A Giant Arc On The Sky

We present the discovery of the Giant Arc, a large-scale structure marked by MgII absorbers spanning approximately 1 Gpc in length and 100 Mpc in width (present epoch values), at a redshift of z ~ 0.8. The structure forms a highly symmetric arc in the plane of the sky, but shows more intricate structuring along the redshift axis, with a depth of ~340 Mpc. Two different statistical analyses show the Giant Arc to be significant at > 4 sigma. We discuss the implications for the Giant Arc in context with other known large structures for the fundamental assumption of homogeneity in cosmology.