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The circumgalactic medium (CGM) in $\gtrsim 10^{12}$ $\mathrm{M}_{\odot}$ halos is dominated by a hot phase ($T \gtrsim 10^{6}$ K). While many models exist for the hot gas structure, there is as yet no consensus. We compare cooling flow models, in which the hot CGM flows inward due to radiative cooling, to the CGM of $\sim 10^{12}-10^{13}$ $\mathrm{M}_{\odot}$ halos in galaxy formation simulations from the FIRE project at $z\sim0$. The simulations include realistic cosmological evolution and feedback from stars but neglect AGN feedback. At both mass scales, CGM inflows are typically dominated by the hot phase rather than by the `precipitation' of cold gas. Despite being highly idealized, we find that cooling flows describe $\sim 10^{13}$ $\mathrm{M}_{\odot}$ halos very well, with median agreement in the density and temperature profiles of $\sim 20\%$ and $\sim 10\%$, respectively. This indicates that stellar feedback has little impact on CGM scales in those halos. For $\sim 10^{12}$ $\mathrm{M}_{\odot}$ halos, the thermodynamic profiles are also accurately reproduced in the outer CGM. For some of these lower-mass halos, cooling flows significantly overpredict the hot gas density in the inner CGM. This could be due to multidimensional angular momentum effects not well captured by our 1D cooling flow models and/or to the larger cold gas fractions in these regions. Turbulence, which contributes $\sim 10-40\%$ of the total pressure, must be included to accurately reproduce the temperature profiles. Overall, cooling flows predict entropy profiles in better agreement with the FIRE simulations than other idealized models in the literature., Comment: 20 pages, 14 figures. Submitted to MNRAS

The properties of warm-hot gas around $\sim L_{*}$ galaxies can be studied with absorption lines from highly ionized metals. We predict Ne VIII column densities from cosmological zoom-in simulations of halos with masses in $\sim 10^{12}$ and $\sim 10^{13}\,\mathrm{M}_{\odot}$ from the FIRE project. Ne VIII traces the volume-filling, virial-temperature gas in $\sim 10^{12}\,\mathrm{M}_{\odot}$ halos. In $\sim 10^{13}\,\mathrm{M}_{\odot}$ halos the Ne VIII gas is clumpier, and biased towards the cooler part of the warm-hot phase. We compare the simulations to observations by the CASBaH and CUBS surveys. We show that when inferring halo masses from stellar masses to compare simulated and observed halos, it is important to account for the scatter in the stellar-mass-halo-mass relation, especially at $\,\mathrm{M}_{\star} \gtrsim 10^{10.5} \,\mathrm{M}_{\odot}$. Median Ne VIII columns in the fiducial FIRE-2 model are about as high as observed upper limits allow, while the simulations analyzed do not reproduce the highest observed columns. This suggests that the median Ne VIII profiles predicted by the simulations are consistent with observations, but that the simulations may underpredict the scatter. We find similar agreement with analytical models that assume a product of the halo gas fraction and metallicity (relative to solar) $\sim 0.1$-$0.3$, indicating that observations are consistent with plausible CGM temperatures, metallicities, and gas masses. Variants of the FIRE simulations with a modified supernova feedback model and/or AGN feedback included (as well as some other cosmological simulations from the literature) more systematically underpredict Ne VIII columns. The circumgalactic Ne VIII observations therefore provide valuable constraints on simulations that otherwise predict realistic galaxy properties., Comment: version accepted by ApJ

The warm-hot plasma in cosmic web filaments is thought to comprise a large fraction of the gas in the local Universe. So far, the search for this gas has focused on mapping its emission, or detecting its absorption signatures against bright, point-like sources. Future, non-dispersive, high spectral resolution X-ray detectors will, for the first time, enable absorption studies against extended objects. Here, we use the Hydrangea cosmological hydrodynamical simulations to predict the expected properties of intergalactic gas in and around massive galaxy clusters, and investigate the prospects of detecting it in absorption against the bright cores of nearby, massive, relaxed galaxy clusters. We probe a total of $138$ projections from the simulation volumes, finding $16$ directions with a total column density $N_{O VII} > 10^{14.5}$ cm$^{-2}$. The strongest absorbers are typically shifted by $\pm 1000$ km/s with respect to the rest frame of the cluster they are nearest to. Realistic mock observations with future micro-calorimeters, such as the Athena X-ray Integral Field Unit or the proposed Line Emission Mapper (LEM) X-ray probe, show that the detection of cosmic web filaments in O VII and O VIII absorption against galaxy cluster cores will be feasible. An O VII detection with a $5\sigma$ significance can be achieved in $10-250$ ks with Athena for most of the galaxy clusters considered. The O VIII detection becomes feasible only with a spectral resolution of around $1$ eV, comparable to that envisioned for LEM., Comment: 17 pages, 10 figures, 4 tables, accepted for publication in MNRAS

