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

Abstract

The nebular recombination line H$\hat{I}\pm$ is widely used as a star-formation rate (SFR) indicator in the local and high-redshift Universe. We present a detailed H$\hat{I}\pm$ 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$\hat{I}\pm$ emission, and its connection to the underlying gas and star formation properties. The H$\hat{I}\pm$ and H${\hat{I}}^2$ radial and vertical surface brightness profiles are in excellent agreement with observations of nearby galaxies. We find that the fraction of H$\hat{I}\pm$ emission from collisional excitation amounts to $f_{\mathrm{col}}\sim 5-10\%$, only weakly dependent on radius and vertical height, and that scattering boosts the H$\hat{I}\pm$ luminosity by $\sim 40\%$. The dust correction via the Balmer decrement works well (intrinsic H$\hat{I}\pm$ 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$\hat{I}\pm$-SFR connection is the dust and helium absorption of ionizing radiation (Lyman continuum [LyC] photons), which are about $f_{\mathrm{abs}}\approx 28\%$ and $f_{\mathrm{He}}\approx 9\%$, respectively. Together with an escape fraction of $f_{\mathrm{esc}}\approx 6\%$, this reduces the available budget for hydrogen line emission by nearly half ($f_{\mathrm{H}}\approx 57\%$). We discuss the impact of the diffuse ionized gas, showing - among other things - that the extraplanar H$\hat{I}\pm$ 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.

26 pages, 20 figures, matches version accepted for publication in MNRAS