2018MNRAS.480.2094L

The Slug Nebula is one of the largest and most luminous Lyman-α (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 H α 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 H α emission. We also obtained a deep, moderate resolution Ly α spectrum of the nearby brightest region of the Slug. We detected an H α flux of F_ H α_= 2.62±0.47 ×10^–17 erg cm^–2 s^–1 (SB_ H α_=2.70±0.48 ×10^–18 erg cm^–2 s^–1 arcsec^–2 at the expected spatial and spectral location. Combining the H α detection with its corresponding Ly α flux (determined from the narrow-band imaging), we calculate a flux ratio of F_ Lyα_/F H α_=5.5±1.1. The presence of a skyline at the location of the H α emission decreases the signal-to-noise ratio of the detection and our ability to put stringent constraints on the H α 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.