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

Abstract

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 Ly α nebula, a dust-obscured, broad-absorption-line quasar, and neighboring galaxies. At a 3 σ surface brightness contour of 1.6 × 10 −17 erg s −1 cm −2 arcsec −2 , the Ly α nebula extends over 17 arcsec (≳140 physical kpc) and has a total Ly α 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, Ly α 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 Ly α rest-frame equivalent width greater than 312 Å (5 σ ). 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 β , He ii , C iv , etc, and with polarimetry will further reveal the origin of the Ly α nebula and nature of the dark cloud.