The Impact of Circumgalactic Rotation on Lyα Radiative Transfer
Abstract
Hydrogen Lyman-alpha (Lyα) is a prominent emission line from the circumgalactic medium (CGM). Due to its resonant nature, Lyα carries imprints of the physical properties and kinematics of the cold CGM. In particular, CGM rotation can modify the Lyα peak separation, which is often interpreted as a tracer of H I column density. We present 3D Monte Carlo Lyα radiative-transfer simulations in a CGM-like rotating medium and examine how the emergent spectra depend on rotational velocity (Vrot), H I column density (NHI), 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 NHI, numerous scatterings reduce the sensitivity of integrated spectra to rotation, producing a degeneracy between Vrot and NHI. Consequently, Lyα peak separation alone can overestimate NHI 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 Lyα 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 Lyα observation is essential for disentangling CGM rotation from radiative-transfer effects.
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BibTeX
@misc{kym2026impact,
title = {{The Impact of Circumgalactic Rotation on Ly$α$ Radiative Transfer}},
author = {Hee-Gyeong Kym and Seok-Jun Chang and Max Gronke and Kwang-Il Seon},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.08396},
eprint = {2609.08396},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.08396},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - The Impact of Circumgalactic Rotation on Lyα Radiative Transfer
AU - Hee-Gyeong Kym
AU - Seok-Jun Chang
AU - Max Gronke
AU - Kwang-Il Seon
PY - 2026
DA - 2026/09/08
PB - arXiv
DO - 10.48550/arxiv.2609.08396
UR - https://arxiv.org/abs/2609.08396
AB - Hydrogen Lyman-alpha (Lyα) is a prominent emission line from the circumgalactic medium (CGM). Due to its resonant nature, Lyα carries imprints of the physical properties and kinematics of the cold CGM. In particular, CGM rotation can modify the Lyα peak separation, which is often interpreted as a tracer of H I column density. We present 3D Monte Carlo Lyα 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, Lyα 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 Lyα 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 Lyα observation is essential for disentangling CGM rotation from radiative-transfer effects.
ER -