The MOSDEF Survey: Direct Observational Constraints on the Ionizing Photon Production Efficiency, ξ ion, at z ∼ 2

Irene Shivaei, Naveen A. Reddy, Brian Siana, Alice E. Shapley, Mariska Kriek, Bahram Mobasher, William R. Freeman, Ryan L. Sanders, Alison L. Coil, Sedona H. Price, Tara Fetherolf, Mojegan Azadi, Gene Leung, Tom Zick

Research output: Contribution to journalArticlepeer-review

75 Scopus citations


We combine Hα and Hβ spectroscopic measurements and UV photometry for a sample of 673 galaxies from the MOSDEF survey to constrain hydrogen-ionizing photon production efficiencies (ζion) at z = 1.4-2.6. We find 〈log(ζion/[s-1erg s-1Hz-1])〉=25.06(25.34), assuming the Calzetti (SMC) curve for the UV dust correction and a scatter of 0.28 dex in the ζiondistribution. After accounting for observational uncertainties and variations in dust attenuation, we conclude that the remaining scatter in ζionis likely dominated by galaxy-to-galaxy variations in stellar populations, including the slope and upper-mass cutoff of the initial mass function, stellar metallicity, star formation burstiness, and stellar evolution (e.g., single/binary star evolution). Moreover, ζionis elevated in galaxies with high ionization states (high [O iii]/[O ii]) and low oxygen abundances (low [N ii]/Hα and high [O iii]/Hβ) in the ionized ISM. However, ζiondoes not correlate with the offset from the z ∼ 0 star-forming locus in the BPT diagram, suggesting no change in the hardness of the ionizing radiation accompanying the offset from the z ∼ 0 sequence. We also find that galaxies with blue UV spectral slopes (〈β〉=-2.1) have ζionelevated by a factor of ∼2 relative to the average ζionof the sample (〈β〉=-1.4). If these blue galaxies are similar to those at z > 6, our results suggest that a lower Lyman-continuum escape fraction is required for galaxies to maintain reionization, compared to the canonical predictions from stellar population models. Furthermore, we demonstrate that even with robustly dust-corrected Hα, the UV dust attenuation can cause on average a ∼0.3 dex systematic uncertainty in calculations.

Original languageEnglish
Article number42
JournalAstrophysical Journal
Issue number1
StatePublished - Mar 1 2018

Bibliographical note

Funding Information:
I.S. thanks Rychard Bouwens and George Rieke for helpful comments. Funding for the MOSDEF survey is provided by NSF AAG grants AST-1312780, 1312547, 1312764, and 1313171 and archival grant AR-13907, provided by NASA through a grant from the Space Telescope Science Institute. The data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

Publisher Copyright:
© 2018. The American Astronomical Society. All rights reserved..


  • H II regions
  • dark ages, reionization, first stars
  • galaxies: general
  • galaxies: high-redshift
  • galaxies: star formation

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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