We present the results of spectroscopic observations of galaxies associatedwith the diffuse X-ray emitting structure discovered by Zappacosta et al.(2002). After measuring the redshifts of 161 galaxies, we confirm anoverdensity of galaxies with projected dimensions of at least 2 Mpc, determineits spectroscopic redshift in z=0.401+/-0.002, and show that it is spatiallycoincident with the diffuse X-ray emission. This confirms the original claimthat this X-ray emission has an extragalactic nature and is due to the Warm-HotIntergalactic Medium (WHIM). We used this value of the redshift to compute thetemperature of the emitting gas. The resulting value depends on the metallicitythat is assumed for the IGM, and is constrained to be between 0.3 and 0.6 keVfor metallicities between 0.05 and 0.3 solar, in good agreement with theexpectations from the WHIM.
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