Feedback from supermassive black holes is believed to be a critical driver of the observed color bimodality of galaxies above the Milky Way mass scale. AGN feedback has been modeled in many galaxy formation simulations, but most implementations have involved simplified prescriptions or a coarse-grained interstellar medium (ISM). We present the first set of FIRE-3 cosmological zoom-in simulations with AGN feedback evolved to $z\sim0$, examining the impact of AGN feedback on a set of galaxies with halos in the mass range $10^{12}-10^{13} M_{\odot}$. These simulations combine detailed stellar and ISM physics with multi-channel AGN feedback including radiative feedback, mechanical outflows, and in some simulations, cosmic rays (CRs). We find that massive (>L*) galaxies in these simulations can match local scaling relations including the stellar mass-halo mass relation and the $M_{\rm BH}$-$\sigma$ relation; in the stronger model with CRs, they also match the size-mass relation and the Faber-Jackson relation. Many of the massive galaxies in the simulations with AGN feedback have quenched star formation and elliptical morphologies, in qualitative agreement with observations. In contrast, simulations at the massive end without AGN feedback produce galaxies that are too massive and form stars too rapidly, are order-of-magnitude too compact, and have velocity dispersions well above Faber-Jackson. Despite these successes, the AGN models analyzed do not produce uniformly realistic galaxies when the feedback parameters are held constant: while the stronger model produces the most realistic massive galaxies, it tends to over-quench the lower-mass galaxies. This indicates that further refinements of the AGN modeling are needed., Comment: 18 pages, 8 figures, accepted by ApJ

We study the impact of resonantly scattered X-ray line emission on the observability of the hot circumgalactic medium (CGM) of galaxies. We apply a Monte Carlo radiative transfer post-processing analysis to the high-resolution TNG50 cosmological magnetohydrodynamical galaxy formation simulation. This allows us to model the resonant scattering of OVII(r) X-ray photons within the complex, multi-phase, multi-scale CGM. The resonant transition of the OVII He-like triplet is one of the brightest, and most promising, X-ray emission lines for detecting the hot CGM and measuring its physical properties. We focus on galaxies with stellar masses 10 < log(M*/Msun) < 11 at z ~ 0. After constructing a model for OVII(r) emission from the central galaxy as well as from CGM gas, we forward model these intrinsic photons to derive observable surface brightness maps. We find that scattering significantly boosts the observable OVII(r) surface brightness of the extended and diffuse CGM. This enhancement can be large -- an order of magnitude on average at a distance of 200 projected kpc for high-mass M* = 10^10.7 Msun galaxies. The enhancement is larger for lower mass galaxies, and can even reach a factor of 100, across the extended CGM. Galaxies with higher star formation rates, AGN luminosities, and central OVII(r) luminosities all have larger scattering enhancements, at fixed stellar mass. Our results suggest that next-generation X-ray spectroscopic missions including XRISM, LEM, ATHENA, and HUBS -- which aim to detect the hot CGM in emission -- could specifically target halos with significant enhancements due to resonant scattering., Comment: Published in MNRAS. See https://www.lem-observatory.org/ and https://www.tng-project.org/ for more details; 2023MNRAS.522.3665N

