ALMA survey of a massive node of the Cosmic Web at z 3. I. Discovery of a large overdensity of CO emitters

Sub-mm surveys toward overdense regions in the early Universe are essential to uncover the obscured star-formation and the cold gas content of assembling galaxies within massive dark matter halos. In this work, we present deep ALMA mosaic observations covering an area of ∼ 2'×2' around MQN01 (MUSE Quasar Nebula 01), one of the largest and brightest Ly-α emitting nebulae discovered thus far surrounding a radio-quiet quasar at z≃3.25. Our observations target the 1.2- and the 3-mm dust continuum, as well as the carbon monoxide CO(4-3) transition in galaxies in the vicinity of the quasar. We identify a robust sample of eleven CO line-emitting galaxies (including a closely-separated quasar companion) which lie within ± 4000 km s^-1 relatively to the quasar systemic redshift. A fraction of these objects are missed in previous deep rest-frame optical/UV surveys thus highlighting the critical role of (sub-)mm imaging. We also detect a total of eleven sources revealed in their 1.2-mm dust continuum with six of them having either high-fidelity spectroscopic redshift information from rest-frame UV metal absorptions, or CO line which place them in the same narrow redshift range. A comparison of the CO luminosity function (LF) and 1.2-mm number count density with that of the general fields points to a galaxy overdensity of δ > 10. We find evidence of a systematic flattening at the bright-end of the CO LF with respect to the trend measured in blank fields. Our findings reveal that galaxies in dense regions at z∼3 are more massive and significantly richer in molecular gas than galaxies in fields, hence enabling a faster and accelerated assembly. This is the first of a series of studies to characterize one of the densest regions of the Universe found so far at z > 3.