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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. 

We present new JWST observations of the nearby, prototypical edge-on, spiral galaxy NGC 891. The northern half of the disk was observed with NIRCam in its F150W and F277W filters. Absorption is clearly visible in the mid-plane of the F150W image, along with vertical dusty plumes that closely resemble the ones seen in the optical. A ∼10 × 3 kpc²area of the lower circumgalactic medium (CGM) was mapped with MIRI F770W at 12 pc scales. Thanks to the sensitivity and resolution of JWST, we detect dust emission out to ∼4 kpc from the disk, in the form of filaments, arcs, and super-bubbles. Some of these filaments can be traced back to regions with recent star formation activity, suggesting that feedback-driven galactic winds play an important role in regulating baryonic cycling. The presence of dust at these altitudes raises questions about the transport mechanisms at play and suggests that small dust grains are able to survive for several tens of million years after having been ejected by galactic winds in the disk-halo interface. We lay out several scenarios that could explain this emission: dust grains may be shielded in the outer layers of cool dense clouds expelled from the galaxy disk, and/or the emission comes from the mixing layers around these cool clumps where material from the hot gas is able to cool down and mix with these cool cloudlets. This first set of data and upcoming spectroscopy will be very helpful to understand the survival of dust grains in energetic environments, and their contribution to recycling baryonic material in the mid-plane of galaxies.