TDCOSMO

XXIV. First spatially resolved kinematics of the lens galaxy obtained using JWST-NIRSpec to improve time-delay cosmography

¹ Department of Astronomy & Astrophysics, University of Chicago, Chicago, IL 60637, USA
² Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA
³ Center for Astronomy, Space Science and Astrophysics, Independent University, Bangladesh, Dhaka 1229, Bangladesh
⁴ Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
⁵ Technical University of Munich, TUM School of Natural Sciences, Physics Department, James-Franck-Str. 1, Garching 85748, Germany
⁶ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, Garching 85748, Germany
⁷ Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA
⁸ Sub-Department of Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁹ Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794, USA
¹⁰ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí i Franquès 1 08028, Barcelona, Spain
¹¹ Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig de Lluís Companys 23 08010, Barcelona, Spain
¹² Department of Physics and Astronomy, UC Davis, 1 Shields Ave., Davis, CA 95616, USA
¹³ SLAC National Laboratory, 2575 Sand Hill Rd, Menlo Park, CA 94025, USA
¹⁴ IPAC, California Institute of Technology, MC 314-6, 1200 E. California Boulevard, Pasadena, CA 91125, USA
¹⁵ STAR Institute, Liège Université, Quartier Agora, Allée du Six Août 19c, B-4000 Liège, Belgium

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 27 June 2025
Accepted: 30 December 2025

Abstract

Spatially resolved stellar kinematics has become a key ingredient in time-delay cosmography to break the mass-sheet degeneracy in the mass profile and in turn provide a precise constraint on the Hubble constant and other cosmological parameters. In this paper, we present the first measurements of 2D resolved stellar kinematics for the lens galaxy in the quadruply lensed quasar system RXJ1131−1231 using integral field spectroscopy from JWST’s Near-Infrared Spectrograph (NIRSpec), marking the first such measurement conducted with JWST. In extracting robust kinematic measurements from this first-of-its-kind dataset, we have made methodological improvements both in the data reduction and kinematic extraction. In our kinematic extraction procedure, we performed joint modeling of the lens galaxy, the quasar, and its host galaxy’s contributions in the spectra to deblend the lens galaxy component and robustly constrain its stellar kinematics. Our improved methodological frameworks are released as software pipelines for future use: SQUIRREL, for extracting stellar kinematics, and REGALJUMPER, for JWST-NIRSpec data reduction. We incorporated additional artifact cleaning beyond the standard JWST pipeline. We compared our measured stellar kinematics from the JWST NIRSpec with previously obtained ground-based measurements from the Keck Cosmic Web Imager integral field unit and find that the two datasets are statistically consistent at a ∼1.1σ confidence level. Our measured kinematics will be used in a future study to improve the precision of the Hubble constant measurement.