The Impact of Circumgalactic Rotation on Ly alpha Radiative Transfer

Hydrogen Lyman-alpha (Ly α) is a prominent emission line from the circumgalactic medium (CGM). Due to its resonant nature, Ly α carries imprints of the physical properties and kinematics of the cold CGM. In particular, CGM rotation can modify the Ly α peak separation, which is often interpreted as a tracer of H I column density. We present 3D Monte Carlo Ly α radiative-transfer simulations in a CGM-like rotating medium and examine how the emergent spectra depend on rotational velocity (V rot), H I column density (N HI), viewing angle, clumpiness, and intrinsic source width. We find that rotation broadens the integrated spectra and increases the peak separation, with the strongest viewing-angle dependence when rotational Doppler shifts dominate over frequency diffusion. At high N HI, numerous scatterings reduce the sensitivity of integrated spectra to rotation, producing a degeneracy between V rot and N HI. Consequently, Ly α peak separation alone can overestimate N HI in a rotating medium. Spatially resolved halo spectra provide a clearer diagnostic: opposite sides of the rotating medium show systematic redshifted and blueshifted asymmetries associated with the line-of-sight velocity of the last-scattering gas. Such rotation-driven signatures can also contribute to velocity-map patterns often interpreted in terms of inflow or outflow, highlighting the need to consider rotation in spatially resolved Ly α observations. We further show that the main signatures persist in simple clumpy media, while the halo signatures are largely insensitive to the intrinsic source width. Our results demonstrate that spatially resolved Ly α observation is essential for disentangling CGM rotation from radiative-transfer effects.