Search

In the Halo21 absorption modelling challenge we generated synthetic absorption spectra of the circumgalactic medium (CGM), and attempted to estimate the metallicity, temperature, and density (Z, T , and n H) of the underlying gas using observational methods. We iteratively generated and analysed 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 n H retrieved within 0.1 dex of the source value for |$\gtrsim 90~{{\ \rm per\ cent}}$| 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. [ABSTRACT FROM AUTHOR]

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 probed a total of 138 projections from the simulation volumes, finding 16 directions with a total column density |$N_{{\rm O\, {\small VII}}} > 10^{14.5}$|  cm−2. The strongest absorbers are typically shifted by ±1000 km s−1 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 |${\rm O\, {\small VII}}$| and |${\rm O\, {\small VIII}}$| absorption against galaxy cluster cores will be feasible. An |${\rm O\, {\small VII}}$| detection with a 5σ significance can be achieved in 10–250 ks with Athena for most of the galaxy clusters considered. The |${\rm O\, {\small VIII}}$| detection becomes feasible only with a spectral resolution of around 1 eV, comparable to that envisioned for LEM. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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  O  vii (r) X-ray photons within the complex, multiphase, multiscale CGM. The resonant transition of the O  vii 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 |$10.0\lt \log {(M_\star /\rm {M_\odot })}\lt 11.0$| at z ≃ 0. After constructing a model for  O  vii (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  O  vii (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 ⋆ = 1010.7 M⊙ 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  O  vii (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 haloes with significant enhancements due to resonant scattering. [ABSTRACT FROM AUTHOR]

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 photoionization processes must be accounted for. In this paper, we study the ionization by photons emitted by the intracluster medium in addition to the photoionization by the cosmic ultraviolet/X-ray background on gas in the vicinity of galaxy clusters. For near-massive clusters such as A2029, the ionization parameter can no longer describe the ionization balance uniquely. The ionization 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 photoionization by the cosmic background are either an upper or lower limit to the ionization 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. [ABSTRACT FROM AUTHOR]

We estimate the detectability of X-ray metal-line emission from the circumgalactic medium (CGM) of galaxies over a large halo mass range (⁠|$\, {M}_{{\rm 200c}}=10^{11.5}$| – |$10^{14.5}\, {\rm M}_{\odot }$|⁠), using the EAGLE simulations. With the XRISM Resolve instrument, a few bright (K α or Fe L-shell) lines from |$\, {M}_{{\rm 200c}}\gtrsim 10^{13} \, {\rm M}_{\odot }$| 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 |$\, {M}_{{\rm 200c}}\gtrsim 10^{12.5}\, {\rm M}_{\odot }$| 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 |$\, {L}_{*}$| galaxies is accessible. These estimates do assume long exposure times (∼1 Ms) and large spatial bins (∼1– |$10\, \mathrm{arcmin}^{2}$|⁠). 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 (T peak), and high densities and metallicities. However, for the K α lines, emission can come from hotter gas in haloes where the virialized, volume-filling gas is hotter than T peak. Trends of emission with halo mass can largely be explained by differences in virial temperature. Differences in the mass trends of K α, He α-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 on to the hot CGM. [ABSTRACT FROM AUTHOR]

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 their 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 × 108 sightlines at 131 redshift (z) bins between 0 ≤ z ≤ 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 that ∼104 localized 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 lognormal redshift distribution at z  = 0.5 requires approximately 50 per cent fewer localized FRBs than a distribution centred at z  = 1. With the Square Kilometre Array expected to detect >103 FRBs per day, in the future, FRBs will be able to provide constraints on AGN feedback. [ABSTRACT FROM AUTHOR]

