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

Submillimeter surveys toward overdense regions in the early Universe are essential for uncovering 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 ∼20 × 20 around MUSE Quasar Nebula 01 (MQN01), one of the largest and brightest Ly-α emitting nebulae discovered thus far; it surrounds 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 11 CO-line-emitting galaxies (including a closely separated quasar companion) that lie within ±4000 km s−1 of the quasar systemic redshift. A fraction of these objects were missed in previous deep rest-frame optical/UV surveys, which highlights the critical role of (sub)millimeter imaging. We also detect a total of 11 sources revealed in the dust continuum at 1.2 mm; six of them have either high-fidelity spectroscopic redshift information from rest-frame UV metal absorptions or the CO(4-3) line that places them in the same narrow redshift range. A comparison of the CO luminosity function and 1.2 mm number count density with those 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 luminosity function 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, which enables a faster and accelerated assembly. This is the first in a series of studies aimed at characterizing one of the densest regions of the Universe found so far at z > 3.