Warm-hot Intergalactic Medium Research Articles

The SRG/eROSITA all-sky survey. X-ray emission from the warm-hot phase gas in long cosmic filaments

The properties of the warm-hot intergalactic medium (WHIM) in cosmic filaments are among the least quantified units in modern astrophysics. The Spectrum Roentgen Gamma/eROSITA All Sky Survey ((SRG/eRASS) provides a unique opportunity to study the X-ray emission of the WHIM. We applied both imaging and spectroscopic stacking techniques to the data of the first four eRASS scans to inspect the X-ray emissions from 7817 cosmic filaments identified from Sloan Digital Sky Survey (SDSS) optical galaxy samples. We obtained a $9 significant detection of the total X-ray signal from filaments in the 0.3--1.2 keV band. Here, we introduce a novel method to estimate the contamination fraction from unmasked X-ray halos, active galactic nuclei, and X-ray binaries associated with filament galaxies. We found an approximately 40<!PCT!> contamination fraction for these unmasked sources, suggesting that the remaining 60<!PCT!> of the signal could be coming from the WHIM and a $5.4 detection significance of the WHIM. Moreover, we modeled the temperature and baryon density contrast of the detected WHIM by fitting the stacked spectrum and surface brightness profile. The best-fit temperature $ K obtained by using a single temperature model, is marginally higher than in the simulation results. This could be due to the fitting of a single temperature model on a multi-temperature spectrum. Assuming a 0.2 solar abundance, the best-fit baryon density contrast $ b is in general agreement with the X-ray emitting phases in the IllustrisTNG simulation. This result suggests that the broadband X-ray emission traces the high end of the temperature and density values that characterize the entire WHIM population.

Probing the magnetized gas distribution in galaxy groups and the cosmic web with POSSUM Faraday rotation measures

ABSTRACT We present initial results from the Polarization Sky Survey of the Universe’s Magnetism (POSSUM), analysing 22 817 Faraday rotation measures (RMs) with median uncertainties of 1.2 rad m$^{-2}$ across 1520 deg2 to study magnetized gas associated with 55 nearby galaxy groups ($z\lesssim 0.025$) with halo masses between $10^{12.5}$ and $10^{14.0}$ M$_\odot$. We identify two distinct gas phases: the intragroup medium (IGrM) within 0–2 splashback radii and the warm-hot intergalactic medium (WHIM) extending from 2 to 7 splashback radii. These phases enhance the standard deviation of residual (i.e. Galactic foreground RM-subtracted) RMs by $6.9\pm 1.8$ rad m$^{-2}$ and $4.2 \pm 1.2$ rad m$^{-2}$, respectively. Estimated magnetic field strengths are several μG within the IGrM and 0.1–1 μG in the WHIM. We estimate the plasma $\beta$ in both phases, and show that magnetic pressure might be more dynamically important than in the ICM of more massive clusters or sparse cosmic web filaments. Our findings indicate that ‘missing baryons’ in the WHIM likely extend beyond the gravitational radii of group-mass haloes to Mpc scales, consistent with large-scale, outflow-driven ‘magnetized bubbles’ seen in cosmological simulations. We demonstrate that RM grids are an effective method for detecting magnetized thermal gas at galaxy group interfaces and within the cosmic web. This approach complements X-ray and Sunyaev-Zel’dovich effect methods, and when combined with fast radio burst dispersion measures, data from the full POSSUM survey – comprising approximately a million RMs – will allow direct magnetic field measurements to further our understanding of baryon circulation in these environments and the magnetized universe.

