Discovery of a Lyα-emitting Dark Cloud within the z ∼ 2.8 SMM J02399-0136 System

We present Keck/Keck Cosmic Web Imager (KCWI) integral field spectrograph observations of the complex system surrounding SMM J02399-0136 (a lensed z = 2.8 sub-mm galaxy), including an associated Lyα nebula, a dust-obscured, broad-absorption-line quasar, and neighboring galaxies. At a 3σ surface brightness contour of 1.6 × 10^-17 erg s^-1 cm^-2 arcsec^-2, the Lyα nebula extends over 17 arcsec (≈140 physical kpc) and has a total Lyα luminosity of (uncorrected for lensing). The nebula exhibits a kinematic shear of over 100 pkpc with lowest velocities east of SMM J02399-0136 and increasing to the southwest. We also discover a bright, Lyα emitter, separated spatially and kinematically from the nebula, at a projected separation of ≈60 kpc from the quasar. This source has no clear central counterpart in deep Hubble Space Telescope imaging, giving an intrinsic Lyα rest-frame equivalent width greater than 312 A (5σ). We argue that this "dark cloud" is illuminated by the quasar with a UV flux that is orders of magnitude brighter than the emission along our sightline. This result confirms statistical inferences that luminous quasars at z < 2 emit UV radiation anisotropically. Future KCWI observations of other lines, e.g., Lyβ, He ii, C iv, etc, and with polarimetry will further reveal the origin of the Lyα nebula and nature of the dark cloud.