A detailed view of the filament of hot gas between the clusters of galaxies Abell 2029 and Abell 2033

Cosmological numerical simulations suggest that the bulk of the missing baryons is in the form of a diffuse gas that resides in the cluster outskirts beyond the cluster’s virial radius within filaments that connect clusters to the cosmic web. The warm-hot intergalactic medium (WHIM) is considered an ideal candidate to search for missing baryons. Observationally, however, it is highly challenging to detect the WHIM due to its very low X-ray surface brightness. One approach, which attempts to get around this limitation, is to observe large-scale filaments between a close pair of galaxy clusters which are aligned such that our line of sight looks along the length of the filament, which maximizes the volume of gas we are looking through. Here, we examine Suzaku, XMM–Newton, and Chandra observations of the Abell 2029/2033 system to investigate the nature of a bridge of X-ray emission joining the two galaxy clusters. By modelling the contributions from the outskirts of the two clusters, we find a significant excess of X-ray emission between the two clusters at the level of 6.5–7.0σ, depending on the choice of model, that cannot be explained by the overlap of the clusters. This excess component to the surface brightness is consistent with being emission from a filament with roughly 1.0 Mpc wide. The derived emission measure for the gas associated with the filament yields an average gas density of 3.7 (-0.7,+1.0) × 10⁻⁵ cm⁻³, corresponding roughly to 160 times the mean baryon density of the Universe. The Suzaku X-ray spectrum of the excess emission indicates that it is significantly colder (1.4 ± 0.6 keV) than the cluster outskirts emission from the two clusters (5 keV), statistically consistent with the temperature expected from the hottest and densest parts of the WHIM.