionized, we cannot find solutions in thermal pressure equilibrium. By introducing hotter, collisionally ionized phases, however, we can achieve balanced pressures. The best models indicate moderate metallicities, often with subsolar N/ α , and, in two cases, ionizing flux fields that are softer and brighter than the fiducial Haardt & Madau UV background model. [ABSTRACT FROM AUTHOR]

Davies et al. established that for L * galaxies the fraction of baryons in the circumgalactic medium (CGM) is inversely correlated with the mass of their central supermassive black holes (BHs) in the EAGLE hydrodynamic simulation. The interpretation is that, over time, a more massive BH has provided more energy to transport baryons beyond the virial radius, which additionally reduces gas accretion and star formation. We continue this research by focusing on the relationship between the (1) BH masses (M BH), (2) physical and observational properties of the CGM, and (3) galaxy colours for Milky Way-mass systems. The ratio of the cumulative BH feedback energy over the gaseous halo binding energy is a strong predictor of the CGM gas content, with BHs injecting significantly higher than the binding energy resulting in gas-poor haloes. Observable tracers of the CGM, including |$\rm {C\, \small{IV}}$|⁠ , |$\rm {O\, \small{VI}}$|⁠ , and |${\rm {H\, \small{I}}}$| absorption line measurements, are found to be effective tracers of the total z ∼ 0 CGM halo mass. We use high-cadence simulation outputs to demonstrate that BH feedback pushes baryons beyond the virial radius within 100 Myr time-scales, but that CGM metal tracers take longer (0.5–2.5 Gyr) to respond. Secular evolution of galaxies results in blue, star-forming or red, passive populations depending on the cumulative feedback from BHs. The reddest quartile of galaxies with M * = 1010.2−10.7 M⊙ (median u − r  = 2.28) has a CGM mass that is 2.5 times lower than the bluest quartile (u − r  = 1.59). We propose observing strategies to indirectly ascertain f CGM via metal lines around galaxies with measured M BH. We predict statistically detectable declines in |$\rm {C\, \small{IV}}$| and |$\rm {O\, \small{VI}}$| covering fractions with increasing M BH for central galaxies with M * = 1010.2−10.7M⊙. [ABSTRACT FROM AUTHOR]

Quasar absorption-line studies in the ultraviolet (UV) can uniquely probe the nature of the multiphase cool–warm (104 < T < 106 K) gas in and around galaxy clusters, promising to provide unprecedented insights into (1) interactions between the circumgalactic medium (CGM) associated with infalling galaxies and the hot (T > 106 K) X-ray emitting intracluster medium (ICM), (2) the stripping of metal-rich gas from the CGM, and (3) a multiphase structure of the ICM with a wide range of temperatures and metallicities. In this work, we present results from a high-resolution simulation of an |$\sim 10^{14} \, \mathrm{M}_{\odot }$| galaxy cluster to study the physical properties and observable signatures of this cool–warm gas in galaxy clusters. We show that the ICM becomes increasingly multiphased at large radii, with the cool–warm gas becoming dominant in cluster outskirts. The diffuse cool–warm gas also exhibits a wider range of metallicity than the hot X-ray emitting gas. We make predictions for the covering fractions of key absorption-line tracers, both in the ICM and in the CGM of cluster galaxies, typically observed with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope (HST). We further extract synthetic spectra to demonstrate the feasibility of detecting and characterizing the thermal, kinematic, and chemical composition of the cool–warm gas using H  i , O  vi , and C  iv lines, and we predict an enhanced population of broad Ly α absorbers tracing the warm gas. Lastly, we discuss future prospects of probing the multiphase structure of the ICM beyond HST. [ABSTRACT FROM AUTHOR]