In the Halo21 absorption modeling challenge we generated synthetic absorption spectra of the circumgalactic medium (CGM), and attempted to estimate the metallicity, temperature, and density (Z, T, and nH) of the underlying gas using observational methods. We iteratively generated and analyzed three increasingly-complex data samples: ion column densities of isolated uniform clouds, mock spectra of 1--3 uniform clouds, and mock spectra of high-resolution turbulent mixing zones. We found that the observational estimates were accurate for both uniform cloud samples, with Z, T, and nH retrieved within 0.1 dex of the source value for >90% of absorption systems. In the turbulent-mixing scenario, the mass, temperature, and metallicity of the strongest absorption components were also retrieved with high accuracy. However, the underlying properties of the subdominant components were poorly constrained because the corresponding simulated gas contributed only weakly to the H I absorption profiles. On the other hand, including additional components beyond the dominant ones did improve the fit, consistent with the true existence of complex cloud structures in the source data., Comment: 16 pages, 10 figures, submitted. Data and additional figures available upon request

The physical properties of the faint and extremely tenuous plasma in the far outskirts of galaxy clusters, the circumgalactic media of normal galaxies, and filaments of the cosmic web, remain one of the biggest unknowns in our story of large-scale structure evolution. Modelling the spectral features due to emission and absorption from this very diffuse plasma poses a challenge, as both collisional and photo-ionisation processes must be accounted for. In this paper, we study the ionisation by photons emitted by the intra-cluster medium in addition to the photo-ionisation by the cosmic UV/X-ray background on gas in the vicinity of galaxy clusters. For near massive clusters such as A2029, the ionisation parameter can no longer describe the ionisation balance uniquely. The ionisation fractions (in particular of C IV, C V, C VI, N VII, O VI, O VII, O VIII, Ne VIII, Ne IX, and Fe XVII) obtained by taking into account the photoionisation by the cosmic background are either an upper or lower limit to the ionisation fraction calculated as a function of distance from the emission from the cluster. Using a toy model of a cosmic web filament, we predict how the cluster illumination changes the column densities for two different orientations of the line of sight. For lines of sight passing close to the cluster outskirts, O VI can be suppressed by a factor of up to $4.5$, O VII by a factor of $2.2$, C V by a factor of $3$, and Ne VIII can be boosted by a factor of $2$, for low density gas., Comment: 12 pages, 9 figures, 3 tables, accepted for publication in MNRAS

Fast Radio Bursts (FRBs) are a promising tool for studying the low-density universe as their dispersion measures (DM) are extremely sensitive probes of electron column density. Active Galactic Nuclei (AGN) inject energy into the intergalactic medium, affecting the DM and its scatter. To determine the effectiveness of FRBs as a probe of AGN feedback, we analysed three different AGN models from the EAGLE simulation series. We measured the mean DM-redshift relation, and the scatter around it, using $2.56 \times 10^8$ sightlines at 131 redshift ($z$) bins between $0 \leq z \leq 3$. While the DM-redshift relation itself is highly robust against different AGN feedback models, significant differences are detected in the scatter around the mean: weaker feedback leads to more scatter. We find $\sim 10^4$ localised FRBs are needed to discriminate between the scatter in standard feedback and stronger, more intermittent feedback models. The number of FRBs required is dependent on the redshift distribution of the detected population. A log-normal redshift distribution at $z=0.5$ requires approximately 50% fewer localised FRBs than a distribution centred at $z=1$. With the Square Kilometre Array expected to detect $>10^3$ FRBs per day, in the future, FRBs will be able to provide constraints on AGN feedback., Comment: 5 pages, 4 figures; Submitted to MNRAS Letters. Comments welcome

