  We present the results of spectroscopic observations of galaxies associated
with the diffuse X-ray emitting structure discovered by Zappacosta et al.
(2002). After measuring the redshifts of 161 galaxies, we confirm an
overdensity of galaxies with projected dimensions of at least 2 Mpc, determine
its spectroscopic redshift in z=0.401+/-0.002, and show that it is spatially
coincident with the diffuse X-ray emission. This confirms the original claim
that this X-ray emission has an extragalactic nature and is due to the Warm-Hot
Intergalactic Medium (WHIM). We used this value of the redshift to compute the
temperature of the emitting gas. The resulting value depends on the metallicity
that is assumed for the IGM, and is constrained to be between 0.3 and 0.6 keV
for metallicities between 0.05 and 0.3 solar, in good agreement with the
expectations from the WHIM.
