Discovery of a Lya emitting dark-cloud within z ~ 2.8 SMMJ02399-0136 system

Abstract

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$\hat{\mathrm{I}}\pm$ nebula, a dust-obscured, broad-absorption-line quasar, and neighboring galaxies. At a 3$\ddot{I}ƒ$ surface brightness contour of 1.6$\times$ 10${}^{-17}$ erg s${}^{-1}$ cm${}^{-2}$ arcsec${}^{-2}$, the Ly$\hat{\mathrm{I}}\pm$ nebula extends over 17 arcsec ($\gtrsim 140$ physical kpc) and has a total Ly$\hat{\mathrm{I}}\pm$ luminosity of $2.5\times 10^{44}\mathrm{erg}{\mathrm{s}}^{-1}$ (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$-$0136 and increasing to the southwest. We also discover a bright, Ly$\hat{\mathrm{I}}\pm$ 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 $HubbleSpaceTelescope$ imaging, giving an intrinsic Ly$\hat{\mathrm{I}}\pm$ rest-frame equivalent width greater than 312 $\mathring{\mathrm{A}}$ (5-$\ddot{I}ƒ$). 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${\hat{\mathrm{I}}}^2$, He${}_{\mathrm{II}}$, C${}_{\mathrm{IV}}$, etc, and with polarimetry will further reveal the origin of the Ly$\hat{\mathrm{I}}\pm$ nebula and nature of the dark cloud.

8 pages, 4 figures, accepted by ApJ