Detection of hot gas in the filament connecting the clusters of galaxies Abell 222 and Abell 223

Abstract

Context. About half of the baryons in the local Universe are invisible and – according to simulations – their dominant fraction resides in filaments connecting clusters of galaxies in the form of low density gas with temperatures in the range of 105 <T <107 K. This warm-hot intergalactic medium has never been detected indisputably using X-ray observations. Aims. We aim to probe the low gas densities expected in the large-scale structure filaments by observing a filament connecting the massive clusters of galaxies A 222 and A 223 (z = 0.21), which has a favorable orientation approximately along our line-of-sight. This filament has been previously detected using weak lensing data and as an over-density of colour-selected galaxies. Methods. We analyse X-ray images and spectra obtained from a deep observation (144 ks) of A 222/223 with XMM-Newton. Results. We present observational evidence of X-ray emission from the filament connecting the two clusters. We detect the filament in the wavelet-decomposed soft-band (0.5–2.0 keV) X-ray image with a 5σ significance. Following the emission down to the 3σ significance level, the observed filament is ≈1.2 Mpc wide. The temperature of the gas associated with the filament, determined from the spectra, is k T = 0.91±0.25 keV, and its emission measure corresponds to a baryon density of (3.4±1.3)×10−5(l/15 Mpc)−1/2 cm−3, where l is the length of the filament along the line-of-sight. This density corresponds to a baryon over-density of ρ/ ρC ≈ 150. The properties of the gas in the filament are consistent with results of simulations of the densest and hottest parts of the warm-hot intergalactic medium.