Cosmic variance of z > 7 galaxies: prediction from BLUETIDES

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 greater than or similar to 6), and therefore have significant cosmic variance. In this work, we use BLIJETIDES, 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 similar to -16 and M-UV similar to -22 over survey areas similar to 0.1 arcmin(2) to similar to 10 deg(2). Within this regime, the cosmic variance decreases with survey area/ volume as a power law with exponents between similar to-0.25 and similar to-0.45. For the planned 10 deg(2) 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 similar to 100 arcmin(2) will have cosmic variance ranging from similar to 20 to 50 per cent. Lensed surveys have the highest cosmic variance greater than or similar to 40 per cent; the cosmic variance of M-UV less than or similar to -16 galaxies is less than or similar to 100 per cent up to z similar to 11. At higher redshifts such as z similar to 12 (14), effective volumes of greater than or similar to (8 Mpc h(-1))(3) (greater than or similar to(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_AT_COSMIC_DAWN) that we publicly release.