Discovery of a Lya emitting dark-cloud within z ~ 2.8 SMMJ02399-0136
  system
  We present Keck/KCWI integral field spectrograph observations of the complex
system surrounding SMM J02399$-$0136 (a lensed $z=2.8$ sub-mm galaxy),
including an associated Ly$\rm \alpha$ nebula, a dust-obscured,
broad-absorption-line quasar, and neighboring galaxies. At a 3$\sigma$ surface
brightness contour of 1.6$\times$ 10$^{-17}$ erg s$^{-1}$ cm$^{-2}$
arcsec$^{-2}$, the Ly$\rm \alpha$ nebula extends over 17 arcsec ($\gtrsim 140$
physical kpc) and has a total Ly$\rm \alpha$ luminosity of $2.5 \times 10^{44}
\, \rm erg \, s^{-1}$ (uncorrected for lensing). The nebula exhibits a
kinematic shear of $\sim$ 1000 km s$^{-1}$ over 100 pkpc with lowest velocities
east of SMM J02399$-$0136 and increasing to the southwest. We also discover a
bright, Ly$\rm \alpha$ emitter, separated spatially and kinematically from the
nebula, at a projected separation of $\approx$60 kpc from the quasar. This
source has no clear central counterpart in deep $Hubble \, Space \, Telescope$
imaging, giving an intrinsic Ly$\rm \alpha$ rest-frame equivalent width greater
than 312 $ \rm \mathring{A}$ (5-$\sigma$). 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. Ly$\rm \beta$, He$\rm _{II}$,
C$\rm _{IV}$, etc, and with polarimetry will further reveal the origin of the
Ly$\rm \alpha$ nebula and nature of the dark cloud.
