Population III Host Candidates at z ∼ 2: Strong He II λ 1640 and Absent Ultraviolet Metal Lines in HETDEX Ly α Emitters
9 affiliations
- 1The University of Texas at Austin
- 2The Pennsylvania State University
- 3University of Hull
- 4Universität Göttingen
- 5Yunnan University
- 6Waseda Universit
- 7MIRAI Technology Institute
- 8Missouri University of Science and Technology
- 9The University of Tokyo Institutes for Advanced Study (UTIAS)
Abstract
Population III (Pop III) stars are expected to produce extremely hard ionizing spectra, yet direct evidence for their signature—strong narrow He ii λ 1640 emission with weak or absent metal lines—remains elusive. Although Pop III formation peaks at z ≳ 10, models predict that nearly pristine gas pockets can survive to later times, making intermediate-redshift searches a probe of metal mixing across cosmic time. We search 109,545 high-confidence Ly α -emitting galaxies at 1.9 < z < 2.3 in the Hobby–Eberly Telescope Dark Energy Experiment’s database for systems with strong He ii λ 1640 and no detected UV metal lines, recovering eight candidates and a first-order, tentative comoving number density of ∼30 Gpc ⁻³ . The eight-object stack yields He ii equivalent width (EW) EW rest = 28.7 ± 6.7 Å, He ii /Ly α = 0.354 ± 0.094, and a resolution-corrected He ii FWHM = 449 ± 105 km s ⁻¹ . We find no significant N v , C iv , or O iii ] emission; using the He ii width to set the scale, we derive 3 σ upper limits of N v /He ii < 0.167, C iv /He ii < 0.128, and O iii ]/He ii < 0.140. This combination of strong He ii , elevated He ii /Ly α , moderate line width, and weak metal lines is difficult to reproduce with metal-enriched stellar populations, active galactic nucleus (AGN) narrow-line regions, shocks, or classical Wolf–Rayet features. We interpret the sample as candidates for very metal-poor or Pop III–like ionizing sources at z ∼ 2. Definitive confirmation requires deeper rest-frame UV spectroscopy to measure He ii and metal-line limits in individual objects, followed by rest-frame optical spectroscopy of H β , [O iii ] λλ 4959, 5007, [O ii ] λ 3727, and H α to constrain gas-phase metallicity, ionization conditions, and AGN activity.
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BibTeX
@article{khanlari2026population,
title = {{Population III Host Candidates at z ∼ 2: Strong He II λ 1640 and Absent Ultraviolet Metal Lines in HETDEX Ly α Emitters}},
author = {Mahan Mirza Khanlari and Dustin Davis and Karl Gebhardt and Tiger Yu-Yang Hsiao and Julian B. Muñoz and Alessandra Venditti and Erin Mentuch Cooper and Robin Ciardullo and Olivia Curtis and Daniel J. Farrow and Steven L. Finkelstein and Caryl Gronwall and Gary J. Hill and Wolfram Kollatschny and Chenxu Liu and Shiro Mukae and Shun Saito and Donald P. Schneider},
journal = {The Astrophysical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-4357/aea6d9},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/aea6d9},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Population III Host Candidates at z ∼ 2: Strong He II λ 1640 and Absent Ultraviolet Metal Lines in HETDEX Ly α Emitters
AU - Mahan Mirza Khanlari
AU - Dustin Davis
AU - Karl Gebhardt
AU - Tiger Yu-Yang Hsiao
AU - Julian B. Muñoz
AU - Alessandra Venditti
AU - Erin Mentuch Cooper
AU - Robin Ciardullo
AU - Olivia Curtis
AU - Daniel J. Farrow
AU - Steven L. Finkelstein
AU - Caryl Gronwall
AU - Gary J. Hill
AU - Wolfram Kollatschny
AU - Chenxu Liu
AU - Shiro Mukae
AU - Shun Saito
AU - Donald P. Schneider
PY - 2026
DA - 2026/10/08
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/aea6d9
UR - https://iopscience.iop.org/article/10.3847/1538-4357/aea6d9
AB - Population III (Pop III) stars are expected to produce extremely hard ionizing spectra, yet direct evidence for their signature—strong narrow He ii λ 1640 emission with weak or absent metal lines—remains elusive. Although Pop III formation peaks at z ≳ 10, models predict that nearly pristine gas pockets can survive to later times, making intermediate-redshift searches a probe of metal mixing across cosmic time. We search 109,545 high-confidence Ly α -emitting galaxies at 1.9 < z < 2.3 in the Hobby–Eberly Telescope Dark Energy Experiment’s database for systems with strong He ii λ 1640 and no detected UV metal lines, recovering eight candidates and a first-order, tentative comoving number density of ∼30 Gpc ⁻³ . The eight-object stack yields He ii equivalent width (EW) EW rest = 28.7 ± 6.7 Å, He ii /Ly α = 0.354 ± 0.094, and a resolution-corrected He ii FWHM = 449 ± 105 km s ⁻¹ . We find no significant N v , C iv , or O iii ] emission; using the He ii width to set the scale, we derive 3 σ upper limits of N v /He ii < 0.167, C iv /He ii < 0.128, and O iii ]/He ii < 0.140. This combination of strong He ii , elevated He ii /Ly α , moderate line width, and weak metal lines is difficult to reproduce with metal-enriched stellar populations, active galactic nucleus (AGN) narrow-line regions, shocks, or classical Wolf–Rayet features. We interpret the sample as candidates for very metal-poor or Pop III–like ionizing sources at z ∼ 2. Definitive confirmation requires deeper rest-frame UV spectroscopy to measure He ii and metal-line limits in individual objects, followed by rest-frame optical spectroscopy of H β , [O iii ] λλ 4959, 5007, [O ii ] λ 3727, and H α to constrain gas-phase metallicity, ionization conditions, and AGN activity.
SN - 0004-637X
SN - 1538-4357
ER -