2014A&A...572A..31F

Simulations predict that gas in the intergalactic medium (IGM) is distributed in filamentary structures that connect dense galaxy clusters and form the cosmic web. These structures of predominantly ionized hydrogen are difficult to observe directly due to their lack of emitting regions. We serendipitously detected an overdensity of log N(HI)>18.0 absorbers at z≃2.69 along the lines of sight toward a pair of background quasars. Three main absorption regions spanning ∼2000km/s (corresponding to 6.4h₇₀^–1Mpc proper) are coincident in the two lines of sight, which are separated by ∼90h₇₀^–1kpc transverse proper distance. Two regions have [Fe/H]←1.9 and correspond to mild overdensities in the IGM gas. The third region is a sub-DLA with [Fe/H]=-1.1 that is probably associated with a galaxy. We discuss the possibility that the lines of sight probe along the length of a filament or intercept a galaxy protocluster.