A ~6 Mpc overdensity at z = 2.7 detected along a pair of quasar sight
  lines: filament or protocluster?
  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 ~2000 km/s (corresponding to 6.4 (h_70)^(-1) Mpc proper) are
coincident in the two lines of sight, which are separated by ~90 (h_70)^(-1)
kpc 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.