OLIMPO: A balloon-borne SZE imager to probe ICM dynamics and the WHIM

OLIMPO is a proposed Antarctic balloon-borne Sunyaev-Zel’dovich effect (SZE) imager to study gas dynamics associated with structure formation along with the properties of the warm-hot intergalactic medium (WHIM) residing in the connective filaments. During a 25 day flight OLIMPO will image a total of 10 z∼0.05 galaxy clusters and 8 bridges at 145, 250, 365, and 460 GHz at an angular resolution of 1.0′–3.3′. The maps will be significantly deeper than those planned from CMB-S4 and CCAT-P, and will have excellent fidelity to the large angular scales of our low-z targets, which are difficult to probe from the ground. In combination with X-ray data from eROSITA and XRISM we will transform our current static view of galaxy clusters into a full dynamic picture by measuring the internal intra-cluster medium (ICM) velocity structure with the kinematic SZE, X-ray spectroscopy, and the power spectrum of ICM fluctuations. Radio observations from ASKAP and MeerKAT will be used to better understand the connection between ICM turbulence and shocks with the relativistic plasma. Beyond the cluster boundary, we will combine thermal SZE maps from OLIMPO with X-ray imaging from eROSITA to measure the thermodynamics of the WHIM residing in filaments, providing a better understanding of its properties and its contribution to the total baryon budget.

Cross Correlation between the Thermal Sunyaev–Zeldovich Effect and the Integrated Sachs–Wolfe Effect

We present a joint cosmological analysis of the power spectra measurement of the Planck Compton parameter and the integrated Sachs–Wolfe (ISW) maps. We detect the statistical correlation between the Planck thermal Sunyaev–Zeldovich (tSZ) map and ISW data with a significance of a 3.6σ confidence level (CL), with the autocorrelation of the Planck tSZ data being measured at a 25σ CL. The joint auto- and cross-power spectra constrain the matter density to be Ωm=0.317−0.031+0.040 , the Hubble constant to be H0=66.5−1.9+2.0kms−1Mpc−1 , and the rms matter density fluctuations to be σ8=0.730−0.037+0.040 at the 68% CL. The derived large-scale structure S 8 parameter is S8≡σ8(Ωm/0.3)0.5=0.755±0.060 . If using only the diagonal blocks of covariance matrices, the Hubble constant becomes H0=69.7−1.5+2.0kms−1Mpc−1 . In addition, we obtain the constraint of the product of the gas bias, gas temperature, and density as bgasTe/(0.1keV)n¯e/1m−3=3.09−0.380+0.320 . We find that this constraint leads to an estimate on the electron temperature today as Te=(2.40−0.300+0.250)×106K , consistent with the expected temperature of the warm–hot intergalactic medium. Our studies show that the ISW–tSZ cross correlation is capable of probing the properties of the large-scale diffuse gas.

Soft X-ray emission from warm gas in IllustrisTNG circum-cluster environments

Context. Whereas X-ray clusters are extensively used for cosmology, their idealised modelling, through the hypotheses of spherical symmetry and hydrostatic equilibrium, is being questioned more and more. The soft X-ray emission detected in tens of clusters with ROSAT was found to be higher than what was expected from the idealised hot gas modelling, pointing to our incomplete understanding of these objects. Aims. Given that cluster environments are at the interface between the hot intra-cluster medium (ICM), warm circum-galactic medium (WCGM), and warm-hot intergalactic medium (WHIM), we aim to explore the relative soft X-ray emission of different gas phases in circum-cluster environments. Method. By using the most massive halos in IllustrisTNG at z = 0, we have predicted the hydrodynamical properties of the gas from cluster centres to their outskirts (5 R200), and modelled their X-ray radiation for various plasma phases. Results. First, we found that the radial profile of temperature, density, metallicity and clumpiness of the ICM are in good agreement with recent X-ray observations of clusters. Secondly, we have developed a method to predict the radial profile of soft X-ray emission in different bands, the column density of ions, and the X-ray absorption lines (O VIII, O VII, Ne IX, and Ne IX) of warm-hot gas inside and around clusters. Conclusion. The warm gas (in the form of both WCGM and WHIM gas) is a strong emitter in soft X-ray bands and is qualitatively consistent with the observational measurements. Our results suggest that the cluster soft excess is induced by the thermal emission of warm gas in the circum-cluster environments.