We estimate the detectability of X-ray metal-line emission from the circumgalactic medium (CGM) of galaxies over a large halo mass range ($\mathrm{M}_{\mathrm{200c}} = 10^{11.5}$-$10^{14.5}\,\mathrm{M}_{\odot}$) using the EAGLE simulations. With the XRISM Resolve instrument, a few bright (K-$\alpha$ or Fe L-shell) lines from $\mathrm{M}_{\mathrm{200c}} \gtrsim 10^{13}\,\mathrm{M}_{\odot}$ haloes should be detectable. Using the Athena X-IFU or the Lynx Main Array, emission lines (especially from O VIII and O VII) from the inner CGM of $\mathrm{M}_{\mathrm{200c}} \gtrsim10^{12.5}\,\mathrm{M}_{\odot}$ haloes become detectable, and intragroup and intracluster gas will be detectable out to the virial radius. With the Lynx Ultra-high Resolution Array, the inner CGM of haloes hosting $\mathrm{L}_{*}$ galaxies is accessible. These estimates do assume long exposure times ($\sim 1 \,$Ms) and large spatial bins ($\sim1$-$10\,\mathrm{arcmin}^{2}$). This emission is dominated by collisionally ionized (CI) gas, and tends to come from halo centres. The emission is biased towards temperatures close to the maximum emissivity temperature for CI gas ($\mathrm{T}_\mathrm{peak}$), and high densities and metallicities. However, for the K-$\alpha$ lines, emission can come from hotter gas in haloes where the virialized, volume-filling gas is hotter than $\mathrm{T}_\mathrm{peak}$. Trends of emission with halo mass can largely be explained by differences in virial temperature. Differences in the mass trends of K-$\alpha$, He-$\alpha$-like, and Fe L-shell lines mirror differences in their emissivities as a function of temperature. We conclude that upcoming X-ray missions will open up a new window onto the hot CGM., Comment: published in MNRAS; this is the version accepted for publication

Context. A significant fraction of the predicted baryons remains undetected in the local universe. We adopted the common assumption that a large fraction of the missing baryons corresponds to the hot (log T(K) = 5.5-7) phase of the Warm Hot Intergalactic Medium (WHIM). We base our missing baryons search on the scenario whereby the WHIM has been heated up via accretion shocks and galactic outflows, and is concentrated towards the filaments of the Cosmic Web. Aims. Our aim is to improve the observational search of the poorly detected hot WHIM. Methods. We detect the filamentary structure within the EAGLE simulation by applying the Bisous formalism to the galaxy distribution. In addition, we use the MMF/NEXUS+ classification of the large scale environment of the dark matter component in EAGLE. We then study the spatio-thermal distribution of the hot baryons within the extracted filaments. Results. While the filaments occupy only 5% of the full simulation volume, the diffuse hot intergalactic medium in filaments amounts to 23% $-$ 25% of the total baryon budget, or 79% $-$ 87% of all the hot WHIM. The most optimal filament sample, with a missing baryon mass fraction of 82%, is obtained by selecting Bisous filaments with a high galaxy luminosity density. For these filaments we derived analytic formulae for the radial gas density and temperature profiles, consistent with recent Planck SZ and CMB lensing observations within the central $r$~ 1 Mpc. Conclusions. Results from EAGLE suggest that the missing baryons are strongly concentrated towards the filament axes. Since the filament finding methods used here are applicable to galaxy surveys, a large fraction of the missing baryons can be localised by focusing the observational efforts on the central 1 Mpc regions of the filaments. Moreover, focusing on high galaxy luminosity density regions will optimise the observational signal., Comment: 22 pages, 16 figures, accepted for publication in Astronomy and Astrophysics (A&A)

The dispersion measure (DM) of fast radio bursts (FRBs) provides a unique way to probe ionised baryons in the intergalactic medium (IGM). Cosmological models with different parameters lead to different DM-redshift ($\mathrm{DM}-z$) relations. Additionally, the over/under-dense regions in the IGM and the circumgalactic medium of intervening galaxies lead to scatter around the mean $\mathrm{DM}-z$ relations. We have used the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulations to measure the mean $\mathrm{DM}-z$ relation and the scatter around it using over one billion lines-of-sight between redshifts $0

