Weak Gravitational Lensing and Cluster Mass Estimates

Hierarchical theories of structure formation predict that clusters ofgalaxies should be embedded in a web like structure, with filaments emanatingfrom them to large distances. The amount of mass contained within suchfilaments near a cluster can be comparable to the collapsed mass of the clusteritself. Diffuse infalling material also contains a large amount of mass. Boththese components can contribute to the cluster weak lensing signal. This``projection bias'' is maximized if a filament lies close to the line-of-sightto a cluster. Using large--scale numerical simulations of structure formationin a cosmological constant dominated cold dark matter model, we show that theprojected mass typically exceeds the actual mass by several tens of percent.This effect is significant for attempts to estimate cluster masses through weaklensing observations, and will affect weak lensing surveys aimed atconstructing the cluster mass function.Comment: 4 pages, 3 figures. LaTeX2e, uses emulateapj.sty and onecolfloat.sty. To be submitted to the Astrophysical Journal Letter