Observational determination of the galaxy bias from cosmic variance with a random pointing survey: clustering of z ∼ 2 galaxies from Hubble’s BoRG survey

Gravitational clustering broadens the count-in-cells distribution of galaxies for surveys along uncorrelated (well-separated) lines of sight beyond Poisson noise. A number of methods have proposed to measure this excess 'cosmic' variance to constrain the galaxy bias (i.e. the strength of clustering) independently of the two-point correlation function. Here we present an observational application of these methods using data from 141 uncorrelated fields (similar to 700 arcmin(2) total) from Hubble's Brightest of Reionizing Galaxies (BoRG) survey. We use BoRG's broad-band imaging in optical and near-infrared to identify N similar to 1000 photometric candidates at z similar to 2 through a combination of colour selection and photometric redshift determination, building a magnitude-limited sample with m(AB) <= 24.5 in F160W. We detect a clear excess in the variance of the galaxy number counts distribution compared to Poisson expectations, from which we estimate a galaxy bias b approximate to 3.63 +/- 0.57. When divided by SED-fit classification into similar to 400 early-type and similar to 600 late-type candidates, we estimate biases of b(early) approximate to 4.06 +/- 0.67 and b(late) approximate to 2.98 +/- 0.98, respectively. These estimates are consistent with previous measurements of the bias from the two-point correlation function, and demonstrate that with N greater than or similar to 100 sightlines, each containing N greater than or similar to 5 objects, the counts-in-cell analysis provides a robustmeasurement of the bias. This implies that the method can be applied effectively to determine clustering properties (and characteristic dark-matter halo masses) of z similar to 6-9 galaxies from a pure-parallel James Webb Space Telescope survey similar in design to Hubble's BoRG survey.