2014ApJ...793...17B

Lyman-break galaxy (LBG) samples observed during reionization (z ≳ 6) with the Hubble Space Telescope's Wide Field Camera 3 are reaching sizes sufficient to characterize their clustering properties. Using a combined catalog from the Hubble eXtreme Deep Field and CANDELS surveys, containing N = 743 LBG candidates at z ≥ 6.5 at a mean redshift of  = 7.2, we detect a clear clustering signal in the angular correlation function (ACF) at ≳ 4σ, corresponding to a real-space correlation length r₀ = 6.7_-1.0+0.9^ h^–1 cMpc. The derived galaxy bias b = 8.6_–1.0^+0.9 is that of dark matter halos of M = 10^11.1-0.3+0.2M_☉ at z = 7.2, and highlights that galaxies below the current detection limit (M_AB∼ -17.7) are expected in lower-mass halos (M ∼ 10⁸-10^10.5 M_☉). We compute the ACF of LBGs at z ∼ 3.8 - z ∼ 5.9 in the same surveys. A trend of increasing bias is found from  = 3.8 (b∼3.0) to  = 7.2 (b ∼ 8.6), broadly consistent with galaxies at fixed luminosity being hosted in dark matter halos of similar mass at 4 ≲ z ≲ 6, followed by a slight rise in halo masses at z ≳ 7 (∼2σ confidence). Separating the data at the median luminosity of the  = 7.2 sample (M_UV= -19.4) shows higher clustering at  = 5.9 for bright galaxies (r₀ = 5.5_–1.6^+1.4h^–1 cMpc, b = 6.2_–1.5^+1.2) compared to faint galaxies (r₀ = 1.9_–1.0^+1.1h^–1 cMpc, b = 2.7_–1.2^+1.2) implying a constant mass-to-light ratio (dlogM/dlogL) ∼ 1.2_–0.8^+1.8. A similar trend is present in the  = 7.2 sample with larger uncertainty. Finally, our bias measurements allow us to investigate the fraction of dark matter halos hosting UV-bright galaxies (the duty cycle, ε_DC). At  = 7.2 values near unity are preferred, which may be explained by the shortened halo assembly time at high redshift.