We use the EAGLE (Evolution and Assembly of GaLaxies and their Environments) cosmological simulation to study the distribution of baryons, and far-ultraviolet (O VI), extreme-ultraviolet (Ne VIII) and X-ray (O VII, O VIII, Ne IX, and Fe XVII) line absorbers, around galaxies and haloes of mass $\mathrm{M}_{200c}=10^{11}$-$10^{14.5}\,\mathrm{M}_{\odot}$ at redshift 0.1. EAGLE predicts that the circumgalactic medium (CGM) contains more metals than the interstellar medium across halo masses. The ions we study here trace the warm-hot, volume-filling phase of the CGM, but are biased towards temperatures corresponding to the collisional ionization peak for each ion, and towards high metallicities. Gas well within the virial radius is mostly collisionally ionized, but around and beyond this radius, and for O VI, photoionization becomes significant. When presenting observables we work with column densities, but quantify their relation with equivalent widths by analysing virtual spectra. Virial-temperature collisional ionization equilibrium ion fractions are good predictors of column density trends with halo mass, but underestimate the diversity of ions in haloes. Halo gas dominates the highest column density absorption for X-ray lines, but lower density gas contributes to strong UV absorption lines from O VI and Ne VIII. Of the O VII (O VIII) absorbers detectable in an Athena X-IFU blind survey, we find that 41 (56) per cent arise from haloes with $\mathrm{M}_{200c}=10^{12.0}$-$10^{13.5}\,\mathrm{M}_{\odot}$. We predict that the X-IFU will detect O VII (O VIII) in 77 (46) per cent of the sightlines passing $\mathrm{M}_{\star}=10^{10.5}$-$10^{11.0}\,\mathrm{M}_{\odot}$ galaxies within 100 pkpc (59 (82) per cent for $\mathrm{M}_{\star}>10^{11.0}\,\mathrm{M}_{\odot}$). Hence, the X-IFU will probe covering fractions comparable to those detected with the Cosmic Origins Spectrograph for O VI., Comment: 22 pages, 17 figures + 6 pages references and appendices. Accepted for publication in MNRAS

We simulate stacked observations of nearby hot X-ray coronae associated with galaxies in the EAGLE and Illustris-TNG hydrodynamic simulations. A forward modeling pipeline is developed to predict 4-year eROSITA observations and stacked image analysis, including the effects of instrumental and astrophysical backgrounds. We propose an experiment to stack z~0.01 galaxies separated by specific star-formation rate (sSFR) to examine how the hot (T>=10^6 K) circumgalactic medium (CGM) differs for high- and low-sSFR galaxies. The simulations indicate that the hot CGM of low-mass (M_*~10^{10.5} Msol), high-sSFR (defined as the top one-third ranked by sSFR) central galaxies will be detectable to a galactocentric radius r~30-50 kpc. Both simulations predict lower luminosities at fixed stellar mass for the low-sSFR galaxies (the lower third of sSFR) with Illustris-TNG predicting 3x brighter coronae around high-sSFR galaxies than EAGLE. Both simulations predict detectable emission out to r~150-200 kpc for stacks centered on high-mass (M_*~10^{11.0} Msol) galaxies, with EAGLE predicting brighter X-ray halos. The extended soft X-ray luminosity correlates strongly and positively with the mass of circumgalactic gas within the virial radius (f_{CGM}). Prior analyses of both simulations have established that f_{CGM} is reduced by expulsive feedback driven mainly by black hole growth, which quenches galaxy growth by inhibiting replenishment of the ISM. Both simulations predict that eROSITA stacks should not only conclusively detect and resolve the hot CGM around L^* galaxies for the first time, but provide a powerful probe of how the baryon cycle operates, for which there remains an absence of consensus between state-of-the-art simulations., Comment: 10 pages, 4 figures, 1 table, Accepted ApJL

We explore the high spectral resolution X-ray data towards the quasar 3C273 to search for signals of hot ($\sim10^{6-7}$ K) X-ray-absorbing gas co-located with two established intergalactic FUV OVI absorbers. We analyze the soft X-ray band grating data of all XMM-Newton and Chandra instruments to search for the hot phase absorption lines at the FUV predicted redshifts. The viability of potential line detections is examined by adopting the constraints of a physically justified absorption model. The WHIM hypothesis is investigated with a complementary 3D galaxy distribution analysis, and by comparison of the measurement results to the WHIM properties in the EAGLE cosmological, hydrodynamical simulation. At FUV redshift z=0.09017, we measured signals of two hot ion species, OVIII and NeIX, with a $3.9\sigma$ combined significance level. Considering the line features in all instruments collectively and assuming collisional equilibrium for absorbing gas, we were able to constrain the temperature ($kT=0.26\pm0.03$ keV) and the column density ($N_H\times{Z_\odot/Z}=1.3_{-0.5}^{+0.6}\times10^{19}$ cm$^{-2}$) of the absorber. Thermal analysis indicates that FUV and X-ray absorption relate to different phases, with estimated temperatures $T_{FUV}\approx3\times10^5$ and $T_{X-ray}\approx3\times10^6$ K, which match the EAGLE predictions for WHIM at the FUV/X-ray measured $N_{ion}$-ranges. We detected a large scale galactic filament crossing the sightline at the redshift of the absorption, linking the absorption to this structure. This study provides insights into co-existing warm and hot gas within a WHIM filament and estimates the ratio of the hot and warm phases. Because the hot phase is thermally distinct from the OVI gas, the estimated baryon content of the absorber is increased, conveying the promise of X-ray follow-up studies of FUV detected WHIM in refining the picture of the missing baryons., Comment: 17 pages, 10 figures, accepted for publication in A&A

