Line-of-Sight Structure Toward Strong Lensing Galaxy Clusters
  We present an analysis of the line-of-sight structure toward a sample of ten
strong lensing cluster cores. Structure is traced by groups that are identified
spectroscopically in the redshift range, 0.1 $\leq$ z $\leq$ 0.9, and we
measure the projected angular and comoving separations between each group and
the primary strong lensing clusters in each corresponding line of sight. From
these data we measure the distribution of projected angular separations between
the primary strong lensing clusters and uncorrelated large scale structure as
traced by groups. We then compare the observed distribution of angular
separations for our strong lensing selected lines of sight against the
distribution of groups that is predicted for clusters lying along random lines
of sight. There is clear evidence for an excess of structure along the line of
sight at small angular separations ($\theta \leq 6'$) along the strong lensing
selected lines of sight, indicating that uncorrelated structure is a
significant systematic that contributes to producing galaxy clusters with large
cross sections for strong lensing. The prevalence of line-of-sight structure is
one of several biases in strong lensing clusters that can potentially be folded
into cosmological measurements using galaxy cluster samples. These results also
have implications for current and future studies -- such as the Hubble Space
Telescope Frontier Fields -- that make use of massive galaxy cluster lenses as
precision cosmological telescopes; it is essential that the contribution of
line-of-sight structure be carefully accounted for in the strong lens modeling
of the cluster lenses.
