Spatially Resolved Gas Kinematics within a Ly$α$ Nebula: Evidence for Large-scale Rotation

We use spatially extended measurements of Ly$α$ as well as less optically thick emission lines from an $\approx$80 kpc Ly$α$ nebula at $z\approx1.67$ to assess the role of resonant scattering and to disentangle kinematic signatures from Ly$α$ radiative transfer effects. We find that the Ly$α$, 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$α$ photons are produced in situ instead of being resonantly scattered from a central source. Second, we see low kinematic offsets between Ly$α$ and the less optically thick HeII line ($\sim$100-200 km s$^{-1}$), providing further support for the argument that the Ly$α$ 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$50 kpc of the nebula. The kinematic profiles are broadly consistent with large-scale rotation in a gas disk that is at least partially stable against collapse. These observations suggest that the Ly$α$ 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.

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