The A3391/95 system observed by eROSITA at a giant crossroad

The german-built X-ray telescope eROSITA, aboard the Russian-German mission Spektr-RG, is scanning the full sky since December 2019 and will perform 8 full-sky surveys in the soft X-ray band by the end of 2023. During the Performance Verification phase observations, the enormous potential of eROSITA has been tested, for instance with the eFEDS mini-survey (covering 140 square degrees of the sky) and with the Abell 3391/95 system of galaxy clusters, a close pair of clusters. eROSITA observations of the A3391/95 field allowed us to detect for the first time a 15 Mpc long gas emission filament extending for 4 degrees on the sky. This connects A3391 and A3395, extending north of A3391 to the Northern Clump (MCXC J0621.7-5242) cluster and down to another cluster south of A3395.
The Northern Clump, in particular, shows features consistent with it moving towards the A3391/95 system.  eROSITA has allowed us to capture the Cosmic Web and a snapshot the Large Scale Structure of the Universe in the process of formation, as shown in the studies by Prof. T. Reiprich (A&A, 2021) and A. Veronica (A&A, submitted).  With the study I led, we used the Magneticum Pathfinder cosmological hydrodynamical simulations to study a pair of clusters very similar to A3391/95.
With simulations, we investigated the origin of the system in time, how the pair clusters formed and where the diffuse gas within and around them came from. This was especially important for the gas in the interconnecting bridge between the clusters. Such cluster pairs, in fact, could be potentially used to unveil the missing baryons that fill cosmic filaments and are so elusive in the X-rays. We found that, in such a pair configuration, the bridge gas has not been stripped from the two clusters but indeed closely resembles true filament gas.
Also in the simulations, we find evidences for the surroundings structures to move towards the central pair, finally meeting in such a giant cosmic crossroad. A galaxy group, in particular, is already bound to one of the pair clusters but 3 Gyr ago shared a very similar configuration with the Northern Clump relative to A3391/95. We expect the Northern Clump is also moving towards the central system and will possibly finally merge with it.


Comparison between the simulated system (right and left zoom) and the Abell 3391/95 field observed by eROSITA (central panel). Credit: V.Biffi et al., A&A, submitted.