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

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 10(5) < T < 10(7) 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 5 sigma significance. Following the emission down to the 3 sigma significance level, the observed filament is approximate to 1.2 Mpc wide. The temperature of the gas associated with the filament, determined from the spectra, is kT = 0.91 +/- 0.25 keV, and its emission measure corresponds to a baryon density of (3.4 +/- 1.3) x 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 rho/ approximate to 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.