Analytic Model for Scattered Ly$\boldsymbolα$ Emission in the Post-reionization Intergalactic Medium
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arXiv
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| Main Authors: | , , , , , |
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| Format: | Preprint |
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2025
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| _version_ | 1866915634306088960 |
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| author | Park, Hyunbae Song, Hyunmi Byrohl, Chris Smith, Aaron Hidenobu, Yajima Lukić, Zarija |
| author_facet | Park, Hyunbae Song, Hyunmi Byrohl, Chris Smith, Aaron Hidenobu, Yajima Lukić, Zarija |
| contents | Ly$α$ intensity mapping is emerging as a new probe of faint galaxies consisting the cosmic web that elude traditional surveys. However, the resonant nature of Ly$α$ radiative transfer complicates the interpretation of observed data. In this study, we develop a fast and accurate analytic prescription for computing the Ly$α$ intensity field on Mpc scales in the post-reionization Universe. Motivated by insights from Monte Carlo radiative transfer (MCRT) experiments, we exploit the fact that in a highly ionized intergalactic medium (IGM) with negligible damping-wing opacity, cosmological redshifting quickly drives Ly$α$ photons out of resonance, terminating the scattering process and simplifying their large-scale behavior. Photons emitted blueward of the Ly$α$ line center tend to scatter on a thin, nearly spherical surface of last scattering, with a radius determined by the redshifting distance to resonance. Based on this behavior, we derive closed-form expressions for the scattered emissivity and projected surface brightness that depend only on the source spectrum, the HI density, and the peculiar velocity field. When applied to a source in a realistically simulated IGM at $z = 3$, our model shows mild discrepancies with MCRT results within a physical Mpc of the host halo, where strong gravitational infall redistributes the scattered photons, but achieves better than 5% accuracy beyond that distance in angle-averaged radial surface brightness profile. Our prescription offers a computationally efficient alternative to MCRT for forward-modeling Ly$α$ intensity maps from cosmological simulations, enabling the inference of underlying cosmological and astrophysical parameters from future observations |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2509_11268 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Analytic Model for Scattered Ly$\boldsymbolα$ Emission in the Post-reionization Intergalactic Medium Park, Hyunbae Song, Hyunmi Byrohl, Chris Smith, Aaron Hidenobu, Yajima Lukić, Zarija Cosmology and Nongalactic Astrophysics Ly$α$ intensity mapping is emerging as a new probe of faint galaxies consisting the cosmic web that elude traditional surveys. However, the resonant nature of Ly$α$ radiative transfer complicates the interpretation of observed data. In this study, we develop a fast and accurate analytic prescription for computing the Ly$α$ intensity field on Mpc scales in the post-reionization Universe. Motivated by insights from Monte Carlo radiative transfer (MCRT) experiments, we exploit the fact that in a highly ionized intergalactic medium (IGM) with negligible damping-wing opacity, cosmological redshifting quickly drives Ly$α$ photons out of resonance, terminating the scattering process and simplifying their large-scale behavior. Photons emitted blueward of the Ly$α$ line center tend to scatter on a thin, nearly spherical surface of last scattering, with a radius determined by the redshifting distance to resonance. Based on this behavior, we derive closed-form expressions for the scattered emissivity and projected surface brightness that depend only on the source spectrum, the HI density, and the peculiar velocity field. When applied to a source in a realistically simulated IGM at $z = 3$, our model shows mild discrepancies with MCRT results within a physical Mpc of the host halo, where strong gravitational infall redistributes the scattered photons, but achieves better than 5% accuracy beyond that distance in angle-averaged radial surface brightness profile. Our prescription offers a computationally efficient alternative to MCRT for forward-modeling Ly$α$ intensity maps from cosmological simulations, enabling the inference of underlying cosmological and astrophysical parameters from future observations |
| title | Analytic Model for Scattered Ly$\boldsymbolα$ Emission in the Post-reionization Intergalactic Medium |
| topic | Cosmology and Nongalactic Astrophysics |
| url | https://arxiv.org/abs/2509.11268 |