Letter to the Editor

The COSMOS-Web ring: Spectroscopic confirmation of the background source at z = 5.1

¹ Cosmic Dawn Center (DAWN), Denmark
² Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen, Denmark
³ DTU Space, Technical University of Denmark, Elektrovej 327, 2800 Kgs., Lyngby, Denmark
⁴ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
⁵ Laboratory for Multiwavelength Astrophysics, School of Physics and Astronomy, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY 14623, USA
⁶ Department of Physics and Astronomy, University of Kentucky, 505 Rose Street, Lexington, KY 40506, USA
⁷ Institut d’Astrophysique de Paris, UMR 7095, CNRS, Sorbonne Université, 98 bis boulevard Arago, F-75014 Paris, France
⁸ Institute for Computational Cosmology, Durham University, South Road, Durham DH1 3LE, UK
⁹ The University of Texas at Austin, 2515 Speedway Blvd Stop, C1400 Austin, TX 78712, USA
¹⁰ Broida Hall, Department of Physics, University of California, Santa Barbara, Santa Barbara, CA 93106, USA
¹¹ University of Bologna – Department of Physics and Astronomy “Augusto Righi” (DIFA), Via Gobetti 93/2, I-40129 Bologna, Italy
¹² INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti 93/3, I-40129 Bologna, Italy
¹³ Caltech/IPAC, 1200 E. California Blvd., Pasadena, CA 91125, USA
¹⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁵ Centre for Extragalactic Astronomy, Durham University, South Road, Durham DH1 3LE, UK
¹⁶ Inter-University Institute for Data Intensive Astronomy, Department of Astronomy, University of Cape Town, 7701 Rondebosch, Cape Town, South Africa
¹⁷ Inter-University Institute for Data Intensive Astronomy, Department of Physics and Astronomy, University of the Western Cape, Robert Sobukwe Road, 7535 Bellville, Cape Town, South Africa
¹⁸ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy

^⋆ Corresponding authors: marko.shuntov@nbi.ku.dk; shuji@dtu.dk

Received: 26 February 2025
Accepted: 26 March 2025

Abstract

We report the spectroscopic confirmation of the background source of the most distant Einstein ring known to date, the COSMOS-Web ring. This system consists of a complete Einstein ring at z = 5.1, which is lensed by a massive early-type galaxy at z ∼ 2. The redshift z = 5.1043 ± 0.0004 is unambiguously identified with our NOEMA and Keck/MOSFIRE spectroscopy; the NOEMA observations reveal the CO(4–3) and CO(5–4) lines at > 8σ, and the MOSFIRE data detect [O II] at ∼6σ. Using multiwavelength photometry spanning near-infrared to radio bands, we find that the lensed galaxy is a dust-obscured starburst (M_⋆ ∼ 1.8 × 10¹⁰ M_⊙, SFR_IR ∼ 60 M_⊙ yr⁻¹) with a high star formation efficiency (gas depletion time τ_dep < 100 Myr), as indicated by the [C I](1–0) non-detection. The redshift confirmation re-validates that the total lens mass budget within the Einstein radius is fully accounted for by the stellar and dark matter components, without the need of modifying the initial mass function or dark matter distribution profile. This work paves the way for detailed studies and future follow-ups of this unique lensing system, which is an ideal laboratory for studying mass distribution at z ∼ 2 and physical conditions of star formation at z ∼ 5.