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

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83 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

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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