The dispersion measure (DM) of fast radio bursts (FRBs) provides a unique way to probe ionized baryons in the intergalactic medium (IGM). Cosmological models with different parameters lead to different DM–redshift (DM– z) relations. Additionally, the over/underdense regions in the IGM and the circumgalactic medium of intervening galaxies lead to scatter around the mean DM– z relations. We have used the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulations to measure the mean DM– z relation and the scatter around it using over 1 billion lines of sight at redshifts 0 < z < 3. We investigated two techniques to estimate line-of-sight DM: pixel scrambling and box transformations. We find that using box transformations (a technique from the literature) causes strong correlations due to repeated replication of structure. Comparing a linear and a non-linear model, we find that the non-linear model with a dependence on cosmological parameters provides a better fit to the DM– z relation. The differences between these models are the most significant at low redshifts (z < 0.5). The scatter around the DM– z relation is highly asymmetric, especially at low redshift (z < 0.5), and becomes more Gaussiana as redshift approaches z = 3, the limit of this study. The increase in Gaussianity with redshift is indicative of the large-scale structure that is better sampled with longer lines of sight. The DM– z relation measured in EAGLE is available with an easy-to-use python interface in the open-source FRB redshift estimation package fruitbat. [ABSTRACT FROM AUTHOR]

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 |$\,{M}_{\rm {200c}}= 10^{11}$| – |$10^{14.5} \, \rm {M}_{\odot}$| 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 |$\,{M}_{\rm {200c}}= 10^{12.0}{-}10^{13.5} \, \rm {M}_{\odot}$|⁠. We predict that the X-IFU will detect O  vii (O  viii) in 77 (46) per cent of the sightlines passing |$\,{M}_{\star }= 10^{10.5}{-}10^{11.0} \, \rm {M}_{\odot}$| galaxies within |$100 \, \rm {pkpc}$| (59 (82) per cent for |$\,{M}_{\star }\gt 10^{11.0} \, \rm {M}_{\odot}$|⁠). Hence, the X-IFU will probe covering fractions comparable to those detected with the Cosmic Origins Spectrograph for O  vi. [ABSTRACT FROM AUTHOR]

At low redshifts, the observed baryonic density falls far short of the total number of baryons predicted. Cosmological simulations suggest that these baryons reside in filamentary gas structures, known as the warm-hot intergalactic medium (WHIM). As a result of the high temperatures of these filaments, the matter is highly ionised such that it absorbs and emits far-UV and soft X-ray photons. Athena, the proposed European Space Agency X-ray observatory, aims to detect the "missing" baryons in the WHIM up to redshifts of z = 1 through absorption in active galactic nuclei and gamma-ray burst (GRB) afterglow spectra, allowing for the study of the evolution of these large-scale structures of the Universe. This work simulates WHIM filaments in the spectra of GRB X-ray afterglows with Athena using the SImulation of X-ray TElescopes framework. We investigate the feasibility of their detection with the X-IFU instrument, through O VII (E = 573 eV) and O VIII (E = 674 eV) absorption features, for a range of equivalent widths imprinted onto GRB afterglow spectra of observed starting fluxes ranging between 10−12 and 10−10 erg cm−2 s−1, in the 0.3−10 keV energy band. The analyses of X-IFU spectra by blind line search show that Athena will be able to detect O VII−O VIII absorption pairs with EWO VII > 0.13 eV and EWO VIII > 0.09 eV for afterglows with F > 2 × 10−11 erg cm−2 s−1. This allows for the detection of ≈ 45−137 O VII−O VIII absorbers during the four-year mission lifetime. The work shows that to obtain an O VII−O VIII detection of high statistical significance, the local hydrogen column density should be limited at NH < 8 × 1020 cm−2. [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]

We use the EAGLE cosmological, hydrodynamical simulations to predict the column density and equivalent width distributions of intergalactic O  vii (⁠|$E=574 \, \rm {eV}$|⁠) and O  viii (⁠|$E=654 \, \rm {eV}$|⁠) absorbers at low redshift. These two ions are predicted to account for |$40 \, \hbox{ per cent}$| 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 active galactic nucleus feedback, which strongly reduces the number of strong (column density |$N \gtrsim 10^{16} \, \rm {cm}^{-2}$|⁠) absorbers. The distributions have a break at |$N \sim 10^{16}\, \rm {cm}^{-2}$|⁠ , corresponding to overdensities of ∼102, likely caused by the transition from sheet/filament to halo gas. Absorption systems with |$N \gtrsim 10^{16} \, \rm {cm}^{-2}$| 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 |${N}_{\rm {H}\, \rm {I}} \gtrsim 10^{15} \, \rm {cm}^{-2}$|⁠ , 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. [ABSTRACT FROM AUTHOR]