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

The nebular recombination line H alpha is widely used as a star formation rate (SFR) indicator in the local and high-redshift Universe. We present a detailed H alpha 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 alpha emission, and its connection to the underlying gas and star formation properties. The H alpha and H beta radial and vertical surface brightness profiles are in excellent agreement with observations of nearby galaxies. We find that the fraction of H alpha emission from collisional excitation amounts to f(col) similar to 5-10 per cent, only weakly dependent on radius and vertical height, and that scattering boosts the H alpha luminosity by similar to 40 per cent. The dust correction via the Balmer decrement works well (intrinsic Ha 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 alpha-SFR connection is the dust and helium absorption of ionizing radiation (Lyman continuum [LyC] photons), which are about f(abs) approximate to 28 per cent and f(He) approximate to 9 per cent, respectively. Together with an escape fraction of f(esc) approximate to 6 per cent, this reduces the available budget for hydrogen line emission by nearly half (f(H) approximate to 57 per cent). We discuss the impact of the diffuse ionized gas, showing - among other things - that the extraplanar H alpha 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.