TDCOSMO

I. An exploration of systematic uncertainties in the inference of H₀ from time-delay cosmography

¹ Institute of Physics, Laboratory of Astrophysics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
e-mail: martin.millon@epfl.ch
² Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
³ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
⁴ Physik-Department, Technische Universität München, James-Franck-Straße 1, 85748 Garching, Germany
⁵ Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), 11F of ASMAB, No.1, Section 4, Roosevelt Road, Taipei 10617, Taiwan
⁶ Kavli Institute for Particle Astrophysics and Cosmology and Department of Physics, Stanford University, Stanford, CA 94305, USA
⁷ Department of Physics, University of California, Davis, CA 95616, USA
⁸ STAR Institute, Quartier Agora – Allée du six Août, 19c B-4000 Liège, Belgium
⁹ Kavli IPMU (WPI), UTIAS, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan
¹⁰ DARK, Niels Bohr Institute, Lyngbyvej 2, 2100 Copenhagen, Denmark
¹¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
¹² Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Avda. Gran Bretaña 1111, Valparaíso, Chile
¹³ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁴ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁵ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁶ Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA, Leiden, The Netherlands
¹⁷ University of Portsmouth, Institute of Cosmology and Gravitation, Portsmouth PO1 3FX, UK
¹⁸ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello, 16, 80131 Napoli, Italy
¹⁹ European Southern Observatory, Karl-Schwarschild-Str. 2, 85748 Garching, Germany
²⁰ Fermi National Accelerator Laboratory, PO Box 500, Batavia, IL 60510, USA

Received: 18 December 2019
Accepted: 20 May 2020

Abstract

Time-delay cosmography of lensed quasars has achieved 2.4% precision on the measurement of the Hubble constant, H₀. As part of an ongoing effort to uncover and control systematic uncertainties, we investigate three potential sources: 1- stellar kinematics, 2- line-of-sight effects, and 3- the deflector mass model. To meet this goal in a quantitative way, we reproduced the H0LiCOW/SHARP/STRIDES (hereafter TDCOSMO) procedures on a set of real and simulated data, and we find the following. First, stellar kinematics cannot be a dominant source of error or bias since we find that a systematic change of 10% of measured velocity dispersion leads to only a 0.7% shift on H₀ from the seven lenses analyzed by TDCOSMO. Second, we find no bias to arise from incorrect estimation of the line-of-sight effects. Third, we show that elliptical composite (stars + dark matter halo), power-law, and cored power-law mass profiles have the flexibility to yield a broad range in H₀ values. However, the TDCOSMO procedures that model the data with both composite and power-law mass profiles are informative. If the models agree, as we observe in real systems owing to the “bulge-halo” conspiracy, H₀ is recovered precisely and accurately by both models. If the two models disagree, as in the case of some pathological models illustrated here, the TDCOSMO procedure either discriminates between them through the goodness of fit, or it accounts for the discrepancy in the final error bars provided by the analysis. This conclusion is consistent with a reanalysis of six of the TDCOSMO (real) lenses: the composite model yields H₀ = 74.0_−1.8^+1.7 km s⁻¹ Mpc⁻¹, while the power-law model yields 74.2_−1.6^+1.6 km s⁻¹ Mpc⁻¹. In conclusion, we find no evidence of bias or errors larger than the current statistical uncertainties reported by TDCOSMO.