An EAGLE view of the missing baryons

Abstract

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.

Properties of gas phases around cosmic filaments atz= 0 in the IllustrisTNG simulation

“…Precisely, we report in Table 2 the fractions of each gas phase in the first radial bin, that is r = 0.05 Mpc. We notice that, despite the decrease, WHIM remains the dominant phase in all types of filaments (accounting for ∼70% of the baryons), which is in qualitative agreement with previous findings at z = 0, using different simulations and filament finders (e.g., Nevalainen et al 2015;Cui et al 2018Cui et al , 2019Martizzi et al 2019;Tuominen et al 2021). We note that cores of filaments are also composed of hotter and denser gas (WCGM and especially hot gas).…”

“…5. We note that the temperature ranges of cores of filaments found in our analysis agree with the recent work of Tuominen et al (2021), who studied filaments detected with the Bisous algorithm (Tempel et al 2016) in the EAGLE simulation (Schaye et al 2015).…”

Gas distribution from clusters to filaments in IllustrisTNG

“…For these reasons, the gaseous component of the cosmic web is becoming the subject of more and more studies, with the aim to construct a comprehensive picture of the baryonic physical processes (heating, cooling, shocks, etc.) that gas undergoes during its transit from one cosmic environment to another (Gheller & Vazza 2019;Martizzi et al 2019;Tuominen et al 2021;Zhu et al 2021a). This interest on cosmic gas is further enhanced by the prospect of future missions which will allow us to explore the hidden cosmic gas with unprecedented accuracy in the coming years (Simionescu et al 2021).…”

Sunyaev Zel’dovich high resolution view of filamentary structures between galaxy clusters pairs

“…Observations of the baryonic matter based on HI absorption, X-ray emission, gas, stars, and galaxies counts provide a smaller baryonic fraction with respect to nucleosynthesis predictions, high-redshift measures of the Lyman-alpha forest absorption, and CMB power spectrum fits [10,21]. Hydrodynamical simulations suggest that the fraction of the baryonic matter not directly observable, about half of the total budget, should lie outside galaxy clusters in the form of filamentary structures connecting the clusters forming the socalled cosmic web [7,23]. These baryons must be characterized by a low-density phase and temperature in the range: 10 5 < T < 10 7 K. The direct observation of missing baryons with X-rays and radio instruments is very challenging because:…”