2020MNRAS.496..754B

In the coming decade, a new generation of telescopes, including JWST and WFIRST, will probe the period of the formation of first galaxies and quasars, and open up the last frontier for structure formation. Recent simulations and observations have suggested that these galaxies are strongly clustered (with large-scale bias >=6), and therefore have significant cosmic variance. In this work, we use BLUETIDES, the largest volume cosmological simulation of galaxy formation, to directly estimate the cosmic variance for current and upcoming surveys. Given its resolution and volume, BLUETIDES can probe the bias and cosmic variance of z > 7 galaxies between magnitude M_UV ∼ -16 and M_UV ∼ -22 over survey areas ∼0.1 arcmin² to ∼10 deg². Within this regime, the cosmic variance decreases with survey area/ volume as a power law with exponents between ∼-0.25 and ∼-0.45. For the planned 10 deg² field of WFIRST, the cosmic variance is between 3 per cent and 10 per cent. Upcoming JWST medium/ deep surveys with areas up to A ∼ 100 arcmin² will have cosmic variance ranging from ∼20 to50 per cent. Lensed surveys have the highest cosmic variance ≳40 per cent; the cosmic variance of M_UV <= -16 galaxies is ≲100 per cent up to z ∼ 11. At higher redshifts such as z ∼ 12 (14), effective volumes of >= (8 Mpc h^–1)³ (>=(12 Mpc h^–1)³) are required to limit the cosmic variance to within 100 per cent. Finally, we find that cosmic variance is larger than Poisson variance and forms the dominant component of the overall uncertainty in all current and upcoming surveys. We present our calculations in the form of simple fitting functions and an online cosmic variance calculator (CV_ATCOSMIC_DAWN) that we publicly release.