The relationship between galaxies and the state/chemical enrichment of the warm-hot intergalactic medium (WHIM) expected to dominate the baryon budget at low-z provides sensitive constraints on structure formation and galaxy evolution models. We present a deep redshift survey in the field of 1ES1553+113, a blazar with a unique combination of UV+X-ray spectra for surveys of the circum-/intergalactic medium (CGM/IGM). Nicastro et al. 2018 reported the detection of two O VII WHIM absorbers at $z=0.4339$ and $0.3551$ in its spectrum, suggesting that the WHIM is metal-rich and sufficient to close the missing baryons problem. Our survey indicates that the blazar is a member of a $z=0.433$ group and that the higher-$z$ O VII candidate arises from its intragroup medium. The resulting bias precludes its use in baryon censuses. The $z=0.3551$ candidate occurs in an isolated environment 630 kpc from the nearest galaxy (with stellar mass $\log M_*/M_\odot \approx 9.7$) which we show is unexpected for the WHIM. Finally, we characterize the galactic environments of broad H I Ly$\alpha$ absorbers (Doppler widths of $b=40-80$ \kms; $T\lesssim4\times10^5$ K) which provide metallicity independent WHIM probes. On average, broad Ly$\alpha$, absorbers are ${\approx}2\times$ closer to the nearest luminous ($L>0.25 L_*$) galaxy (700 kpc) than narrow ($b<30$ \kms; $T\lesssim4\times10^5$ K) ones (1300 kpc) but ${\approx}2\times$ further than O\,VI absorbers (350 kpc). These observations suggest that gravitational collapse heats portions of the IGM to form the WHIM but with feedback that does not enrich the IGM far beyond galaxy/group halos to levels currently observable in UV/X-ray metal lines., Comment: Accepted to ApJ Letters; 10 pages, 4 figures, 1 table with the full version available on the journal webpage (or by request); v2 corrects one reference

We describe a new approach to studying the intergalactic and circumgalactic medium in the local Universe: direct detection through narrow-band imaging of ultra-low surface brightness visible-wavelength line emission. We use the hydrodynamical cosmological simulation EAGLE to investigate the expected brightness of this emission at low redshift ($z$ $\lesssim$ 0.2). H$\alpha$ emission in extended halos (analogous to the extended Ly$\alpha$ halos/blobs detected around galaxies at high redshifts) has a surface brightness of $\gtrsim700$ photons cm$^{-2}$ sr$^{-1}$ s$^{-1}$ out to $\sim$100 kpc. Mock observations show that the Dragonfly Telephoto Array, equipped with state-of-the-art narrow-band filters, could directly image these structures in exposure times of $\sim$10 hours. H$\alpha$ fluorescence emission from this gas can be used to place strong constraints on the local ultra-violet background, and on gas flows around galaxies. Detecting H$\alpha$ emission from the diffuse intergalactic medium (the "cosmic web") is beyond current capabilities, but would be possible with a hypothetical 1000-lens Dragonfly array., Comment: 18 pages, 12 figures. Accepted for publication in ApJ

Davies et al. (2019) 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, 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 >~10x the binding energy resulting in gas-poor haloes. Observable tracers of the CGM, including CIV, OVI, and HI 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 timescales, 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_*=10^{10.2-10.7} M_solar (median u-r = 2.28) has a CGM mass that is 2.5x lower than the bluest quartile (u-r=1.59). We propose strategies for observing the predicted lower CGM column densities and covering fractions around galaxies hosting more massive BHs using the Cosmic Origins Spectrograph on Hubble., Comment: 15 pages, 10 figures, 1 table, submitted to MNRAS

