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H-alpha emission in local galaxies: star formation, time variability and the diffuse ionized gas

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 $f_{\rm col}\sim5-10\%$, only weakly dependent on radius and vertical height, and that scattering boosts the H$α$ luminosity by $\sim40\%$. The dust correction via the Balmer decrement works well (intrinsic H$α$ emission recoverable within $25\%$), 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 $f_{\rm abs}\approx28\%$ and $f_{\rm He}\approx9\%$, respectively. Together with an escape fraction of $f_{\rm esc}\approx6\%$, this reduces the available budget for hydrogen line emission by nearly half ($f_{\rm H}\approx57\%$). 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.