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

TL;DR

This study uses spatially extended emission line measurements from an 80 kpc Ly$ ext{a}$ nebula at z≈1.67 to investigate gas kinematics, revealing evidence for large-scale rotation and in situ emission, suggesting early galaxy formation processes.

Contribution

It provides the first detailed kinematic analysis of a Ly$ ext{a}$ nebula showing large-scale rotation and in situ emission, advancing understanding of galaxy formation at high redshift.

Findings

Ly$ ext{a}$ and other emission lines have similar spatial extent.

Low velocity offsets support in situ emission origin.

Velocity shear indicates large-scale rotation.

Abstract

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.