QSO Museum I: A sample of 61 extended Ly α-emission nebulae surrounding z ∼ 3 quasars

Motivated by the discovery of rare enormous Lyman α nebulae (ELAN) around z ̃ 2 quasars, we initiated a long-term campaign with the MUSE/VLT instrument to directly uncover the astrophysics of the gas around quasars. We present here the first 61 targets under the acronym QSO MUSEUM. These quasars are characterized by a median redshift z = 3.17, absolute i magnitude in the range -29.67 ≤ Mi(z = 2) ≤ -27.03, and different levels of radio-loudness. This sample unveils diverse specimens of Ly α nebulosities extending for tens of kpc around these quasars (average maximum projected distance of 80 kpc) above a surface brightness SB > 8.8 × 10-19 erg s-1 cm-2 arcsec-2(2σ). The bulk of the Ly α emission is within R < 50 kpc, and is characterized by relatively quiescent kinematics, with average velocity dispersions < 400 km s-1. Therefore, the motions within all these Ly α nebulosities have amplitudes consistent with gravitational motions expected in dark matter haloes hosting quasars at these redshifts, possibly reflecting the complexity in propagating a fast wind on large scales. Our current data suggest a combination of photoionization and resonant scattering as powering mechanisms of the Ly α emission. We discover the first z ̃ 3 ELAN, confirming a very low probability ({̃ }1{{ per cent}}) of occurrence of such systems at these redshifts. Finally, we discuss the redshift evolution currently seen in extended Ly α emission around radio-quiet quasars from z ̃ 3 to z ̃ 2, concluding that it is possibly linked to a decrease of cool gas mass within the circumgalactic medium of quasars from z ̃ 3 to z ̃ 2, and thus to the balance of cool versus hot media. Overall, QSO MUSEUM opens the path to statistical surveys targeting the gas phases in quasars' haloes along cosmic times.