The eROSITA view of the Abell 3391/95 field: Case study from the Magneticum cosmological simulation

Abstract

Context. Clusters of galaxies reside at the nodes of the cosmic web, interconnected by filamentary structures that contain tenuous diffuse gas, especially in the warm-hot phase. Galaxy clusters grow by mergers of smaller objects and gas that are mainly accreted through these large-scale filaments. For the first time, the large-scale cosmic structure and a long gas-emission filament have been captured by eROSITA on board the Spectrum-Roentgen-Gamma mission in a direct X-ray observation of the A3391/95 field. Aims. We investigate the assembly history of an A3391/95-like system of clusters and the thermo-chemical properties of the diffuse gas in it by connecting simulation predictions to the eROSITA observations with the aim to constrain the origin and nature of the gas in the pair-interconnecting bridge. Methods. We analysed the properties of a system resembling A3391/95, extracted from the (352 h−1 cMpc)3 volume of the Magneticum Pathfinder cosmological simulations at z = 0.07. We tracked the main progenitors of the pair clusters and of surrounding groups back in time to study the assembly history of the system and its evolution. Results. Similarly to the observed A3391/95 system, the simulated cluster pair is embedded in a complex network of gas filaments, with structures aligned over more than 20 projected Mpc, and the whole region collapses towards the central overdense node. The spheres of influence (3 × R200) of the two main clusters already overlap at z = 0.07, but their virial boundaries are still physically separated. The diffuse gas located in the interconnecting bridge closely reflects the warm-hot intergalactic medium, with a typical temperature of ~1 keV and an overdensity δ ~ 100 with respect to the mean baryon density of the Universe, and a lower enrichment level compared to the intra-cluster medium in clusters. We find that most of the bridge gas collapsed from directions roughly orthogonal to the intra-cluster gas accretion directions, and its origin is mostly unrelated to the two cluster progenitors. We find clear signatures in the surrounding groups of infall motion towards the pair, such as significant radial velocities and a slowdown of gas compared to dark matter. These findings further support the hypothesis that the Northern Clump (MCXC J0621.7-5242) cluster infalls along a cosmic gas filament towards Abell 3391 and might be merging with it. Conclusions. We conclude that in this configuration, the pair clusters of the A3391/95-like system are in a pre-merger phase and have not yet interacted. The diffuse gas in the interconnecting bridge is mostly warm filament gas and not tidally stripped cluster gas.

eROSITA spectro-imaging analysis of the Abell 3408 galaxy cluster

“…Furthermore, if an emission bridge does connect the two clusters, the eRASS image will provide its observation. More generally, given the early eROSITA results on cluster outskirts and infall regions (e.g., Reiprich et al 2021;Ghirardini et al 2021;Churazov et al 2021;Veronica et al 2022;Whelan et al 2021;Sanders et al 2022), and given the close correspondence to similar structures observed in cosmological hydrodynamical simulations (e.g., Biffi et al 2022), we may expect that bridges and clumpy filaments will be discovered in several more nearby systems in the future in the eRASS. These systems can then be studied in detail after the eight surveys are completed using the SRG/eROSITA scanning mode, with which areas of tens of square degrees can be scanned efficiently, homogeneously, and deeply.…”

Relative distribution of dark matter, gas, and stars around cosmic filaments in the IllustrisTNG simulation

“…After analysing the full baryonic content of cosmic filaments, let us now focus our analysis only on the gaseous component, whose observation is the most challenging nowadays. Indeed, despite recent detection of the SZ effect around filaments (see e.g., de Graaff et al 2019 andTanimura et al 2020a results), and studies of X-ray emission of filament gas (see e.g., Tanimura et al 2020b andBiffi et al 2022 respectively in ROSAT and in the very recent eROSITA data), the gaseous content of cosmic filaments is still not fully constrained in observations. In order to help future studies, we provide scaling relations between gas density, temperature, and pressure, derived from the analysis of the TNG300-1 simulation.…”

“…The detection of baryons around filaments thus represents a challenge which demands different observational approaches and techniques. For example, by observing individual pairs of clusters, Tittley & Henriksen (2001), Werner et al (2008), Planck Collaboration VIII (2013), Sugawara et al (2017), Akamatsu et al (2017), Alvarez et al (2018), Bonjean et al (2018), Connor et al (2018), Connor et al (2019), Govoni et al (2019), Hincks et al (2022), and Biffi et al (2022) (among others) have detected filaments of scales of a few megaparsecs, acting as bridges of matter between the cluster pairs. Using statistical approaches, some of the properties of these bridges (such as their density, temperature, or magnetic field strength) have been constrained thanks to stacking techniques (e.g., Tanimura et al 2019;de Graaff et al 2019;Vernstrom et al 2021).…”

Hydrostatic mass profiles of galaxy clusters in the eROSITA survey

“…The Magneticum Pathfinder simulations 5 (see Biffi et al 2022) are cosmological hydrodynamical simulations of a large cosmic volume. A light cone is constructed from the so-called Box2 at high resolution ("hr"), which comprises a comoving volume of (352 h −1 cMpc) 3 , resolved with 2 × 1584 3 particles.…”