H <i>α</i> 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 detailed radiative transfer study of high-resolution isolated Milky-Way Large Magellanic Cloud simulations that include transfer, non-equilibrium thermochemistry, dust evolution. focus on spatial morphology temporal variability emission, its connection to underlying gas star formation properties. H$\beta$ radial vertical surface brightness profiles are excellent agreement with observations nearby galaxies. find fraction emission from collisional excitation amounts $f_{\rm col}\sim5-10\%$, only weakly dependent radius height, scattering boosts luminosity by $\sim40\%$. correction via Balmer decrement works well (intrinsic recoverable within $25\%$), though attenuation law depends amount itself both spatially resolved integrated scales. Important for understanding H$\alpha$-SFR helium absorption ionizing radiation (Lyman continuum [LyC] photons), which about abs}\approx28\%$ He}\approx9\%$, respectively. Together an escape esc}\approx6\%$, this reduces available budget hydrogen nearly half ($f_{\rm H}\approx57\%$). discuss impact diffuse ionized gas, showing - among other things extraplanar powered LyC photons escaping disc. Future applications framework cosmological (zoom-in) will assist interpretation spectroscopy galaxies upcoming James Webb Space Telescope.