Two modes of LyC escape from bursty star formation: implications for [C II] deficits and the sources of reionization

Harley Katz, Aayush Saxena, Joki Rosdahl, Taysun Kimm, Jeremy Blaizot, Thibault Garel, Leo Michel-Dansac, Martin Haehnelt, Richard S. Ellis, Laura Penterrici, Julien Devriendt, Adrianne Slyz

Research output: Contribution to journalArticlepeer-review


We use the SPHINX20 cosmological radiation hydrodynamics simulation to study how Lyman continuum (LyC) photons escape from galaxies and the observational signatures of this escape. We define two classes of LyC leaker: Bursty Leakers and Remnant Leakers, based on their star formation rates (SFRs) that are averaged over 10 Myr (SFR10) or 100 Myr (SFR100). Both have fesc > 20 per cent and experienced an extreme burst of star formation, but Bursty Leakers have SFR10 > SFR100, while Remnant Leakers have SFR10 < SFR100. The maximum SFRs in these bursts were typically ∼100 times greater than the SFR of the galaxy prior to the burst, a rare 2σ outlier among the general high-redshift galaxy population. Bursty Leakers are qualitatively similar to ionization-bounded nebulae with holes, exhibiting high ionization parameters and typical H II region gas densities. Remnant Leakers show properties of density-bounded nebulae, having normal ionization parameters but much lower H II region densities. Both types of leaker exhibit [C II]158μm deficits on the [C II]–SFR100 relation, while only Bursty Leakers show deficits when SFR10 is used. We predict that [C II] luminosity and SFR indicators such as H α and M1500Å can be combined to identify both types of LyC leaker and the mode by which photons are escaping. These predictions can be tested with [C II] observations of known z = 3–4 LyC leakers. Finally, we show that leakers with fesc > 20 per cent dominate the ionizing photon budget at z ≳ 7.5 but the contribution from galaxies with fesc < 5 per cent becomes significant at the tail-end of reionization.

Original languageEnglish
Pages (from-to)270-285
Number of pages16
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Publication statusPublished - 2022 Jan 1

Bibliographical note

Funding Information:
We thank the referee for their comments which improved the manuscript. AS and RSE acknowledge financial support from European Research Council Advanced Grant FP7/669253. TG is supported by the ERC Starting grant 757258 ‘TRIPLE’. TK was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (No. 2020R1C1C1007079 and No. 2022R1A6A1A03053472). Computing time for this work was provided by the Partnership for Advanced Computing in Europe (PRACE) as part of the ‘First luminous objects and reionization with SPHINX (cont.)’ (2016153539, 2018184362, 2019215124) project. We thank Philipp Otte and Filipe Guimaraes for helpful support throughout the project and for the extra storage they provided us. We also thank GENCI for providing additional computing resources under GENCI grant A0070410560. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 693024). Some of this work used the DiRAC@Durham facility managed by the Institute for Computational Cosmology on behalf of the STFC DiRAC HPC Facility ( The equipment was funded by BEIS capital funding via STFC capital grants ST/P002293/1, ST/R002371/1, and ST/S002502/1, Durham University, and STFC operations grant ST/R000832/1. Some of this work was performed using the DiRAC Data Intensive service at Leicester, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility ( The equipment was funded by BEIS capital funding via STFC capital grants ST/K000373/1 and ST/R002363/1 and STFC DiRAC Operations grant ST/R001014/1. DiRAC is part of the National e-Infrastructure.

Publisher Copyright:
© 2022 The Author(s).

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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