Recently Vernstrom et al. (2021, MNRAS) claimed the first definitive detection of the synchrotron cosmic web, obtained by ‘stacking’ hundreds of thousands of pairs of close-proximity clusters in low-frequency radio observations and looking for a residual excess signal spanning the intracluster bridge. A reproduction study by Hodgson et al. (2022, PASA, 39, e013), using both the original radio data as well as new observations with the Murchison Widefield Array, failed to confirm these findings. Whilst the detection remains unsure, we here turn to stacking a simulated radio sky to understand what kind of excess radio signal is predicted by our current best cosmological models of the synchrotron cosmic web. We use the FIlaments & GAlactic RadiO (FIGARO; Hodgson et al. 2021a, PASA, 38, e047) simulation, which models both the synchrotron cosmic web as well as various subtypes of active galactic nucleii and star-forming galaxies. Being a simulation, we have perfect knowledge of the location of clusters and galaxy groups which we use in our own stacking experiment. Whilst we do find an excess radio signature in our stacks that is attributable to the synchrotron cosmic web, its distribution is very different to that found by Vernstrom et al. (2021, MNRAS). Instead, we observe the appearance of excess emission on the immediate interiors of cluster pairs as a result of asymmetric, ‘radio relic’-like shocks surrounding cluster cores, whilst the excess emission spanning the intracluster region—attributable to filaments proper—is two orders of magnitude lower and undetectable in our experiment even under ideal conditions.

We follow up on the surprising recent announcement by Vernstrom et al. (2021, MNRAS) of the detection of the synchrotron cosmic web. We attempt to reproduce their detection with new observations with the Phase II, extended configuration of the Murchison Widefield Array at 118.5 MHz. We reproduce their detection methodology by stacking pairs of nearby luminous red galaxies (LRGs)—used as tracers for clusters and galaxy groups—contained in our low-frequency radio observations. We show that our observations are significantly more sensitive than those used in Vernstrom et al. and that our angular sensitivity is sufficient. And yet, we make no statistically significant detection of excess radio emission along the bridge spanning the LRG pairs. This non-detection is true both for the original LRG pair catalogue as used in Vernstrom et al., as well as for other larger catalogues with modified selection criteria. Finally, we return to the original data sets used in Vernstrom et al., and find that whilst we clearly reproduce the excess X-ray emission from ROSAT, we are not able to reproduce any kind of broad and extended excess intercluster filamentary emission using the original 118.5 MHz MWA survey data. In the interests of understanding this result, as part of this paper we release images of the 14 fields used in this study, the final stacked images, as well as key components of our stacking and modelling code.

We produce the first low to mid-frequency radio simulation that incorporates both traditional extragalactic radio sources as well as synchrotron cosmic web emission. The FIlaments & GAlactic RadiO (FIGARO) simulation includes 10 unique 4° × 4° fields, incorporating active galactic nucleii (AGNs), star-forming galaxies (SFGs), and synchrotron cosmic web emission out to a redshift of z = 0.8 and over the frequency range 100–1 400 MHz. To do this, the simulation brings together a recent 1003 Mpc3 magnetohydrodynamic simulation (Vazza et al. 2019, A&A, 627, A5), calibrated to match observed radio relic population statistics, alongside updated ‘T-RECS’ code for simulating extragalactic radio sources (Bonaldi et al. 2019, MNRAS, 482, 2). Uniquely, the AGNs and SFGs are populated and positioned in accordance with the underlying matter density of the cosmological simulation. In this way, the simulation provides an accurate understanding of the apparent morphology, angular scales, and brightness of the cosmic web as well as—crucially—the clustering properties of the cosmic web with respect to the embedded extragalactic radio population. We find that the synchrotron cosmic web does not closely trace the underlying mass distribution of the cosmic web, but is instead dominated by shocked shells of emission surrounding dark matter halos and resembles a large, undetected population of radio relics. We also show that, with accurate kernels, the cosmic web radio emission is clearly detectable by cross-correlation techniques and this signal is separable from the embedded extragalactic radio population. We offer the simulation as a public resource towards the development of techniques for detecting and measuring the synchrotron cosmic web.