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

doi:10.3847/1538-4357/aaad62

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

Physics And Astronomy

Research output: Contribution to journal › Article › peer-review

75 Scopus citations

Abstract

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 ζ_iondistribution. After accounting for observational uncertainties and variations in dust attenuation, we conclude that the remaining scatter in ζ_ionis 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, ζ_ionis 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, ζ_iondoes 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 ζ_ionelevated by a factor of ∼2 relative to the average ζ_ionof 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 language	English
Article number	42
Journal	Astrophysical Journal
Volume	855
Issue number	1
DOIs	https://doi.org/10.3847/1538-4357/aaad62
State	Published - 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..

Keywords

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

Access to Document

10.3847/1538-4357/aaad62

Cite this

Shivaei, I., Reddy, N. A., Siana, B., Shapley, A. E., Kriek, M., Mobasher, B., Freeman, W. R., Sanders, R. L., Coil, A. L., Price, S. H., Fetherolf, T., Azadi, M., Leung, G., & Zick, T. (2018). The MOSDEF Survey: Direct Observational Constraints on the Ionizing Photon Production Efficiency, ξ _ion, at z ∼ 2. Astrophysical Journal, 855(1), Article 42. https://doi.org/10.3847/1538-4357/aaad62

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

abstract = "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.",

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

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

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: {\textcopyright} 2018. The American Astronomical Society. All rights reserved..",

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doi = "10.3847/1538-4357/aaad62",

language = "English",

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TY - JOUR

T1 - The MOSDEF Survey

T2 - Direct Observational Constraints on the Ionizing Photon Production Efficiency, ξ ion, at z ∼ 2

AU - Shivaei, Irene

AU - Reddy, Naveen A.

AU - Siana, Brian

AU - Shapley, Alice E.

AU - Kriek, Mariska

AU - Mobasher, Bahram

AU - Freeman, William R.

AU - Sanders, Ryan L.

AU - Coil, Alison L.

AU - Price, Sedona H.

AU - Fetherolf, Tara

AU - Azadi, Mojegan

AU - Leung, Gene

AU - Zick, Tom

N1 - 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..

PY - 2018/3/1

Y1 - 2018/3/1

N2 - 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.

AB - 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.

KW - H II regions

KW - dark ages, reionization, first stars

KW - galaxies: general

KW - galaxies: high-redshift

KW - galaxies: star formation

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