Sample Variance in Cosmological Observations with a Narrow Field of View

Surveys with a narrow field of view can play an important role in probing cosmology, but inferences from these surveys suffer from large sample variance, arising from random fluctuations around the cosmic mean. The standard method for computing the sample variance is based on two key approximations: treating perturbations linearly and the survey geometry as a box. We demonstrate that it can lead to a significant underestimate of the sample variance in narrow surveys. We present a new method for accurately computing the sample variance and apply our method to the recent observations of the warm–hot intergalactic medium (WHIM) based on spectroscopic measurements of blazars. We find that the sample variances in these surveys are significantly larger than the quoted measurement errors; for example, the cosmic mean baryon density contained in the WHIM could be lower by 54% at 1σ fluctuation than estimated in one observation. Accurately quantifying the sample variance is essential in deriving correct interpretations of the measurements in surveys with a small field of view.

WHIM-hunting through cross-correlations between optical and SZ effect data in the Virgo cluster filaments

Context. The physical state of most of the baryonic matter in the local universe is unknown, commonly referred to as the “missing baryon problem”. It has been theorized that at least half of these missing baryons are in a warm-hot, low-density phase, outside of the virialized dark-matter halos. Aims. We attempted to find the signature of this warm-hot intergalactic medium (WHIM) phase in the filaments of the nearby Virgo cluster by using optical and Sunyaev-Zeldovich effect data. Methods. Specifically, we used a filament-galaxy catalog created from the HyperLeda database and an all-sky Compton-y map extracted from the Planck satellite data for two-dimensional cross-correlation analysis by applying a spherical harmonics transform. The significance test is based on the null-test simulations, which exploits advanced cut-sky analysis tools for a proper map reconstruction. To place upper limits on the WHIM density in the Virgo filaments, realistic baryonic density modeling within the cosmic filaments was done based on state-of-the-art hydro-simulations, within the signal-boosting routine. Results. The cross-correlation signal is found to be too dim compared to the noise level in the Plancky-map. At a 3 σ confidence level, the upper limit on volume-average WHIM density turns out to be ⟨ ne ⟩< 4 × 10−4 cm−3, which is indeed consistent with the WHIM parameter space, as predicted from simulations.

Probing the hot circumgalactic medium of external galaxies in X-ray absorption II: a luminous spiral galaxy at z ≈ 0.225

ABSTRACT The circumgalactic medium (CGM) is the most massive baryonic component of a spiral galaxy, shock heated to about 106 K for an $\rm L^{\star }$ galaxy. The CGM of the Milky Way has been well-characterized through X-ray absorption line spectroscopy. However, the paucity of bright background sources makes it challenging to probe the CGM of external galaxies. Previously, using broad O vi absorption as a signpost, we successfully detected the CGM of one galaxy in X-rays. Here we report on the detection of the O vii K α absorption line at the redshift of a spiral galaxy at z ≈ 0.225, using 1.2 Ms of Chandra observations. This is a robust detection, clearly showing the presence of the hot gas. The mass in the hot phase is at least an order of magnitude larger than that in the cooler phases detected in the UV. The presence of hot gas 116 h−1 kpc from the centre of this galaxy provides credence to the existence of the extended CGM of the Milky Way. There has been a report of the detection of O vii absorption from the warm-hot intergalactic medium in this sightline, using stacking analysis on older data set. We argue that the absorption line is from the CGM of the z ≈ 0.225 galaxy instead.