We report a tentative detection of the circumgalactic medium (CGM) of Wolf–Lundmark–Melotte (WLM), an isolated, low-mass (log M */M⊙ ≈ 7.6), dwarf irregular galaxy in the Local Group (LG). We analyse an HST /COS archival spectrum of a quasar sightline (PHL2525), which is 45 kpc (0.5 virial radius) from WLM and close to the Magellanic Stream (MS). Along this sightline, two ion absorbers are detected in Si  ii , Si  iii , Si  iv , C  ii , and C  iv at velocities of ∼−220 km s−1 (Component v-220) and ∼−150 km s−1 (Component v-150). To identify their origins, we study the position–velocity alignment of the components with WLM and the nearby MS. Near the magellanic longitude of PHL2525, the MS-related neutral and ionized gas moves at ≲−190 km s−1, suggesting an MS origin for Component v-220, but not for Component v-150. Because PHL2525 passes near WLM and Component v-150 is close to WLM's systemic velocity (∼−132 km s−1), it is likely that Component v-150 arises from the galaxy's CGM. This results in a total Si mass in WLM's CGM of |$M_{\rm Si}^{\rm CGM}\sim (0.2-1.0)\times 10^5~\mathrm{M}_\odot$| using assumption from other COS dwarf studies. Comparing |$M_{\rm Si}^{\rm CGM}$| to the total Si mass synthesized in WLM over its lifetime (∼1.3 × 105 M⊙), we find ∼3 per cent is locked in stars, ∼6 per cent in the ISM, ∼15–77 per cent in the CGM, and the rest (∼14–76 per cent) is likely lost beyond the virial radius. Our finding resonates with other COS dwarf galaxy studies and theoretical predictions that low-mass galaxies can easily lose metals into their CGM due to stellar feedback and shallow gravitational potential. [ABSTRACT FROM AUTHOR]

We explore the circumgalactic metal content traced by commonly observed low ion absorbers, including |${\rm{C\,{\small II}}}$|⁠, |${\rm{Si\,{\small II}}}$|⁠, |${\rm{Si\,{\small III}}}$|⁠, |$\rm{Si\,{\small IV}}$|⁠, and |${\rm{Mg\,{\small II}}}$|⁠. We use a set of cosmological hydrodynamical zoom simulations run with the EAGLE model and including a non-equilibrium ionization and cooling module that follows 136 ions. The simulations of |$z$| ≈ 0.2 L * (M 200 = 1011.7 − 1012.3M⊙) haloes hosting star-forming galaxies and group-sized (M 200 = 1012.7 − 1013.3M⊙) haloes hosting mainly passive galaxies reproduce key trends observed by the COS–Halos survey – low ion column densities show 1) little dependence on galaxy-specific star formation rate, 2) a patchy covering fraction indicative of 104 K clumps with a small volume filling factor, and 3) a declining covering fraction as impact parameter increases from 20−160kpc. Simulated |${\rm{Si\,{\small II}}}$|⁠, |${\rm{Si\,{\small III}}}$|⁠, |$\rm{Si\,{\small IV}}$|⁠, |${\rm{C\,{\small II}}}$|⁠, and |${\rm{C\,{\small III}}}$| column densities show good (mostly within 0.3 dex) agreement with observations, while |${\rm{Mg\,{\small II}}}$| is under-predicted. Low ions trace a significant metal reservoir, ≈108M⊙, residing primarily at 10−100kpc from star-forming and passive central galaxies. These clumps preferentially flow inwards and most will accrete onto the central galaxy within the next several Gyr, while a small fraction are entrained in strong outflows. A multiphase structure describes the inner CGM (<0.5 R 200) with low-ion metal clumps surrounded by a hot, ambient medium. The dense, cool phase is separate from the |$\rm{O\,{\small VI}}$| observed by COS–Halos, which arises from the outer CGM (>0.5 R 200) tracing virial temperature gas around L * galaxies. Our simulations support previous ionization models indicating that cloud covering factors decline while densities and pressures show little decline with increasing impact parameter (typically <0.3 dex from 40 to 160 kpc). We find the cool clumps have lower pressures than the ambient medium they are embedded in, and discuss that numerical effects within the hydrodynamic solver likely play a role. [ABSTRACT FROM AUTHOR]

The gas surrounding galaxies outside their disks or interstellar medium and inside their virial radii is known as the circumgalactic medium (CGM). In recent years this component of galaxies has assumed an important role in our understanding of galaxy evolution owing to rapid advances in observational access to this diffuse, nearly invisible material. Observations and simulations of this component of galaxies suggest that it is a multiphase medium characterized by rich dynamics and complex ionization states. The CGM is a source for a galaxy's star-forming fuel, the venue for galactic feedback and recycling, and perhaps the key regulator of the galactic gas supply. We review our evolving knowledge of the CGM with emphasis on its mass, dynamical state, and coevolution with galaxies. Observations from all redshifts and from across the electromagnetic spectrum indicate that CGM gas has a key role in galaxy evolution. We summarize the state of this field and pose unanswered questions for future research. [ABSTRACT FROM AUTHOR]

