The detection of intergalactic Hα emission from the Slug Nebula at z ~ 2.3

The Slug Nebula is one of the largest and most luminous Lyman-a (Ly α) nebulae discovered to date, extending over 450 kpc around the bright quasar UM287 at z= 2.283. Characterized by high surface brightnesses over intergalactic scales, its Ly α emission may either trace high-density ionized gas ('clumps') or large column densities of neutral material. To distinguish between these two possibilities, information from a non-resonant line such as Hα is crucial. Therefore, we analysed a deep Multi-Object Spectrometer For Infra-Red Exploration (MOSFIRE) observation of one of the brightest Ly α-emitting regions in the Slug Nebula with the goal of detecting associated Hα emission. We also obtained a deep, moderate resolution Ly α spectrum of the nearby brightest region of the Slug. We detected an Hα flux of FHα = 2.62 ± 0.47 × 10-17 erg cm-2 s-1 (SBHα = 2.70 ± 0.48 × 10-18 erg cm-2 s-1 arcsec-2 at the expected spatial and spectral location. Combining the Hα detection with its corresponding Ly α flux (determined from the narrow-band imaging), we calculate a flux ratio of FLyα/FHα = 5.5 ± 1.1. The presence of a skyline at the location of the Hα emission decreases the signal-to-noise ratio of the detection and our ability to put stringent constraints on the Hα kinematics. Our measurements argue for the origin of the Ly α emission being recombination radiation, suggesting the presence of high-density ionized gas. Finally, our high-resolution spectroscopic study of the Ly α emission does not show evidence of a rotating disc pattern and suggest a more complex origin for at least some parts of the Slug Nebula.