Abundance and temperature of the outer hot circumgalactic medium

Context. Despite their vital importance to understanding galaxy evolution and our own Galactic ecosystem, our knowledge of the physical properties of the hot X-ray emitting phase of the Milky Way is still inadequate. However, sensitive SRG/eROSITA large area surveys are now providing us with the long-sought data needed to mend this state of affairs. Aims. Our aim is to constrain the properties of the Milky Way hot halo emission toward intermediate Galactic latitudes close to the Galactic anti-center. Methods. We analyzed the spectral properties of the integrated soft X-ray emission observed by eROSITA in the relatively deep eFEDS field. Results. We observe a flux of 12.6 and 5.1 × 10−12 erg cm−2 s−1 deg−2 in the total (0.3–2) and soft (0.3–0.6 keV) band. We measure the temperature and metal (oxygen) abundance of the hot circumgalactic medium (CGM) to be within kTCGM = 0.153–0.178 keV and ZCGM = 0.052–0.072 Z⊙, depending on the contribution of solar wind charge exchange (SWCX). Slightly higher CGM abundances ZCGM = 0.05–0.10 Z⊙ are possible, considering the uncertain extrapolation of the extragalactic cosmic X-ray background (CXB) emission below ~1 keV. To recover CGM abundances as high as ZCGM = 0.3 Z⊙, the presence of an additional component must be postulated, likely associated with the warm-hot intergalactic medium, providing ~15–20% of the flux in the soft X-ray band. We observe line widths of the CGM plasma smaller than Δυ ≤ 500 km s−1. The emission in the soft band is dominated (~47%) by the circumgalactic medium (CGM), whose contribution reduces to ~30% if heliospheric SWCX contributes at the level of ~15% also during solar minimum. The remaining flux is provided by the CXB (~33%) and the local hot bubble (~18%). Moreover, the eROSITA data require the presence of an additional component associated with the elusive Galactic corona plus a possible contribution from unresolved M dwarf stars. This component has a temperature of kT ~ 0.4– 0.7 keV, a considerable (~ kiloparsec) scale height, and might be out of thermal equilibrium. It contributes ~9% to the total emission in the 0.6—2 keV band, and is therefore a likely candidate to produce part of the unresolved CXB flux observed in X-ray ultra-deep fields. We also observe a significant contribution to the soft X-ray flux due to SWCX, during periods characterized by stronger solar wind activity, and causing the largest uncertainty on the determination of the CGM temperature. Conclusions. We constrain temperature, emission measure, abundances, thermal state, and spectral shape of the outer hot CGM of the Milky Way.

A search for the missing baryons with X-ray absorption lines towards the blazar 1ES 1553+113

ABSTRACT This paper presents an analysis of XMM–Newton and Chandra X-ray spectra of the quasar 1ES 1553+113, in search for absorption lines from the intervening warm–hot intergalactic medium (WHIM). A search for O vii, O viii, and Ne ix resonance absorption lines was performed at eight fixed redshifts that feature O vi or H i broad Lyman α absorption lines that were previously detected from Hubble Space Telescope (HST) data. The search yielded one possible detection of O vii at a redshift z ≃ 0.1877 with an O vi prior, with a statistical significance that is equivalent to a 2.6σ confidence level. The spectra were also stacked at the wavelengths of the expected redshifted O vii and O viii lines, but the analysis did not reveal evidence for the presence of additional X-ray absorbing WHIM. Moreover, the spectra were used to investigate two putative O vii absorption lines that were detected serendipitously in an earlier analysis of the same data by F. Nicastro and collaborators. The paper also presents a comprehensive statistical framework for cosmological inferences from the analysis of absorption lines, which makes use of cosmological simulations for the joint probability distributions of far-ultraviolet (FUV) and X-ray ions. Accordingly, we conclude that the new possible O vii absorption at z ≃ 0.1877 is consistent with a contribution from the hot WHIM to the baryon density in an amount of ΩWHIM, X/Ωb = 44 ± 22 per cent. However, there are large systematic uncertainties associated with the temperature and abundances of the absorbers, and only a larger sample of X-ray sources can provide an accurate determination of the cosmological density of the WHIM.

