A JWST/NIRSpec Exploration of the Connection between Ionization Parameter, Electron Density, and Star-formation-rate Surface Density in z = 2.7-6.3 Galaxies

Naveen A. Reddy; Michael W. Topping; Ryan L. Sanders; Alice E. Shapley; Gabriel Brammer

doi:10.3847/1538-4357/acd754

A JWST/NIRSpec Exploration of the Connection between Ionization Parameter, Electron Density, and Star-formation-rate Surface Density in z = 2.7-6.3 Galaxies

Naveen A. Reddy
, Michael W. Topping
, Ryan L. Sanders
, Alice E. Shapley
, Gabriel Brammer

Physics And Astronomy

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

74 Scopus citations

Abstract

We examine the factors responsible for the variation in the ionization parameter (U) of high-redshift star-forming galaxies based on medium-resolution JWST/NIRSpec observations obtained by the Cosmic Evolution Early Release Science survey. The sample consists of 48 galaxies with redshifts z _spec = 2.7−6.3, which are largely representative of typical galaxies at these redshifts. The [S ii] λ λ6718, 6733 doublet is used to estimate electron densities (n _e), and dust-corrected Hα luminosities are used to compute ionizing photon rates (Q). Using composite spectra of galaxies in bins of [O iii] λ λ4960, 5008/[O ii] λ λ3727, 3730 (O32) as a proxy for U, we determine that galaxies with higher O32 have 〈n _e〉 ≃ 500 cm⁻³ that are ≳5 × larger than that of lower-O32 galaxies. We do not find a significant difference in 〈Q〉 between low- and high-O32 galaxies. Photoionization modeling indicates a large spread in log U of ≈1.5 dex at a fixed Z _neb. On the other hand, the data indicate a highly significant correlation between U and star-formation-rate surface density (Σ_SFR), which appears to be redshift invariant at z ∼ 1.6−6.3, and possibly up to z ∼ 9.5. We consider several avenues through which metallicity and Σ_SFR (or gas density) may influence U, including variations in n _e and Q, internal dust extinction of ionizing photons, and the effects of gas density on the volume filling fraction. Based on these considerations, we conclude that gas density may play a more central role than metallicity in modulating U at these redshifts.

Original language	English
Article number	167
Journal	Astrophysical Journal
Volume	952
Issue number	2
DOIs	https://doi.org/10.3847/1538-4357/acd754
State	Published - Aug 1 2023

Bibliographical note

Publisher Copyright:
© 2023. The Author(s). Published by the American Astronomical Society.

Funding

We acknowledge the entire CEERS team for their effort to design and execute this Early Release Science observational program. We thank Natascha Förster Schreiber for useful conversations. This work is based on observations made with the NASA/ESA/CSA JWST. The data were obtained from the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-03127 for JWST. The specific observations analyzed can be accessed via DOI:10.17909/z7p0-8481. Support for this work was also provided through the NASA Hubble Fellowship grant HST-HF2-51469.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. The Cosmic Dawn Center is funded by the Danish National Research Foundation (DNRF) under grant No. 140. Cloud-based data processing and file storage for this work is provided by the AWS Cloud Credits for Research program. We acknowledge the entire CEERS team for their effort to design and execute this Early Release Science observational program. We thank Natascha Förster Schreiber for useful conversations. This work is based on observations made with the NASA/ESA/CSA JWST. The data were obtained from the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-03127 for JWST. The specific observations analyzed can be accessed via DOI: 10.17909/z7p0-8481 . Support for this work was also provided through the NASA Hubble Fellowship grant HST-HF2-51469.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. The Cosmic Dawn Center is funded by the Danish National Research Foundation (DNRF) under grant No. 140. Cloud-based data processing and file storage for this work is provided by the AWS Cloud Credits for Research program.

