Inspiraling halo accretion mapped in Lyα emission around a z ∼ 3 quasar

In an effort to search for Ly alpha emission from circum-and intergalactic gas on scales of hundreds of kpc around z similar to 3 quasars, and thus characterize the physical properties of the gas in emission, we have initiated an extensive fast survey with the Multi-Unit Spectroscopic Explorer (MUSE): Quasar Snapshot Observations withMUse: Search for Extended Ultraviolet eMission (QSO MUSEUM). In this work, we report the discovery of an enormous Ly alpha nebula (ELAN) around the quasar SDSS J102009.99+ 104002.7 at z = 3.164, which we followedup with deeper MUSE observations. This ELAN spans similar to 297 projected kpc, has an average Ly alpha surface brightness SBLy alpha similar to 6.04 x 10(-18) erg s(-1) cm(-2) arcsec(-2)(within the 2 sigma isophote) and is associated with an additional four previously unknown embedded sources: two Ly a emitters and two faint active galactic nuclei (one type-1 and one type(-2) quasar). By mapping at high significance, the line-of-sight velocity in the entirety of the observed structure, we unveiled a large-scale coherent rotation-like pattern spanning similar to 300 km s(-1) with a velocity dispersion of < 270 km s(-1), which we interpret as a signature of the inspiraling accretion of substructures within the quasar's host halo. Future multiwavelength data will complement our MUSE observations and are definitely needed to fully characterize such a complex system. None the less, our observations reveal the potential of newsensitive integral-field spectrographs to characterize the dynamical state of diffuse gas on large scales in the young Universe, and thereby witness the assembly of galaxies.