Prospects of detecting soft X-ray emission from typical WHIM filaments around massive clusters and the coma cluster soft excess

ABSTRACT While hot ICM in galaxy clusters makes these objects powerful X-ray sources, the cluster’s outskirts and overdense gaseous filaments might give rise to much fainter sub-keV emission. Cosmological simulations show a prominent ‘focusing’ effect of rich clusters on the space density of the warm-hot intergalactic medium (WHIM) filaments up to a distance of $\sim 10\, {\rm Mpc}$ (∼ turnaround radius, rta) and beyond. Here, we use Magneticum simulations to characterize their properties in terms of integrated emission measure for a given temperature and overdensity cut and the level of contamination by the more dense gas. We suggest that the annuli $(\sim 0.5-1)\times \, r_{ta}$ around massive clusters might be the most promising sites for the search of the gas with overdensity ≲ 50. We model spectral signatures of the WHIM in the X-ray band and identify two distinct regimes for the gas at temperatures below and above $\sim 10^6\, {\rm K}$. Using this model, we estimate the sensitivity of X-ray telescopes to the WHIM spectral signatures. We found that the WHIM structures are within reach of future high spectral resolution missions, provided that the low-density gas is not extremely metal-poor. We then consider the Coma cluster observed by SRG/eROSITA during the CalPV phase as an example of a nearby massive object. We found that beyond the central r ∼ 40 arcmin ($\sim 1100\, {\rm kpc}$) circle, where calibration uncertainties preclude clean separation of the extremely bright cluster emission from a possible softer component, the conservative upper limits are about an order of magnitude larger than the levels expected from simulations.

Studying the warm hot intergalactic medium in emission: a reprise

ABSTRACT The warm-hot intergalactic medium (WHIM) contains a significant portion of the ‘missing baryons’. Its detection in emission remains a challenge. Integral field spectrometers like X-IFU on board of the Athena satellite will secure WHIM detection in absorption and emission and, for the first time, allow us to investigate its physical properties. In our research, we use the CAMELS simulations to model the surface brightness maps of the OVII and OVIII ion lines and compute summary statistics like photon counts and 2-point correlation functions to infer the properties of the WHIM. Our findings confirm that detectable WHIM emission is primarily associated with galaxy haloes, and the properties of the WHIM show minimal evolution from z ∼ 0.5 to the present time. By exploring a wide range of parameters within the CAMELS suite, we investigate the sensitivity of WHIM properties to cosmology and energy feedback mechanisms influenced by active galactic nuclei and stellar activity. This approach allows us to separate the cosmological aspects from the baryonic processes and place constraints on the latter. Additionally, we provide forecasts for WHIM observations using a spectrometer similar to X-IFU. We anticipate detecting 1–3 WHIM emission lines per pixel and mapping the WHIM emission profile around haloes up to a few tens of arcminutes, surpassing the typical size of a WHIM emitter. Overall, our work demonstrates the potential of emission studies to probe the densest phase of the WHIM, shedding light on its physical properties and offering insights into the cosmological and baryonic processes at play.

Boundless baryons: how diffuse gas contributes to anisotropic tSZ signal around simulated Three Hundred clusters

ABSTRACT Upcoming advances in galaxy surveys and cosmic microwave background data will enable measurements of the anisotropic distribution of diffuse gas in filaments and superclusters at redshift z = 1 and beyond, observed through the thermal Sunyaev–Zel’dovich (tSZ) effect. These measurements will help distinguish between different astrophysical feedback models, account for baryons that appear to be ‘missing’ from the cosmic census, and present opportunities for using locally anisotropic tSZ statistics as cosmological probes. This study seeks to guide such future measurements by analysing whether diffuse intergalactic gas is a major contributor to anisotropic tSZ signal in The Three Hundred Gizmo-Simba hydrodynamic simulations. We apply multiple different halo boundary and temperature criteria to divide concentrated from diffuse gas at z = 1, then create mock Compton- y maps for the separated components. The maps from 98 simulation snapshots are centred on massive galaxy clusters, oriented by the most prominent filament axis in the galaxy distribution, and stacked. Results vary significantly depending on the definition used for diffuse gas, indicating that assumptions should be clearly defined when claiming observations of the warm-hot intergalactic medium. In all cases, the diffuse gas is important, contributing 25–60 per cent of the tSZ signal in the far field (>4 h−1 comoving Mpc) from the stacked clusters. The gas 1–2 virial radii from halo centres is especially key. Oriented stacking and environmental selections help to amplify the signal from the warm-hot intergalactic medium, which is aligned but less concentrated along the filament axis than the hot halo gas.

