Inspiraling Halo Accretion Mapped in Lyman-$\hat{I}\pm$ Emission around a $z\sim 3$ Quasar

Abstract

In an effort to search for Ly$\hat{I}\pm$ emission from circum- and intergalactic gas on scales of hundreds of kpc around $z\sim 3$ quasars, and thus characterise 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 with MUse: Search for Extended Ultraviolet eMission (QSO MUSEUM). In this work, we report the discovery of an enormous Ly$\hat{I}\pm$ nebula (ELAN) around the quasar SDSS~J102009.99+104002.7 at $z=3.164$, which we followed-up with deeper MUSE observations. This ELAN spans $\sim 297$ projected kpc, has an average Ly$\hat{I}\pm$ surface brightness ${\mathrm{SB}}_{\mathrm{Ly}\hat{\mathrm{I}}\pm }\sim 6.04\times 10^{-18}$ erg s${}^{-1}$ cm${}^{-2}$ arcsec${}^{-2}$ (within the $2\ddot{I}ƒ$ isophote), and is associated with an additional four, previously unknown embedded sources: two Ly$\hat{I}\pm$ 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 $\sim 300$ 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 characterise such a complex system. None the less, our observations reveal the potential of new sensitive integral-field spectrographs to characterise the dynamical state of diffuse gas on large scales in the young Universe, and thereby witness the assembly of galaxies.

39 pages with 27 figures and 5 appendices. Accepted to MNRAS