Context. A significant fraction of the predicted baryons remain undetected in the local Universe. We adopted the common assumption that a large fraction of the missing baryons correspond 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 it is concentrated towards the filaments of the cosmic web. Aims. Our aim is to improve the observational search for the poorly detected hot WHIM. Methods. We detected the filamentary structure within the EAGLE hydrodynamical simulation by applying the Bisous formalism to the galaxy distribution. To test the reliability of our results, we used the MMF/NEXUS+ classification of the large-scale environment of the dark matter component in EAGLE. We then studied 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 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 Sunyaev-Zeldovich and cosmic microwave background lensing observations within the central r ≈ 1 Mpc. Conclusions. Results from the EAGLE simulation 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. To optimise the observational signal, it is beneficial to focus on the filaments with the highest galaxy luminosity densities detected in the optical data.
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%.
This paper presents an updated scaling relation between the optical luminosity density (LD) of galaxies in the r band and the density of the warm–hot intergalactic medium (WHIM) in cosmic filaments, using the high–resolution EAGLE simulations. We find a strong degree of correlation between the WHIM density and the galaxy luminosity density, resulting in a scaling relation between the two quantities that permits to predict the WHIM density of filaments with a scatter of less than 1/2 dex in a broad range of smoothed filament luminosity densities. In order to estimate the performance of the simulation–based calibration of the LD–WHIM density relation, we applied it to a sample of low–redshift filaments detected with the Bisous method in the Legacy Survey SDSS DR12 data. In the volume covered by the SDSS data, our relation predicts a WHIM density amounting to 31 ± 0.07 ± 0.12 per cent (statistical errors followed by systematic) of cosmic baryon density. This agrees, albeit within the large uncertainties, with the current estimates of the cosmological missing baryon fraction, implying that our LD - WHIM density relation may be a useful tool in the search for the missing baryons. This method of analysis provides a new promising avenue to study the physical properties of the missing baryons, using an observable that is available for large volumes of the sky, complementary and independent from WHIM searches with absorption–line systems in the FUV or X–rays.
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