Cosmic gas highways in C-EAGLE simulations

Context. A substantial fraction of cosmic baryons is expected to hide in the form of diffuse warm-hot intergalactic medium (WHIM) at X-ray temperatures (T = 105 − 107 K). Due to the expected low density of WHIM, it has been very difficult to detect so far. A statistically significant sample of credible detections of the WHIM phase might help solve the problem of the missing cosmic baryons. While the majority of cosmic gas is approximately at rest inside the filaments of the Cosmic Web, the fraction of gas located close to galaxy clusters is falling towards them with substantial velocities. The infalling gas is influenced by the increasing density in the cluster vicinity and eventually undergoes a termination shock, which may boost its X-ray signal. Thus, the cluster outskirts are potential locations for improved detectability of the missing baryons. Aims. The primary goal of this work is to identify optimal locations of the enhanced X-ray emission and absorption, arising from the interaction of infalling filamentary gas with cluster material. Our further goal is to improve our understanding of the various physical processes affecting WHIM as it falls towards clusters of galaxies along the cosmic filaments. We aim to utilise this information for planning future X-ray observations of WHIM in cluster outskirts. Methods. We applied the DisPerSE filament finder to the galaxy distribution in the surroundings of a single Coma-like (M200 ∼ 1015.4 M⊙) simulated C-EAGLE cluster of galaxies. We characterised the distribution of the thermodynamic properties of the gas in such filaments and provided a physical interpretation for the results. This analysis serves as a proof of method to be applied to the full C-EAGLE sample in a future work. Results. We captured a large fraction (∼50%) of the hot (T > 105.5 K) gas falling towards the cluster in the detected filaments in the cluster outskirts. The gas in the filaments is in approximate free fall all the way down to the radial distance of ∼2 r200 from the cluster. At smaller radii, the filament gas begins to slow down due to the increasing pressure of the ambient gas; approximately half of the filament gas nevertheless penetrates into the cluster before being decelerated. The deceleration is accompanied by the conversion of gas bulk kinetic energy into heat. As a result, the density and temperature of the gas in the filaments increase from the general Cosmic Web level of ρ ∼ 10ρav (where ρav is the cosmic mean baryon density) and T = 105 − 106 K at r ∼ 4 r200 towards ρ ∼ 100ρav and T = 107 − 108 K at the virial boundary of the very massive cluster studied in this paper. Conclusions. The detection of the cosmic filaments of galaxies around clusters may provide a practical observational avenue for locating the densest and hottest phase of the missing baryons.

Systematic errors in the maximum-likelihood regression of Poisson count data: introducing the overdispersed χ2 distribution

ABSTRACT This paper presents a new method to estimate systematic errors in the maximum-likelihood regression of count data. The method is applicable in particular to X-ray spectra in situations where the Poisson log-likelihood, or the Cash goodness-of-fit statistic, indicate a poor fit that is attributable to overdispersion of the data. Overdispersion in Poisson data is treated as an intrinsic model variance that can be estimated from the best-fit model, using the maximum-likelihood Cmin statistic. The paper also studies the effects of such systematic errors on the ΔC likelihood-ratio statistic, which can be used to test for the presence of a nested model component in the regression of Poisson count data. The paper introduces an overdispersed χ2 distribution that results from the convolution of a χ2 distribution that models the usual ΔC statistic, and a zero-mean Gaussian that models the overdispersion in the data. This is proposed as the distribution of choice for the ΔC statistic in the presence of systematic errors. The methods presented in this paper are applied to XMM–Newton data of the quasar 1ES 1553+113 that were used to detect absorption lines from an intervening warm-hot intergalactic medium (WHIM). This case study illustrates how systematic errors can be estimated from the data, and their effect on the detection of a nested component, such as an absorption line, with the ΔC statistic.

