Mg ii in the JWST era: a probe of Lyman continuum escape?

Harley Katz, Thibault Garel, Joakim Rosdahl, Valentin Mauerhofer, Taysun Kimm, Jérémy Blaizot, Léo Michel-Dansac, Julien Devriendt, Adrianne Slyz, Martin Haehnelt

Research output: Contribution to journalArticlepeer-review

14 Citations (Scopus)


Limited constraints on the evolution of the Lyman Continuum (LyC) escape fraction represent one of the primary uncertainties in the theoretical determination of the reionization history. Due to the intervening intergalactic medium, the possibility of observing LyC photons directly in the epoch of reionization is highly unlikely. For this reason, multiple indirect probes of LyC escape have been identified, some of which are used to identify low-redshift LyC leakers (e.g. O32), while others are primarily useful at z > 6 (e.g. [O iii]/[C iii] far-infrared emission). The flux ratio of the resonant Mg ii doublet emission at 2796$\mathring{\rm A}$ and 2803$\mathring{\rm A}$ as well as the Mg ii optical depth have recently been proposed as ideal diagnostics of LyC leakage that can be employed at z > 6 with JWST. Using state-of-the-art cosmological radiation hydrodynamics simulations post-processed with CLOUDY and resonant-line radiative transfer, we test whether Mg ii is indeed a useful probe of LyC leakage. Our simulations indicate that the majority of bright, star-forming galaxies with high LyC escape fractions are expected to be Mg ii emitters rather than absorbers at z = 6. However, we find that the Mg ii doublet flux ratio is a more sensitive indicator of dust rather than neutral hydrogen, limiting its use as a LyC leakage indicator to only galaxies in the optically thin regime. Given its resonant nature, we show that Mg ii will be an exciting probe of the complex kinematics in high-redshift galaxies in upcoming JWST observations.

Original languageEnglish
Pages (from-to)4265-4286
Number of pages22
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Publication statusPublished - 2022 Sept 1

Bibliographical note

Publisher Copyright:
© 2022 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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