Spatially Resolved Gas Kinematics within a Ly$\alpha$ Nebula: Evidence
  for Large-scale Rotation
  We use spatially extended measurements of Ly$\alpha$ as well as less
optically thick emission lines from an $\approx$80 kpc Ly$\alpha$ nebula at
$z\approx1.67$ to assess the role of resonant scattering and to disentangle
kinematic signatures from Ly$\alpha$ radiative transfer effects. We find that
the Ly$\alpha$, CIV, HeII, and CIII] emission lines all tell a similar story in
this system, and that the kinematics are broadly consistent with large-scale
rotation. First, the observed surface brightness profiles are similar in extent
in all four lines, strongly favoring a picture in which the Ly$\alpha$ photons
are produced in situ instead of being resonantly scattered from a central
source. Second, we see low kinematic offsets between Ly$\alpha$ and the less
optically thick HeII line ($\sim$100-200 km s$^{-1}$), providing further
support for the argument that the Ly$\alpha$ and other emission lines are all
being produced within the spatially extended gas. Finally, the full velocity
field of the system shows coherent velocity shear in all emission lines:
$\approx$500 km s$^{-1}$ over the central $\approx$50 kpc of the nebula. The
kinematic profiles are broadly consistent with large-scale rotation in a gas
disk that is at least partially stable against collapse. These observations
suggest that the Ly$\alpha$ nebula represents accreting material that is
illuminated by an offset, hidden AGN or distributed star formation, and that is
undergoing rotation in a clumpy and turbulent gas disk. With an implied mass of
M(<R=20 kpc)$\sim3\times10^{11}$ $M_{\odot}$, this system may represent the
early formation of a large Milky Way mass galaxy or galaxy group.
