Cosmic variance in the Great Observatories Origins Deep Survey

Cosmic variance is the uncertainty in observational estimates of the volume density of extragalactic objects such as galaxies or quasars arising from the underlying large-scale density fluctuations. This is often a significant source of uncertainty, especially in deep galaxy surveys, which tend to cover relatively small areas. We present estimates of the relative cosmic variance for one-point statistics (i.e., number densities) for typical scales and volumes sampled by the Great Observatories Origins Deep Survey (GOODS). We use two approaches: (1) For objects with a known two-point correlation function that is well approximated by a power law, one can use the standard analytic formalism to calculate the cosmic variance (in excess of shot noise). We use this approach to estimate the cosmic variance for several populations that are being studied in the GOODS program (extremely red objects [EROs] at zsimilar to1 and Lyman break galaxies [LBGs] at zsimilar to3 and zsimilar to4) using clustering information for similar populations in the literature. (2) For populations with unknown clustering, one can use predictions from cold dark matter theory to obtain a rough estimate of the variance as a function of number density. We present a convenient plot that allows one to use this approach to read off the cosmic variance for a population with a known mean redshift and estimated number density. We conclude that for the volumes sampled by GOODS, cosmic variance is a significant source of uncertainty for strongly clustered objects (similar to40%-60% for EROs) and less serious for less clustered objects (similar to10%-20% for LBGs).