Evidence for X-ray emission from a large-scale filament of galaxies?

Cosmological simulations predict that a large fraction of the baryonic mass of the universe exists as 10(5)-10(7) K diffuse, X-ray-emitting gas, traci ng low-density filament and sheetlike structures exterior to massive cluste rs of galaxies. If present, this gas helps reconcile the current shortfall in observed baryon counts relative to the predictions of the standard big b ang model. We present here the discovery and analysis of a 5 sigma signific ance half-degree filamentary structure, which is present in both the I-band salary surface density and the unresolved X-ray emission in a deep ROSAT P SPC field. The estimated diffuse X-ray emission component of this structure has a surface brightness of similar or equal to 1.6 x 10(-16) ergs s(-1) c m(-2) arcmin(-2) (0.5-2 keV), comparable to the predictions for intercluste r gas, and may represent a direct detection of this currently unconfirmed b aryonic component.