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

Sandro Tacchella; Aaron Smith; Rahul Kannan; Federico Marinacci; Lars Hernquist; Mark Vogelsberger; Paul Torrey; Laura Sales; Hui Li

doi:10.1093/mnras/stac818

paper

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

arXiv:2112.00027 · doi:10.1093/mnras/stac818

Abstract

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 $f_{col} \sim 5 - 10%$ , only weakly dependent on radius and vertical height, and that scattering boosts the H $α$ luminosity by $\sim 40%$ . 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 $f_{abs} \approx 28%$ and $f_{He} \approx 9%$ , respectively. Together with an escape fraction of $f_{esc} \approx 6%$ , this reduces the available budget for hydrogen line emission by nearly half ( $f_{H} \approx 57%$ ). 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.

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

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

Abstract

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