2008A&A...482L..29W

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<10⁷K. This warm-hot intergalactic medium has never been detected indisputably using X-ray observations. 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. We analyse X-ray images and spectra obtained from a deep observation (144 ks) of A 222/223 with XMM-Newton. 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 (0.5-2.0keV) X-ray image with a 5σ significance. Following the emission down to the 3σ significance level, the observed filament is ≃1.2Mpc wide. The temperature of the gas associated with the filament, determined from the spectra, is kT=0.91±0.25keV, and its emission measure corresponds to a baryon density of (3.4±1.3)x10^–5(l/15Mpc)^–1/2cm^–3, where l is the length of the filament along the line-of-sight. This density corresponds to a baryon over-density of ρ/<ρ_C≥∼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.