ALMA reveals large overdensity and strong clustering of galaxies in quasar environments at $z\sim4$

TL;DR

This study uses ALMA to identify and analyze dusty galaxies around $z\sim4$ quasars, revealing significant overdensities and strong clustering, which differ from Lyman alpha emitter distributions, shedding light on galaxy environments in the early universe.

Contribution

First systematic ALMA survey of dusty CO(4--3) emitters in high-redshift quasar environments, measuring their clustering and overdensity, and comparing with Lyman alpha emitters.

Findings

Detected 5 CO emitters in quasar fields, indicating high overdensity.

Measured a strong clustering of CO emitters with a cross-correlation length of ~8.37 h^{-1} cMpc.

Found a lower clustering strength for Lyman alpha emitters, suggesting different galaxy populations or properties.

Abstract

We present an Atacama Large Millimeter/submillimeter Array (ALMA) survey of CO(4--3) line emitting galaxies in 17 quasar fields at $z\sim4$ aimed to perform the first systematic search of dusty galaxies in high$-z$ quasar environments. Our blind search of galaxies around the quasars results in 5 CO emitters with S/N$\geq5.6$ within a projected radius of $R\lesssim1.5\,h^{-1}\,$cMpc and a velocity range of $\rm\Delta\,v=\pm1000\,$km\,s$^{-1}$ around the quasar. In blank fields, we expect to detect only 0.28 CO emitters within the same volume, implying a total overdensity of $17.6^{+11.9}_{-7.6}$ in our fields, and indicating that quasars trace massive structures in the early universe. We quantify this overdensity by measuring the small-scale clustering of CO emitters around quasars, resulting in a cross-correlation length of $r_{\rm 0,QG}=8.37^{+2.42}_{-2.04}\,h^{-1}\,$cMpc, assuming a fixed slope $\gamma=1.8$. This contradicts the reported mild overdensities (x1.4) of Lyman alpha emitters (LAEs) in the same fields at scales of $R\lesssim7\,h^{-1}\,$cMpc which is well described by a cross-correlation length 3 times lower than that measured for CO emitters. We discuss some possibilities to explain this discrepancy, including low star formation efficiency, and excess of dust in galaxies around quasars. Finally, we constrain, for the first time, the clustering of CO emitters at $z\sim4$, finding an auto-correlation length of $r_{\rm 0,CO}=3.14\pm1.71\,h^{-1}\,$cMpc (with $\gamma=1.8$). Our work together with the previous study of LAEs around quasars traces simultaneously the clustering properties of both optical and dusty galaxy populations in quasars fields, stressing the importance of multi-wavelength studies, and highlighting important questions about galaxy properties in high$-z$ dense environments.