A radio ridge connecting two galaxy clusters in a filament of the cosmic web

Abstract

Galaxy clusters are the most massive gravitationally bound structures in the Universe. They grow by accreting smaller structures in a merging process that produces shocks and turbulence in the intra-cluster gas. We observed a ridge of radio emission connecting the merging galaxy clusters Abell 0399 and Abell 0401 with the Low Frequency Array (LOFAR) at 140 MHz. This emission requires a population of relativistic electrons and a magnetic field located in a filament between the two galaxy clusters. We performed simulations to show that a volume-filling distribution of weak shocks may re-accelerate a pre-existing population of relativistic particles, producing emission at radio wavelengths that illuminates the magnetic ridge.One Sentence Summary: Discovery of radio emission from a cosmic web filament located between two massive galaxy clusters. Main Text:The matter distribution of the Universe is not uniform, but forms a cosmic web, with a structure of filaments and galaxy clusters surrounding large voids. Galaxy clusters form at the intersections of the cosmic web filaments and grow by accreting substructures in a merging process, which converts most of the infall kinetic energy into thermal gas energy. A residual fraction of non-thermalised energy is expected to manifest itself in the form of turbulent gas motions, magnetic fields, and relativistic particles. Extended radio sources called radio halos and radio relics are found at the center and the periphery of galaxy clusters, respectively, visible through their emission of synchrotron radiation. Magnetic fields and relativistic particles in the large-scale structure of the Universe can be inferred from observations of these sources.

Second-order Fermi Reacceleration Mechanisms and Large-Scale Synchrotron Radio Emission in Intracluster Bridges

“…[3][4][5][6][7][8][9][10][11][12]. More recently LOFAR observations have discovered diffuse radio emission from regions extending on even larger scales and that connect pairs of massive clusters in a pre-merger phase [13,14]. These observations prove that these regions, where the gas is likely compressed, are filled by relativistic electrons and magnetic fields that are generated on scales which had never been probed before.…”

“…These observations prove that these regions, where the gas is likely compressed, are filled by relativistic electrons and magnetic fields that are generated on scales which had never been probed before. The most spectacular case is the 5 Mpc long radio bridge connecting the two massive clusters A399 and A401 [14], where the radio emission follows a filament of gas connecting the two clusters that was early discovered with the Planck satellite through the Sunyaev-Zeldovich effect [15]. What makes their interpretation challenging is that radio bridges appears as truly diffuse radio emissions on gigantic scales, suggesting that relativistic particles are accelerated in situ by mechanisms that are distributed on very large spatial scales and that are not necessarily powered by the energy dissipated as a consequence of major cluster-cluster mergers.…”

“…Numerical simulations show that relatively weak shocks, M ≤ 2−3, form in these regions and that up to ∼ 10% of the volume has been crossed by these shocks in the last Gyr [e.g. 14] leading to the possibility that radio bridges may result from re-acceleration of a volume filling population of fossil relativistic electrons by these weak shocks under favorable projection effects. However, in order to match the constraints on the spectrum of the emission, this scenario requires assumptions on the age and dynamics of the fossil electrons that are not very plausible (see discussion in [14]).…”

“…14] leading to the possibility that radio bridges may result from re-acceleration of a volume filling population of fossil relativistic electrons by these weak shocks under favorable projection effects. However, in order to match the constraints on the spectrum of the emission, this scenario requires assumptions on the age and dynamics of the fossil electrons that are not very plausible (see discussion in [14]). Virtually all major mergers should undergo a stage in which the remnant of a cosmic filament connecting the two clusters is compressed and pre-processed by gas dynamics, before the two clusters collide.…”

“…Therefore, recent detections [i.e. 13,14] may have unveiled the tip of the iceberg of a common (albeit short-lived, i.e. ∼ Gyr) phenomenology.…”