ALMA Reveals a Large Overdensity and Strong Clustering of Galaxies in Quasar Environments at z ∼4

We present an Atacama Large Millimeter/submillimeter Array (ALMA) survey of CO(4-3) line emitting galaxies in 17 quasar fields at z ∼4 aimed at performing the first systematic search of dusty galaxies in high-z quasar environments. Our blind search of galaxies around the quasars results in five CO emitters with S/N ≥ 5.6 within a projected radius of R ∼2 1.5 h -1 cMpc and a velocity range of δv = ±1000 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 17.6-7.6+11.9 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 r0,QG=8.37-2.04+2.42h-1 cMpc, assuming a fixed slope γ = 1.8. This contradicts the reported mild overdensities (x1.4) of Lyα emitters (LAEs) in the same fields at scales of R ∼2 7 h -1 cMpc, which are well described by a cross-correlation length 3.0-1.4+1.5 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 ∼4, finding an autocorrelation length of r 0,CO = 3.14 ±1.71 h -1 cMpc (with γ = 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 multiwavelength studies, and highlighting important questions about galaxy properties in high-z dense environments.