2017ApJ...848....7G

In the standard picture of structure formation, the first massive galaxies are expected to form at the highest peaks of the density field, which constitute the cores of massive proto-clusters. Luminous quasars (QSOs) at z ∼ 4 are the most strongly clustered population known, and should thus reside in massive dark matter halos surrounded by large overdensities of galaxies, implying a strong QSO-galaxy cross-correlation function. We observed six z ∼ 4 QSO fields with VLT/FORS, exploiting a novel set of narrow-band filters custom designed to select Lyman Break Galaxies (LBGs) in a thin redshift slice of Δz∼0.3, mitigating the projection effects that have limited the sensitivity of previous searches for galaxies around z 4 QSOs. We find that LBGs are strongly clustered around QSOs, and present the first measurement of the QSO-LBG cross-correlation function at z ∼ 4, on scales of 0.1 R 9 h^–1 Mpc (comoving). Assuming a power-law form for the cross-correlation function ξ=(r/r₀^QG)^γ, we measure r₀^QG=8.83_–1.51^+1.39 h^–1 Mpc for a fixed slope of γ=2.0. This result is in agreement with the expected cross-correlation length deduced from measurements of the QSO and LBG auto-correlation function, and assuming a deterministic bias model. We also measure a strong auto-correlation of LBGs in our QSO fields, finding r₀^GG=21.59_–1.69^+1.72 h^–1 Mpc for a fixed slope of γ=1.5, which is ∼4 times larger than the LBG auto-correlation length in blank fields, providing further evidence that QSOs reside in overdensities of LBGs. Our results qualitatively support a picture where luminous QSOs inhabit exceptionally massive (M_halo 10¹² M_☉) dark matter halos at z ∼ 4.