The far-ultraviolet continuum slope as a Lyman Continuum escape estimator at high redshift

J. Chisholm, A. Saldana-Lopez, S. Flury, D. Schaerer, A. Jaskot, R. Amorín, H. Atek, S. L. Finkelstein, B. Fleming, H. Ferguson, V. Fernández, M. Giavalisco, M. Hayes, T. Heckman, A. Henry, Z. Ji, R. Marques-Chaves, V. Mauerhofer, S. Mccandliss, M. S. OeyG. Östlin, michael j rutkowski, C. Scarlata, T. Thuan, M. Trebitsch, B. Wang, G. Worseck, X. Xu

Research output: Contribution to journalArticlepeer-review

40 Scopus citations


Most of the hydrogen in the intergalactic medium (IGM) was rapidly ionized at high redshifts. While observations have established that reionization occurred, observational constraints on the high-redshift ionizing emissivity remain elusive. Here, we present a new analysis of the Low-redshift Lyman Continuum Survey (LzLCS) and literature observations, a combined sample of 89 star-forming galaxies at redshifts near 0.3 with Hubble Space Telescope observations of their ionizing continua (or Lyman Continuum, LyC). We find a strong (6σ significant) inverse correlation between the continuum slope at 1550 Å (defined as Fλ ∝ λ {β 1550 obs) and both the LyC escape fraction (fesc,LyC) and fesc,LyC times the ionizing photon production efficiency (ζion). On average, galaxies with redder continuum slopes have smaller fesc,LyC than galaxies with bluer slopes mainly due to higher dust attenuation. More than 5 per cent (20 per cent) of the LyC emission escapes galaxies with βobs 1550 <-2.1 (-2.6). We find strong correlations between βobs 1550 and the [O iii]/[O ii] flux ratio (at 7.5σ significance), galaxy stellar mass (at 5.9σ), the gas-phase metallicity (at 4.6σ), and the observed far-ultraviolet absolute magnitude (at 3.4σ). Using previous observations of β obs 1550 at high redshift, we estimate the evolution of fesc,LyC with both redshift and galaxy magnitude. The LzLCS observations suggest that fainter and lower mass galaxies dominate the ionizing photon budget at higher redshift, possibly due to their rapidly evolving metal and dust content. Finally, we use our correlation between βobs 1550 and fesc,LyC × ζion to predict the ionizing emissivity of galaxies during the epoch of reionization. Our estimated emissivities match IGM observations, and suggest that star-forming galaxies emit sufficient LyC photons into the IGM to exceed recombinations near redshifts of 7-8.

Original languageEnglish (US)
Pages (from-to)5104-5120
Number of pages17
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
StatePublished - Dec 1 2022
Externally publishedYes

Bibliographical note

Funding Information:
Support for this work was provided by NASA through grant number HST-GO-15626 from the Space Telescope Science Institute.

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


  • dark ages, reionization, first stars
  • galaxies: high-redshift
  • galaxies: starburst
  • ultraviolet: galaxies


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