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

Kavli Affiliate: Mark Vogelsberger

| First 5 Authors: Sandro Tacchella, Aaron Smith, Rahul Kannan, Federico Marinacci, Lars Hernquist

| Summary:

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 $f_{rm
col}sim5-10%$, only weakly dependent on radius and vertical height, and that
scattering boosts the H$alpha$ luminosity by $sim40%$. The dust correction
via the Balmer decrement works well (intrinsic H$alpha$ 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$alpha$-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$alpha$ 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.

| Search Query: ArXiv Query: search_query=au:”Mark Vogelsberger”&id_list=&start=0&max_results=10

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