TDCOSMO

XII. Improved Hubble constant measurement from lensing time delays using spatially resolved stellar kinematics of the lens galaxy^⋆

¹ Department of Astronomy & Astrophysics, University of Chicago, Chicago, IL 60637, USA
e-mail: ajshajib@uchicago.edu
² Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA
³ Department of Physics and Astronomy, University of California, Davis, CA 95616, USA
⁴ Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
⁵ Sub-Department of Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁶ Technical University of Munich, TUM School of Natural Sciences, Department of Physics, James-Franck-Str. 1, Garching 85748, Germany
⁷ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, Garching 85748, Germany
⁸ Institute of Astronomy and Astrophysics, Academia Sinica, 11F of ASMAB, No. 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan
⁹ Physics Department, California Polytechnic State University, San Luis Obispo, CA 93407, USA
¹⁰ Fermi National Accelerator Laboratory, PO Box 500 Batavia, IL 60510, USA
¹¹ STAR Institute, Quartier Agora, Allée du Six Août, 19c, 4000 Liège, Belgium
¹² Kavli Institute for Particle Astrophysics and Cosmology and Department of Physics, Stanford University, Stanford, CA 94305, USA
¹³ SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA
¹⁴ Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794, USA
¹⁵ Institute of Physics, Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland

Received: 10 January 2023
Accepted: 1 February 2023

Abstract

Strong-lensing time delays enable the measurement of the Hubble constant (H₀) independently of other traditional methods. The main limitation to the precision of time-delay cosmography is mass-sheet degeneracy (MSD). Some of the previous TDCOSMO analyses broke the MSD by making standard assumptions about the mass density profile of the lens galaxy, reaching 2% precision from seven lenses. However, this approach could potentially bias the H₀ measurement or underestimate the errors. For this work, we broke the MSD for the first time using spatially resolved kinematics of the lens galaxy in RXJ1131−1231 obtained from the Keck Cosmic Web Imager spectroscopy, in combination with previously published time delay and lens models derived from Hubble Space Telescope imaging. This approach allowed us to robustly estimate H₀, effectively implementing a maximally flexible mass model. Following a blind analysis, we estimated the angular diameter distance to the lens galaxy D_d = 865₋₈₁⁺⁸⁵ Mpc and the time-delay distance D_Δt = 2180₋₂₇₁⁺⁴⁷² Mpc, giving H₀ = 77.1_−7.1^+7.3 km s⁻¹ Mpc⁻¹ – for a flat Λ cold dark matter cosmology. The error budget accounts for all uncertainties, including the MSD inherent to the lens mass profile and line-of-sight effects, and those related to the mass–anisotropy degeneracy and projection effects. Our new measurement is in excellent agreement with those obtained in the past using standard simply parametrized mass profiles for this single system (H₀ = 78.3_−3.3^+3.4 km s⁻¹ Mpc⁻¹) and for seven lenses (H₀ = 74.2_−1.6^+1.6 km s⁻¹ Mpc⁻¹), or for seven lenses using single-aperture kinematics and the same maximally flexible models used by us (H₀ = 73.3_−5.8^+5.8 km s⁻¹ Mpc⁻¹). This agreement corroborates the methodology of time-delay cosmography.