The Hα sky in three dimensions

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 Hα emissivity throughout the local Milky Way. There is good morphological agreement between the predicted and observed Hα all-sky map of the Wisconsin Hα Mapper. We find that our simulation underproduces the observed Hα 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 × 10⁵⁰ 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^◦) Hα 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.