We analyse the mass, temperature, metal enrichment, and OVI abundance of the circumgalactic medium (CGM) around z ~ 0.2 galaxies of mass 109M☉ < M* < 1011.5M☉ in the Illustris simulation. Among star-forming galaxies, the mass, temperature, and metallicity of the CGM increase with stellar mass, driving an increase in the OVI column-density profile of ~0.5 dex with each 0.5 dex increase in stellar mass. Observed OVI column-density profiles exhibit a weaker mass dependence than predicted; the simulated OVI abundance profiles are consistent with those observed for star-forming galaxies of mass M* = 1010.5-11.5M☉, but underpredict the observed OVI abundances by ≳0.8 dex for lower mass galaxies. We suggest that this discrepancy may be alleviated with additional heating of the abundant cool gas in low-mass haloes, or with increased numerical resolution capturing turbulent/conductive mixing layers between CGM phases. Quenched galaxies of mass M* = 1010.5-11.5M☉ are found to have 0.3-0.8 dex lower OVI column-density profiles than star-forming galaxies of the same mass, in qualitative agreement with the observed OVI abundance bimodality. This offset is driven by AGN feedback, which quenches galaxies by heating theCGMand ejecting significant amounts of gas from the halo. Finally, we find that the inclusion of the central-galaxy's radiation field may enhance the photoionization of the CGM within ~50 kpc, further increasing the predicted OVI abundance around star-forming galaxies. [ABSTRACT FROM AUTHOR]

We analyse the low-redshift (z≈0.2) circumgalactic medium (CGM) by comparing absorption-line data from the COS-Halos survey to absorption around a matched galaxy sample from two cosmological hydrodynamic simulations. The models include different prescriptions for galactic outflows, namely hybrid energy/momentum driven wind (ezw), and constant winds (cw). We compare equivalent widths, covering factors, ion ratios, and kinematics. Both wind models show generally ≲1σ agreement with these observations for H I and certain low-ionization metal lines, but poorer agreement with higher ionization metal lines including Si III and O VI that are well observed by COS-Halos. This suggests that both models predict too much cool, metal-enriched gas and not enough hot gas, and / or that metals are not sufficiently mixed. This may reflect our model assumption of ejecting outflows as cool and unmixing gas. Our ezw simulation includes a heuristic prescription to quench massive galaxies by superheating interstellar medium gas. This produces low-ionization absorption broadly consistent with observations, but substantial O VI absorption inconsistent with data, suggesting that gas around quenched galaxies in the real Universe does not cool. At impact parameters of ≲50 kpc, recycling winds dominate the absorption of low ions and even H I, while O VI generally arises from metals ejected ≳1 Gyr ago. The similarity between the wind models is surprising, since they differ substantially in the amount and phase distribution of halo gas.We show that this similarity owes mainly to our comparison at fixed stellar (not halo) mass, suggesting that CGM properties are more closely tied to galaxy's stellar (versus halo) mass. [ABSTRACT FROM AUTHOR]

Modern analyses of structure formation predict a universe tangled in a 'cosmic web' of dark matter and diffuse baryons. These theories further predict that at low z, a significant fraction of the baryons will be shock-heated to T ~ 105-105 K yielding a warm-hot intergalactic medium (WHIM), but whose actual existence has eluded a firm observational confirmation.We present a novel experiment to detect theWHIM, by targeting the putative filaments connecting galaxy clusters. We use HST/COS to observe a remarkable quasi-stellar object (QSO) sightline that passes within Δd = 3 Mpc from the seven intercluster axes connecting seven independent cluster pairs at redshifts 0.1 ≤ z ≤ 0.5.We find tentative excesses of total HI, narrow HI (NLA; Doppler parameters b < 50 km s-1), broad HI (BLA; b ≥ 50 km s-1) and OVI absorption lines within rest-frame velocities of Δv Δ 1000 km s-1 from the cluster-pairs redshifts, corresponding to ~2, ~1.7, ~6 and ~4 times their field expectations, respectively. Although the excess of OVI likely comes from gas close to individual galaxies, we conclude that most of the excesses of NLAs and BLAs are truly intergalactic. We find the covering fractions, fc, of BLAs close to cluster pairs are ~4-7 times higher than the random expectation (at the ~2σ c.l.), whereas the fc of NLAs and OVI are not significantly enhanced. We argue that a larger relative excess of BLAs compared to those of NLAs close to cluster pairs may be a signature of the WHIM in intercluster filaments. By extending this analysis to tens of sightlines, our experiment offers a promising route to detect the WHIM. [ABSTRACT FROM AUTHOR]