Clustering of Lyman-alpha Emitters Around Quasars at $z\sim 4$

Abstract

The strong observed clustering of $z>3.5$ quasars indicates they are hosted by massive ($M_{\mathrm{halo}}\gtrsim 10^{12}{h}^{-1}{\mathrm{M}}_{\odot }$) dark matter halos. Assuming quasars and galaxies trace the same large-scale structures, this should also manifest as strong clustering of galaxies around quasars. Previous work on high-redshift quasar environments, mostly focused at $z>5$, have failed to find convincing evidence for these overdensities. Here we conduct a survey for Lyman alpha emitters (LAEs) in the environs of 17 quasars at $z\sim 4$ probing scales of $R\lesssim 7h^{-1}\mathrm{Mpc}$. We measure an average LAE overdensity around quasars of 1.4 for our full sample, which we quantify by fitting the quasar-LAE cross-correlation function. We find consistency with a power-law shape with correlation length of $r_0^{QG}=2.78_{-1.05}^{+1.16}{h}^{-1}\mathrm{cMpc}$ for a fixed slope of $\gamma =1.8$. We also measure the LAE auto-correlation length and find $r_0^{GG}=9.12_{-1.31}^{+1.32}{h}^{-1}$\,cMpc ($\gamma =1.8$), which is $3.3$ times higher than the value measured in blank fields. Taken together our results clearly indicate that LAEs are significantly clustered around $z\sim 4$ 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. 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 over-interpreting studies of $z\sim 6$ quasar environments based on a single or handful of quasar fields.

19 pages, 12 figures, submitted to the ApJ