2019ApJ...886...79G

The strong observed clustering of z > 3.5 quasars indicates that they are hosted by massive (M_halo≳10¹²h^–1M_☉) dark matter halos. Assuming that quasars and galaxies trace the same large-scale structures, this should manifest as strong clustering of galaxies around quasars. Previous works on high-redshift quasar environments have failed to find convincing evidence for these overdensities. Here we conduct a survey for Lyα emitters (LAEs) in the environs of 17 quasars at z ∼ 4 probing scales of R≲7h^–1Mpc. We measure an average LAE overdensity of 1.4_–0.4^+0.4, which we quantify by fitting the quasar-LAE cross-correlation function. We find consistency with a power-law shape with correlation length r₀^QG=2.78_–1.05^+1.16h^–1cMpc for a fixed slope of γ = 1.8 and rule out a zero clustering hypothesis at the 95% confidence level. We also measure the LAE autocorrelation length and find r₀^GG=9.12_–1.31^+1.32h^–1 cMpc (γ = 1.8), which is 3.3_–1.0^+0.9 times higher than the value measured in blank fields. Taken together, our results clearly indicate that LAEs are significantly clustered around z ∼ 4 quasars. We compare the observed clustering with the expectation from a deterministic bias model, whereby LAEs and quasars probe the same underlying dark matter overdensities, and find that our measurements fall short of the predicted overdensities by a factor of 2.1_–0.5^+0.7. We discuss possible explanations for this discrepancy, including large-scale quenching or the presence of excess dust in galaxies near quasars. Finally, the large cosmic variance from field to field observed in our sample (10/17 fields are actually underdense) cautions one from overinterpreting studies of z ∼ 6 quasar environments based on a single or handful of quasar fields.