Funders	Funder number
National Aeronautics and Space Administration	HST-HF2-51469.001-A
National Aeronautics and Space Administration
Entomological Society of America
Space Telescope Science Institute	NAS5-26555
Space Telescope Science Institute
Canadian Space Agency	NAS5-03127
Canadian Space Agency
Danmarks Grundforskningsfond	140
Danmarks Grundforskningsfond

ASJC Scopus subject areas

Astronomy and Astrophysics
Space and Planetary Science

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10.3847/1538-4357/acd754

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title = "A JWST/NIRSpec Exploration of the Connection between Ionization Parameter, Electron Density, and Star-formation-rate Surface Density in z = 2.7-6.3 Galaxies",

abstract = "We examine the factors responsible for the variation in the ionization parameter (U) of high-redshift star-forming galaxies based on medium-resolution JWST/NIRSpec observations obtained by the Cosmic Evolution Early Release Science survey. The sample consists of 48 galaxies with redshifts z spec = 2.7−6.3, which are largely representative of typical galaxies at these redshifts. The [S ii] λ λ6718, 6733 doublet is used to estimate electron densities (n e ), and dust-corrected Hα luminosities are used to compute ionizing photon rates (Q). Using composite spectra of galaxies in bins of [O iii] λ λ4960, 5008/[O ii] λ λ3727, 3730 (O32) as a proxy for U, we determine that galaxies with higher O32 have 〈n e〉 ≃ 500 cm−3 that are ≳5 × larger than that of lower-O32 galaxies. We do not find a significant difference in 〈Q〉 between low- and high-O32 galaxies. Photoionization modeling indicates a large spread in log U of ≈1.5 dex at a fixed Z neb. On the other hand, the data indicate a highly significant correlation between U and star-formation-rate surface density (ΣSFR), which appears to be redshift invariant at z ∼ 1.6−6.3, and possibly up to z ∼ 9.5. We consider several avenues through which metallicity and ΣSFR (or gas density) may influence U, including variations in n e and Q, internal dust extinction of ionizing photons, and the effects of gas density on the volume filling fraction. Based on these considerations, we conclude that gas density may play a more central role than metallicity in modulating U at these redshifts.",

author = "Reddy, \{Naveen A.\} and Topping, \{Michael W.\} and Sanders, \{Ryan L.\} and Shapley, \{Alice E.\} and Gabriel Brammer",

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AU - Reddy, Naveen A.

AU - Topping, Michael W.

AU - Sanders, Ryan L.

AU - Shapley, Alice E.

AU - Brammer, Gabriel

PY - 2023/8/1

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AB - We examine the factors responsible for the variation in the ionization parameter (U) of high-redshift star-forming galaxies based on medium-resolution JWST/NIRSpec observations obtained by the Cosmic Evolution Early Release Science survey. The sample consists of 48 galaxies with redshifts z spec = 2.7−6.3, which are largely representative of typical galaxies at these redshifts. The [S ii] λ λ6718, 6733 doublet is used to estimate electron densities (n e ), and dust-corrected Hα luminosities are used to compute ionizing photon rates (Q). Using composite spectra of galaxies in bins of [O iii] λ λ4960, 5008/[O ii] λ λ3727, 3730 (O32) as a proxy for U, we determine that galaxies with higher O32 have 〈n e〉 ≃ 500 cm−3 that are ≳5 × larger than that of lower-O32 galaxies. We do not find a significant difference in 〈Q〉 between low- and high-O32 galaxies. Photoionization modeling indicates a large spread in log U of ≈1.5 dex at a fixed Z neb. On the other hand, the data indicate a highly significant correlation between U and star-formation-rate surface density (ΣSFR), which appears to be redshift invariant at z ∼ 1.6−6.3, and possibly up to z ∼ 9.5. We consider several avenues through which metallicity and ΣSFR (or gas density) may influence U, including variations in n e and Q, internal dust extinction of ionizing photons, and the effects of gas density on the volume filling fraction. Based on these considerations, we conclude that gas density may play a more central role than metallicity in modulating U at these redshifts.

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ER -

A JWST/NIRSpec Exploration of the Connection between Ionization Parameter, Electron Density, and Star-formation-rate Surface Density in z = 2.7-6.3 Galaxies

Abstract

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