Searching for the warm-hot intergalactic medium usingXMM–Newtonhigh-resolution X-ray spectra

ABSTRACTThe problem of missing baryons in the local universe remains an open question. One proposed alternative is that at low redshifts missing baryons are in the form of the warm-hot intergalactic medium (WHIM). In order to test this idea, we present a detailed analysis of X-ray high-resolution spectra of six extragalactic sources, Mrk 421, 1ES 1028+511, 1ES 1553+113, H2356−309, PKS 0558−504, and PG 1116+215, obtained with the XMM–Newton Reflection Grating Spectrometer to search for signals of WHIM and/or circumgalactic medium X-ray-absorbing gas. We fit the X-ray absorption with the IONeq model, allowing us to take into account the presence of X-ray spectral features due to the multiphase component of the local interstellar medium (ISM). An additional IONeq component is included to model the WHIM absorption, instead of the traditional Gaussian absorption line modelling. We found no statistical improvement in the fits when including such component in any of the sources, concluding that we can safely reject a successful detection of WHIM absorbers towards these lines of sights. Our simulation shows that the presence of the multiphase ISM absorption features prevents the detection of low-redshift WHIM absorption features in the >17 Å spectral region for moderate exposures using high-resolution spectra.

Cosmic metal invaders: Intergalactic O VII as a tracer of the warm-hot intergalactic medium within cosmic filaments in the EAGLE simulation

Context. The current observational status of the hot (log T(K) > 5.5) intergalactic medium (IGM) remains incomplete. While recent X-ray emission and Sunyaev-Zeldovich effect observations from stacking large numbers of Cosmic Web filaments have yielded statistically significant detections of this phase, direct statistically significant measurements of single objects remain scarce. The lack of such a sample currently prevents a robust analysis of the cosmic baryon content composed of the hot IGM, which would potentially help solve the cosmological missing baryons problem. Aims. In order to improve the observationally challenging search for the missing baryons, we utilise the theoretical avenue afforded by the EAGLE simulations. Our aim is to get insights into the metal enrichment of the Cosmic Web and the distribution of highly ionised metals in the IGM. Our goal is to aid in the planning of future X-ray observations of the hot intergalactic plasma. Methods. We detected the filamentary network by applying the Bisous formalism to galaxies in the EAGLE simulation. We characterised the spatial distributions of oxygen and O VII and studied their mass and volume filling fractions in the filaments. Since oxygen is formed in and expelled from galaxies, we also studied the surroundings of haloes. We used this information to construct maps of the O VII column density and determine the feasibility of detecting it via absorption with Athena X-IFU. Results. Within EAGLE, the oxygen and O VII number densities drop dramatically beyond the virial radii of haloes. In the most favourable scenario, the median extent of O VII above the Athena X-IFU detection limit is ≈700 kpc. Since galaxies are relatively far apart from one another, only ∼1% of the filament volumes are filled with O VII at high enough column densities to be detectable by X-IFU. The highly non-homogeneous distribution of the detectable O VII complicates the usage of the measurements of the intergalactic O VII absorbers for tracing the missing baryons and estimating their contribution to the cosmic baryon budget. Instead, the detectable volumes form narrow and dense envelopes around haloes, while the rest of the O VII is diluted at low densities within the full filament volumes. This localised nature, in turn, results in a low chance (∼10−20% per sight line) of detecting intergalactic O VII with Athena X-IFU within the observational SDSS catalogue of nearby filaments. Fortunately, with deeper filament samples, such as those provided via the future 4MOST 4HS survey, the chances of intercepting an absorbing system are expected to increase up to a comfortable level of ∼50% per sight line. Conclusions. Based on EAGLE results, targeting the Cosmic Web with Athena may only result in tip-of-the-iceberg detections of the intergalactic O VII, which is located in the galaxy outskirts. This would not be enough to conclusively solve the missing baryon problem. However, the projection of many filaments into a single line of sight will enable a useful X-ray observation strategy with Athena X-IFU for the hot cosmic baryon gas, reducing the amount of baryons still missing by up to ∼25%.

