The Pantheon+ analysis: Improving the redshifts and peculiar velocities of Type Ia supernovae used in cosmological analyses

Abstract

We examine the redshifts of a comprehensive set of published Type Ia supernovae, and provide a combined, improved catalogue with updated redshifts. We improve on the original catalogues by using the most up-to-date heliocentric redshift data available; ensuring all redshifts have uncertainty estimates; using the exact formulae to convert heliocentric redshifts into the Cosmic Microwave Background (CMB) frame; and utilising an improved peculiar velocity model that calculates local motions in redshift-space and more realistically accounts for the external bulk flow at high-redshifts. We review 2607 supernova redshifts; 2285 are from unique supernovae and 322 are from repeat-observations of the same supernova. In total, we updated 990 unique heliocentric redshifts, and found 5 cases of missing or incorrect heliocentric corrections, 44 incorrect or missing supernova coordinates, 230 missing heliocentric or CMB frame redshifts, and 1200 missing redshift uncertainties. The absolute corrections range between $10^{-8} \leq \Delta z \leq 0.038$ , and RMS $(\Delta z) \sim 3{\times 10^{-3}}$ . The sign of the correction was essentially random, so the mean and median corrections are small: $4{\times 10^{-4}}$ and $4{\times 10^{-6}}$ respectively. We examine the impact of these improvements for $H_0$ and the dark energy equation of state w and find that the cosmological results change by $\Delta H_0 = -0.12\,\mathrm{km\,s}^{-1}\mathrm{Mpc}^{-1}$ and $\Delta w = 0.003$ , both significantly smaller than previously reported uncertainties for $H_0$ of 1.0 $\mathrm{km\,s}^{-1}\mathrm{Mpc}^{-1}$ and w of 0.04 respectively.

The Pantheon+ Analysis: Evaluating Peculiar Velocity Corrections in Cosmological Analyses with Nearby Type Ia Supernovae

“…The optimal PV corrections can be constrained such that the impact on H 0 is 0.06-0.11 km s −1 Mpc −1 and the impact on w is 0.02-0.03. The data set analyzed in this paper, which is released in Carr et al (2022), will remain valuable for years, as for measurements of H 0 , the current low-z data set is likely irreplaceable because it contains measurements of SNe used in the second rung of the distance ladder, and roughly one of these is discovered per year. Therefore, further study on this set and these corrections are encouraged.…”

“…The process for identifying galaxy groups is depicted in Figure 3. In order to use these grouping catalogs, we match the SNe (N = 584) to their host galaxies as determined by Carr et al (2022). Any galaxy labeled to be in a group in T15 or Lim17 has an identification in the Principal Galaxies Catalog (PGC), so we match our host galaxies to entries in the PGC using the Lyon Extragalactic Database 10 .…”

“…At low z, redshifts in Pantheon have been primarily obtained from host galaxy spectra, and < 6% from the SN spectral features themselves (Steinhardt et al 2020). Here we use the redshifts that we present in a companion paper (Carr et al 2022), which examines and reassigns the host galaxies of all the low-z SNe and query NED for the most recently acquired redshift of all the galaxies.…”

“…The first set of corrections are based on 2M++, which we obtain using a velocity field reconstruction derived from the data from Lavaux & Hudson (2011). In order to extract the CF corrections from 2M++, we follow the general methodology in Carrick et al (2015) 12 (hereafter C15) who create a predicted PV field as a function of position in real space, and Carr et al (2022) use that field and convert it to redshift space in two steps. They convert each grid point from a real space position to a redshift-space position using the predicted CF correction at that point, and then they interpolate the resulting redshift-space grid using inverse distance weighting.…”

“…Importantly, the analysis done here uses the same sample that will be used for both Riess et al (2022;SH0ES), Brout et al (2022), andScolnic et al (2022;hereafter Pantheon+). This paper is a companion paper to Carr et al (2022), which does a full review of all redshift samples and also derives the 2M++ velocities used in this work. In this paper, we study the efficacy of both group and CF corrections using a current compilation of low-z SNe Ia.…”