ZTF SN Ia DR2: Peculiar velocities’ impact on the Hubble diagram

¹ Aix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France
² Department of Physics, Duke University, Durham, NC 27708, USA
³ Université Clermont-Auvergne, CNRS, LPCA, 63000 Clermont-Ferrand, France
⁴ School of Physics, Trinity College Dublin, College Green, Dublin 2, Ireland
⁵ Oskar Klein Centre, Department of Astronomy, Stockholm University, SE-10691 Stockholm, Sweden
⁶ Institut für Physik, Humboldt Universität zu Berlin, Newtonstr 15, 12101 Berlin, Germany
⁷ Univ. Lyon, Univ. Claude Bernard Lyon 1, CNRS, IP2I Lyon/IN2P3, UMR 5822, F-69622 Villeurbanne, France
⁸ Department of Physics, Lancaster University, Lancs LA1 4YB, UK
⁹ National Research Council of Canada, Herzberg Astronomy & Astrophysics Research Centre, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
¹⁰ Sorbonne/Paris Cité Universités, LPNHE, CNRS/IN2P3, LPNHE, 75005 Paris, France
¹¹ Institute of Astronomy and Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹² Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
¹³ Deutsches Elektronen Synchrotron DESY, Platanenallee 6, 15738 Zeuthen, Germany
¹⁴ Lawrence Berkeley National Laboratory, 1 Cyclotron Road MS 50B-4206, Berkeley, CA 94720, USA
¹⁵ Department of Astronomy, University of California, Berkeley, 501 Campbell Hall, Berkeley, CA 94720, USA
¹⁶ Nordic Optical Telescope, Rambla José Ana Fernández Pérez 7, ES-38711 Breña Baja, Spain
¹⁷ IPAC, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
¹⁸ 249-17 Caltech, Pasadena, CA 91125, USA
¹⁹ Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USA
²⁰ Department of Physics, Drexel University, Philadelphia, PA 19104, USA

^⋆ Corresponding authors; bastien.carreres@duke.edu, rosselli@cppm.in2p3.fr

Received: 15 April 2024
Accepted: 19 July 2024

Abstract

Type Ia supernovae (SNe Ia) are used to determine the distance-redshift relation and build the Hubble diagram. Neglecting their host-galaxy peculiar velocities (PVs) may bias the measurement of cosmological parameters. The smaller the redshift, the larger the effect is. We used realistic simulations of SNe Ia observed by the Zwicky Transient Facility (ZTF) to investigate the effect of different methods of taking PVs into account. We studied the impact of neglecting galaxy PVs and their correlations in an analysis of the SNe Ia Hubble diagram. We find that it is necessary to use the PV full covariance matrix computed from the velocity power spectrum to take the sample variance into account. Considering the results we have obtained using simulations, we determine the PV systematic effects in the context of the ZTF SN Ia DR2 sample. We determine the PV impact on the intercept of the Hubble diagram, a_B, which is directly linked to the measurement of H₀. We show that not taking into account PVs and their correlations results in a shift in the H₀ value of about 1.0 km s⁻¹ Mpc⁻¹ and a slight underestimation of the H₀ error bar.