We use the EAGLE cosmological, hydrodynamical simulations to predict the column density and equivalent width distributions of intergalactic O VII ($E=574$ eV) and O VIII ($E=654$ eV) absorbers at low redshift. These two ions are predicted to account for 40% of the gas-phase oxygen, which implies that they are key tracers of cosmic metals. We find that their column density distributions evolve little at observable column densities from redshift 1 to 0, and that they are sensitive to AGN feedback, which strongly reduces the number of strong (column density $N \gtrsim 10^{16} \, \mathrm{cm}^{-2})$ absorbers. The distributions have a break at $N \sim 10^{16} \, \mathrm{cm}^{-2}$, corresponding to overdensities of $\sim 10^{2}$, likely caused by the transition from sheet/filament to halo gas. Absorption systems with $N \gtrsim 10^{16} \mathrm{cm}^{-2}$ are dominated by collisionally ionized O VII and O VIII, while the ionization state of oxygen at lower column densities is also influenced by photoionization. At these high column densities, O VII and O VIII arising in the same structures probe systematically different gas temperatures, meaning their line ratio does not translate into a simple estimate of temperature. While O VII and O VIII column densities and covering fractions correlate poorly with the H I column density at $N_{\mathrm{H \, I}} \gtrsim 10^{15} \, \mathrm{cm}^{-2}$, O VII and O VIII column densities are higher in this regime than at the more common, lower H I column densities. The column densities of O VI and especially Ne VIII, which have strong absorption lines in the UV, are good predictors of the strengths of O VII and O VIII absorption and can hence aid in the detection of the X-ray lines., Comment: Accepted for publication in MNRAS. Minor changes w.r.t version 1 including updated values for Table 2

A large fraction of the baryons and most of the metals in the Universe are unaccounted for. They likely lie in extended galaxy halos, galaxy groups, and the cosmic web, and measuring their nature is essential to understanding galaxy formation. These environments have virial temperatures >10^5.5 K, so the gas should be visible in X-rays. Here we show the breakthrough capabilities of grating spectrometers to 1) detect these reservoirs of hidden metals and mass, and 2) quantify hot gas flows, turbulence, and rotation around the Milky Way and external galaxies. Grating spectrometers are essential instruments for future X-ray missions, and existing technologies provide 50-1500-fold higher throughput compared to current orbiting instruments., Comment: Submitted to the Astro2020 Decadal (6 pages, 5 figures)

The majority of baryons reside beyond the optical extent of a galaxy in the circumgalactic and intergalactic media (CGM/IGM). Gaseous halos are inextricably linked to the appearance of their host galaxies through a complex story of accretion, feedback, and continual recycling. The energetic processes, which define the state of gas in the CGM, are the same ones that 1) regulate stellar growth so that it is not over-efficient, and 2) create the diversity of today's galaxy colors, SFRs, and morphologies spanning Hubble's Tuning Fork Diagram. They work in concert to set the speed of growth on the star-forming Main Sequence, transform a galaxy across the Green Valley, and maintain a galaxy's quenched appearance on the Red Sequence. Most baryons in halos more massive than 10^12 Msolar along with their high-energy physics and dynamics remain invisible because that gas is heated above the UV ionization states. We argue that information on many of the essential drivers of galaxy evolution is primarily contained in this "missing" hot gas phase. Completing the picture of galaxy formation requires uncovering the physical mechanisms behind stellar and SMBH feedback driving mass, metals, and energy into the CGM. By opening galactic hot halos to new wavebands, we not only obtain fossil imprints of >13 Gyrs of evolution, but observe on-going hot-mode accretion, the deposition of superwind outflows into the CGM, and the re-arrangement of baryons by SMBH feedback. A description of the flows of mass, metals, and energy will only be complete by observing the thermodynamic states, chemical compositions, structure, and dynamics of T>=10^6 K halos. These measurements are uniquely possible with a next-generation X-ray observatory if it provides the sensitivity to detect faint CGM emission, spectroscopic power to measure absorption lines and gas motions, and high spatial resolution to resolve structures., Comment: White paper submitted to the Astro2020 Decadal Survey (5 pages, 2 figures)