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ABSTRACT The nebular recombination line H α is widely used as a star formation rate (SFR) indicator in the local and high-redshift Universe. We present a detailed H α radiative transfer study of high-resolution isolated Milky-Way and Large Magellanic Cloud simulations that include radiative transfer, non-equilibrium thermochemistry, and dust evolution. We focus on the spatial morphology and temporal variability of the H α emission, and its connection to the underlying gas and star formation properties. The H α and H β radial and vertical surface brightness profiles are in excellent agreement with observations of nearby galaxies. We find that the fraction of H α emission from collisional excitation amounts to fcol ∼ 5–$$10{{\ \rm per\ cent}}$$, only weakly dependent on radius and vertical height, and that scattering boosts the H α luminosity by $$\sim 40{{\ \rm per\ cent}}$$. The dust correction via the Balmer decrement works well (intrinsic H α emission recoverable within 25 per cent), though the dust attenuation law depends on the amount of attenuation itself both on spatially resolved and integrated scales. Important for the understanding of the H α–SFR connection is the dust and helium absorption of ionizing radiation (Lyman continuum [LyC] photons), which are about $$f_{\rm abs}\approx 28{{\ \rm per\ cent}}$$ and $$f_{\rm He}\approx 9{{\ \rm per\ cent}}$$, respectively. Together with an escape fraction of $$f_{\rm esc}\approx 6{{\ \rm per\ cent}}$$, this reduces the available budget for hydrogen line emission by nearly half ($$f_{\rm H}\approx 57{{\ \rm per\ cent}}$$). We discuss the impact of the diffuse ionized gas, showing – among other things – that the extraplanar H α emission is powered by LyC photons escaping the disc. Future applications of this framework to cosmological (zoom-in) simulations will assist in the interpretation of spectroscopy of high-redshift galaxies with the upcoming James Webb Space Telescope.
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This content will become publicly available on March 26, 2026
The Hα sky in three dimensions
ABSTRACT We combine parallax distances to nearby O stars with parsec-scale resolution three-dimensional dust maps of the local region of the Milky Way (within 1.25 kpc of the Sun) to simulate the transfer of Lyman continuum photons through the interstellar medium (ISM). Assuming a fixed gas-to-dust ratio, we determine the density of ionized gas, electron temperature, and H$$\alpha$$ emissivity throughout the local Milky Way. There is good morphological agreement between the predicted and observed H$$\alpha$$ all-sky map of the Wisconsin H$$\alpha$$ Mapper. We find that our simulation underproduces the observed H$$\alpha$$ emission while overestimating the sizes of H ii regions, and we discuss ways in which agreement between simulations and observations may be improved. Of the total ionizing luminosity of $$5.84 \times 10^{50}~{\rm photons \, s^{-1}}$$, 15 per cent is absorbed by dust, 64 per cent ionizes ‘classical’ H ii regions, 11 per cent ionizes the diffuse warm ionized medium, and 10 per cent escapes the simulation volume. We find that 18 per cent of the high-altitude ($$|b| > 30{}^{\circ }$$) H$$\alpha$$ arises from dust scattered rather than direct emission. These initial results provide an impressive validation of the three-dimensional dust maps and O-star parallaxes, opening a new frontier for studying the ionized ISM’s structure and energetics in three dimensions.
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- Award ID(s):
- 2009276
- PAR ID:
- 10648247
- Publisher / Repository:
- Oxford University Press on behalf of Royal Astronomical Society
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society: Letters
- Volume:
- 540
- Issue:
- 1
- ISSN:
- 1745-3925
- Page Range / eLocation ID:
- L21 to L27
- Subject(s) / Keyword(s):
- radiative transfer methods: numerical H II regions ISM: structure local interstellar matter Galaxy: structure
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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