The production and escape of Lyman-Continuum radiation from star-forming galaxies at z ∼ 2 and their redshift evolution

Jorryt Matthee, David Sobral, Philip Best, Ali Ahmad Khostovan, Iván Oteo, Rychard Bouwens, Huub Röttgering

Research output: Contribution to journalArticlepeer-review


We study the production rate of ionizing photons of a sample of 588 Hα emitters (HAEs) and 160 Lyman-α emitters (LAEs) at z = 2.2 in the COSMOS field in order to assess the implied emissivity from galaxies, based on their ultraviolet (UV) luminosity. By exploring the rest-frame Lyman Continuum (LyC) with GALEX/NUV data, we find fesc <2.8 (6.4) per cent through median (mean) stacking. By combining the Hα luminosity density with intergalactic medium emissivity measurements from absorption studies, we find a globally averaged of 5.9^{+14.5}_{-4.2} per cent at z = 2.2 if we assume HAEs are the only source of ionizing photons. We find similarly low values of the global at z ≈ 3-5, also ruling out a high at z <5. These low escape fractions allow us to measure ξion, the number of produced ionizing photons per unit UV luminosity, and investigate how this depends on galaxy properties. We find a typical ξion ≈ 1024.77 ± 0.04 Hz erg-1 for HAEs and ξion ≈ 1025.14 ± 0.09 Hz erg-1 for LAEs. LAEs and low-mass HAEs at z = 2.2 show similar values of ξion as typically assumed in the reionization era, while the typical HAE is three times less ionizing. Due to an increasing ξion with increasing EW(Hα), ξion likely increases with redshift. This evolution alone is fully in line with the observed evolution of ξion between z ≈ 2 and 5, indicating a typical value of ξion ≈ 1025.4 Hz erg-1 in the reionization era.
Original languageEnglish
Pages (from-to)3637-3655
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Publication statusPublished - 17 Nov 2016


  • galaxies: evolution
  • galaxies: high-redshift
  • cosmology: observations
  • dark ages
  • reionization
  • first stars


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