Population III Host Candidates at z ∼ 2: Strong He II λ 1640 and Absent Ultraviolet Metal Lines in HETDEX Ly α Emitters
Year: 2026
Type: article
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.
Source: The Astrophysical Journal
Authors: Mahan Mirza Khanlari, Dustin Davis, Karl Gebhardt, Tiger Yu-Yang Hsiao, Julian B. Muñoz, Alessandra Venditti, Erin Mentuch Cooper, Robin Ciardullo, Olivia Curtis, Daniel J. Farrow, Steven L. Finkelstein, Caryl Gronwall, Gary J. Hill, Wolfram Kollatschny, Chenxu Liu, Shiro Mukae, Shun Saito, Donald P. Schneider
Institutions: The University of Texas at Austin, Pennsylvania State University, University of Hull, University of Göttingen, Yunnan University, Waseda University, MIREA - Russian Technological University, Missouri University of Science and Technology, The University of Tokyo Institutes for Advanced Study
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $4,499
Funders: National Science Foundation, National Aeronautics and Space Administration, Pennsylvania State University, University of Texas at Austin, Missouri University of Science and Technology, University of Oxford, Georg-August-Universität Göttingen, University of Tokyo, Ludwig-Maximilians-Universität München, Leibniz-Institut für Astrophysik Potsdam, Max-Planck-Institut für Astrophysik, Department of Mechanical Engineering, University of Texas at Austin, U.S. Air Force, Leibniz-Gemeinschaft, Air Force Research Laboratory
Awards: G1245352055, COLLABORATIVE RESEARCH: Hobby-Eberly Telescope Dark Energy Experiment, NSF-BSF: Dark-Matter Interactions at Cosmic Dawn, G3046006893, Collaborative Research: A Partnership in Central Missouri in the Era of Multi-messenger Astrophysics, G5001813018, G5115324995, Collaborative Research: Tracing Cosmic Structures with Galaxies, Quasars, and Gas at Cosmic Noon, Advancing the Science from The Hobby-Eberly Telescope Dark Energy Experiment, Lyman-alpha intensity mapping in the Hobby-Eberly Telescope Dark Energy Experiment, Decoding the First Galaxies via 21-cm Fluctuations, G8944725566
OpenAlex ID: W7221030947