Looking for the signals of the missing baryons in the extragalactic background light

The missing baryons in the universe are assumed to be hidden in the whole space as a warm–hot intergalactic medium (WHIM). Finding them is one of the important subjects in modern cosmology. In this paper, we point out that the very high-energy electron–positron rays may light up the WHIM due to the anomalous bremsstrahlung according to an improved Bethe–Heitler formula. The resulting excess of the extragalactic background light (EBL) can be observed by the direct measurement method. A possible explanation on the difference between the direct and indirect measurements of EBL is also proposed. Thus, we find a new window to probe the WHIM properties via the EBL.

The physical nature of circumgalactic medium absorbers in Simba

ABSTRACT We study the nature of the low-redshift circumgalactic medium (CGM) in the Simba cosmological simulations as traced by ultraviolet absorption lines around galaxies in bins of stellar mass ($\mbox{$M_\star $}\, > 10^{10}{\rm M}_\odot$) for star-forming, green valley and quenched galaxies at impact parameters r⊥ ≤ 1.25r200. We generate synthetic spectra for H i , Mg ii , C ii , Si iii , C iv , and O vi , fit Voigt profiles to obtain line properties, and estimate the density, temperature, and metallicity of the absorbing gas. We find that CGM absorbers are most abundant around star-forming galaxies with $\mbox{$M_\star $}\, < 10^{11}\,\,{\rm M}_{\odot }$, while the abundance of green valley galaxies show similar behaviour to those of quenched galaxies, suggesting that the CGM ‘quenches’ before star formation ceases. H i absorbing gas exists across a broad range of cosmic phases [condensed gas, diffuse gas, hot halo gas, and Warm-Hot Intergalactic Medium (WHIM)], while essentially all low ionization metal absorption arises from condensed gas. O vi absorbers are split between hot halo gas and the WHIM. The fraction of collisionally ionized CGM absorbers is $\sim 25{\text{--}}55{{\ \rm per\ cent}}$ for C iv and $\sim 80{\text{--}}95{{\ \rm per\ cent}}$ for O vi , depending on stellar mass and impact parameter. In general, the highest column density absorption features for each ion arise from dense gas. Satellite gas, defined as that within 10r1/2,⋆, contributes $\sim 3{{\ \rm per\ cent}}$ of overall H i absorption but $\sim 30{{\ \rm per\ cent}}$ of Mg ii absorption, with the fraction from satellites decreasing with increasing ion excitation energy.

X-ray absorption lines in the warm–hot intergalactic medium: probing Chandra observations with the CAMEL simulations

ABSTRACT Known as the ‘Missing Baryon Problem’, about one-third of baryons in the local universe remain unaccounted for. The missing baryons are thought to reside in the warm–hot intergalactic medium (WHIM) of the cosmic web filaments, which are challenging to detect. Recent Chandra X-ray observations used a novel stacking analysis and detected an O vii absorption line towards the sightline of a luminous quasar, hinting that the missing baryons may reside in the WHIM. To explore how the properties of the O vii absorption line depend on feedback physics, we compare the observational results with predictions obtained from the Cosmology and Astrophysics with MachinE Learning (CAMEL) Simulation suite. CAMELS consists of cosmological simulations with state-of-the-art supernova (SN) and active galactic nuclei (AGNs) feedback models from the IllustrisTNG and SIMBA simulations, with varying strengths. We find that the simulated O vii column densities are higher in the outskirts of galaxies than in the large-scale WHIM, but they are consistently lower than those obtained in the Chandra observations, for all feedback runs. We establish that the O vii distribution is primarily sensitive to changes in the SN feedback prescription, whereas changes in the AGN feedback prescription have minimal impact. We also find significant differences in the O vii column densities between the IllustrisTNG and SIMBA runs. We conclude that the tension between the observed and simulated O vii column densities cannot be explained by the wide range of feedback models implemented in CAMELS.