Connected structure in the 2dFGRS
  We describe and apply a simple prescription for defining connected structures
in galaxy redshift surveys. The method is based upon two passes with a
friends-of-friends groupfinder. The first pass uses a cylindrical linking
volume to find galaxy groups and clusters, in order to suppress the
line-of-sight smearing introduced by the large random velocities of galaxies
within these deep potential wells. The second pass, performed with a spherical
linking volume, identifies the connected components. This algorithm has been
applied to the 2dFGRS, within which it picks out a total of 7,603 systems
containing at least two galaxies and having a mean redshift less than 0.12.
Connected systems with many members appear filamentary in nature, and the
algorithm recovers two particularly large filaments within the 2dFGRS. For
comparison, the algorithm is has also been applied to LambdaCDM mock galaxy
surveys. While the model population of such systems is broadly similar to that
in the 2dFGRS, it does not generally contain such extremely large structures.
