The exquisite sensitivity of MUSE on VLT/ESO together with the recent discovery of Enormous (>200 kpc) Lyman-Alpha Nebulae (ELAN) around radio-quiet z~2-3 quasars have fostered the study of the circum/inter-galactic gas in emission around individual systems out to very large scales. Extended Lyman-Alpha emission is now routinely detected around high-z quasars, and it is usually explained as powered by the ionising radiation of these sources.
In this talk I will present a large (>70) census of z~2-3 quasars using the narrow-band and IFU technique with the aim to determine the frequency of the ELAN phenomenon, as well as to study the size, luminosity, covering factor, and the gas physical properties of the average Lyman-alpha nebula around quasars and their evolution through cosmic time.
In particular, I will show the results of the QSO MUSEUM survey (Quasar Snapshot Observations with MUse: Searching for Extended Ultraviolet eMission), which comprises 61 z~3 quasars observed with the MUSE/VLT instrument, and which suggests that Lyman-alpha could be used to trace the inspiraling motions of the cool circum-galactic medium, predicted to take place on hundreds of kpc within massive halos.
I will conclude by showing an example of preliminary results on the study of physical quasar pairs and projected quasar pairs (simultaneously in emission and absorption) studied with MUSE, together with a census of the submillimeter environment around ELAN and around a subsample of the QSO MUSEUM quasars.