![]() ![]() We stacked on data from two radio surveys: the 1.4-GHz total and polarized intensity maps from the recently released High-band north (HBN) Global Magneto-Ionic Medium Survey (GMIMS) survey ( 47) and the 30-GHz maps in total and polarized intensity from the Planck mission ( 48). To compare, we also stacked on pairs of clusters that are separated in physical space by hundreds or thousands of megaparsecs although, due to projection, still appear near to each other on the sky (these pairs are a control sample where strong filamentary emission is not expected and are referred to as “unconnected” pairs). This ensured that the clusters and filaments would line up and any signal would add together. To perform the stacking, cutouts were made of all the pairs, which were then rotated and scaled. These cluster pairs are most likely connected by filaments (from here, these are referred to as the “connected” pairs, although not all of the pairs are in reality connected by filaments). Using LRGs from the Sloan Digital Sky Survey (SDSS) Data Release 7 LRG catalog ( 46), we stacked 612,025 pairs of LRGs or clusters that are physically near each other in three-dimensional (3D) space, i.e., with physical separations between 1 and 15 Mpc. ![]() In some cases, stacking may be the only method of observing such faint or diffuse emission (as direct imaging, even with future telescopes, may still have imaging limitations and be affected by confusion of galaxies). Stacking many clusters or pairs of clusters decreases the noise, allowing one to look for an average signal from the objects well below the noise level of the image. LRGs are early-type massive galaxies that are known to usually reside in the centers of groups or clusters ( 43– 45) and are identified in optical surveys in much larger numbers than galaxy clusters. Pairs of luminous red galaxies (LRGs) are a commonly used tracer. However, in general, the number of known clusters is not large enough for such stacking experiments, and a tracer for clusters and groups must be used. The method of stacking filaments, or pairs of clusters to detect filamentary emission between them, has had success in recent years via the thermal Sunyaev-Zel’dovich effect ( 36, 37), weak lensing ( 38, 39), the dark matter mass-to-light ratio ( 40), thermal x-ray emission ( 27, 41, 42), and, by our group, radio synchrotron emission ( 27). While the overdense matter distribution tracked by galaxies has long since been detected with optical and infrared surveys, the presence of diluted and hot plasmas in between galaxies has so far been detected only in a statistical way. Moreover, it is still unknown how turbulent the environment inside filaments is. The magnetic field strength in filaments is still not well known, with values from diffuse synchrotron emission ranging from 30 to 100 nG (from either equipartition or inverse Compton scattering calculations or comparison with DSA simulations) ( 27– 30) and from Faraday rotation measure (RM) studies ranging from 10 to 100 nG ( 31– 35). The existence of such shocks is still an unproven core concept of our models of cosmic structure formation and growth. A plausible scenario to refill such giant volumes, away from any other galaxy, would be Fermi-type acceleration by shocks forming at the periphery of filaments. While such radio detection is compatible with synchrotron emission by weak magnetic fields organized on scales of megaparsecs, the mechanism that could accelerate ultrarelativistic electrons in such diluted plasmas (or the potential contribution from radio galaxies or other sources) remains more debatable. Even within the now most well-studied radio intercluster bridge, the particle acceleration mechanism for the emission is debated between Fermi-I processes ( 22) and more turbulent Fermi-II processes ( 26). ![]() To date, only a few inter- and intracluster bridges have been found to host detectable diffuse radio emission ( 22– 25). It is thought that similar processes should be present in between clusters in intercluster bridges and filaments as well. ![]()
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