Connected structure in the 2dFGRS

Abstract

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.

10 pages, 11 figures. Accepted for publication in MNRAS

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