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

TL;DR

This paper investigates how neglecting peculiar velocities of host galaxies biases the measurement of the Hubble constant using SNe Ia, emphasizing the importance of covariance matrices derived from the velocity power spectrum.

Contribution

It introduces a method to incorporate galaxy peculiar velocities via the full covariance matrix in SNe Ia analyses, improving the accuracy of cosmological parameter estimation.

Findings

Neglecting PVs biases H0 by about 1.0 km/s/Mpc.

Using the full PV covariance matrix reduces systematic errors.

Peculiar velocities significantly impact low-redshift SNe Ia analyses.

Abstract

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 use realistic simulations of SNe Ia observed by the Zwicky Transient Facility (ZTF) to investigate the effect of different methods to take into account PVs. We study 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 into account the sample variance. Considering the results we have obtained using simulations, we determine the PV systematic effects in the context of the ZTF DR2 SNe Ia sample. We determine the PV impact on the intercept of the Hubble diagram, $a_B$, which is directly linked to the measurement of $H_0$. We show that not taking into account PVs and their correlations results in a shift of the $H_0$ value of about $1.0$km.s$^{-1}$.Mpc$^{-1}$ and a slight underestimation of the $H_0$ error bar.