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 \pm 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 .