  The Slug Nebula is one of the largest and most luminous Lyman-alpha (LyA)
nebulae discovered to date, extending over 450 kiloparsecs (kpc) around the
bright quasar UM287 at z=2.283. Characterized by high surface brightnesses over
intergalactic scales, its LyA 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
Halpha is crucial. Therefore, we analyzed a deep MOSFIRE observation of one of
the brightest LyA emitting regions in the Slug Nebula with the goal of
detecting associated Halpha emission. We also obtained a deep, moderate
resolution LyA spectrum of the nearby brightest region of the Slug. We detected
an Halpha flux of F_(Halpha)= 2.62 +/- 0.47 x 10^-17 erg/cm^2/s
(SB_(Halpha)=2.70 +/- 0.48 x 10^-18 erg/cm^2/s/sq") at the expected spatial and
spectral location. Combining the Halpha detection with its corresponding LyA
flux (determined from the narrow-band imaging) we calculate a flux ratio of
F_(LyA_/F_(Halpha)= 5.5 +/- 1.1. The presence of a skyline at the location of
the Halpha emission decreases the signal to noise ratio of the detection and
our ability to put stringent constraints on the Halpha kinematics. Our
measurements argue for the origin of the LyA emission being recombination
radiation, suggesting the presence of high-density ionized gas. Finally, our
high-resolution spectroscopic study of the LyA emission does not show evidence
of a rotating disk pattern and suggest a more complex origin for at least some
parts of the Slug Nebula.
