The COSMOS-Web ring: In-depth characterization of an Einstein ring lensing system at z ∼ 2

¹ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
e-mail: wilfried.mercier@lam.fr
² Cosmic Dawn Center (DAWN), Denmark
³ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen, Denmark
⁴ Institut d’Astrophysique de Paris, UMR 7095, CNRS, Sorbonne Université, 98 bis boulevard Arago, 75014 Paris, France
⁵ Institute for Computational Cosmology, Durham University, South Road, Durham DH1 3LE, UK
⁶ DTU Space, Technical University of Denmark, Elektrovej, Building 328, 2800 Kgs. Lyngby, Denmark
⁷ Caltech/IPAC, 1200 E. California Blvd, Pasadena, CA 91125, USA
⁸ The University of Texas at Austin, 2515 Speedway Blvd Stop C1400, Austin, TX 78712, USA
⁹ Instituto de Astrofísica de Canarias (IAC), La Laguna 38205, Spain
¹⁰ Observatoire de Paris, LERMA, PSL University, 61 avenue de l’Observatoire, 75014 Paris, France
¹¹ Université Paris-Cité, 5 Rue Thomas Mann, 75014 Paris, France
¹² Universidad de La Laguna. Avda. Astrofísico Fco, Sanchez, La Laguna, Tenerife, Spain
¹³ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
¹⁴ Department of Astronomy and Astrophysics, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA
¹⁵ Institute of Cosmology and Gravitation, University of Portsmouth, PO13FX Portsmouth, UK
¹⁶ Centre for Extragalactic Astronomy, Durham University, South Road, Durham DH1 3LE, UK
¹⁷ Department of Physics and Astronomy, University of Hawaii, Hilo, 200 W Kawili St, Hilo, HI 96720, USA
¹⁸ Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA
¹⁹ Department of Physics, Faculty of Science, University of Helsinki, 00014 Helsinki, Finland
²⁰ Department of Computer Science, Aalto University, PO Box 15400, Espoo 00076, Finland
²¹ Technical University of Munich, TUM School of Natural Sciences, Department of Physics, James-Franck-Str. 1, 85748 Garching, Germany
²² Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
²³ University of California Santa Barbara, Santa Barbara, CA, USA
²⁴ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
²⁵ Laboratory for Multiwavelength Astrophysics, School of Physics and Astronomy, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY 14623, USA
²⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²⁷ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700AV Groningen, The Netherlands
²⁸ University of Bologna – Department of Physics and Astronomy “Augusto Righi” (DIFA), Via Gobetti 93/2, 40129 Bologna, Italy
²⁹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti 93/3, 40129 Bologna, Italy
³⁰ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen, Denmark

Received: 27 September 2023
Accepted: 20 February 2024

Abstract

Aims. We provide an in-depth analysis of the COSMOS-Web ring, an Einstein ring at z ≈ 2 that we serendipitously discovered during the data reduction of the COSMOS-Web survey and that could be the most distant lens discovered to date.

Methods. We extracted the visible and near-infrared photometry of the source and the lens from more than 25 bands. We combined these observations with far-infrared detections to study the dusty nature of the source and we derived the photometric redshifts and physical properties of both the lens and the source with three different spectral energy distribution (SED) fitting codes. Using JWST/NIRCam images, we also produced two lens models to (i) recover the total mass of the lens, (ii) derive the magnification of the system, (iii) reconstruct the morphology of the lensed source, and (iv) measure the slope of the total mass density profile of the lens.

Results. We find the lens to be a very massive elliptical galaxy at z = 2.02 ± 0.02 with a total mass within the Einstein radius of M_tot(<θ_Ein = (3.66 ± 0.36) × 10¹¹ M_⊙ and a total stellar mass of M_⋆ = 1.37_−0.11^+0.14 × 10¹¹ M_⊙. We also estimate it to be compact and quiescent with a specific star formation rate below 10⁻¹³ yr. Compared to stellar-to-halo mass relations from the literature, we find that the total mass of the lens within the Einstein radius is consistent with the presence of a dark matter (DM) halo of total mass M_h = 1.09_−0.57^+1.46 × 10¹³ M_⊙. In addition, the background source is a M_⋆ = (1.26 ± 0.17) × 10¹⁰ M_⊙ star-forming galaxy (SFR ≈ (78 ± 15) M_⊙ yr) at z = 5.48 ± 0.06. The morphology reconstructed in the source plane shows two clear components with different colors. Dust attenuation values from SED fitting and nearby detections in the far infrared also suggest that the background source could be at least partially dust-obscured.

Conclusions. We find the lens at z ≈ 2. Its total, stellar, and DM halo masses are consistent within the Einstein ring, so we do not need any unexpected changes in our description of the lens such as changing its initial mass function or including a non-negligible gas contribution. The most likely solution for the lensed source is at z ≈ 5.5. Its reconstructed morphology is complex and highly wavelength dependent, possibly because it is a merger or a main sequence galaxy with a heterogeneous dust distribution.