Characterising the Standardisation Properties of Type Ia Supernovae in the z band with Hierarchical Bayesian Modelling

TL;DR

This study investigates the standardisation properties of Type Ia supernovae in the z band using hierarchical Bayesian modelling, revealing less residual scatter and consistent mass steps, which can enhance distance measurements for future surveys.

Contribution

It presents the first detailed analysis of SN Ia standardisation in the z band, demonstrating its potential to improve cosmological distance estimates.

Findings

Z-band peak magnitudes show less residual scatter than optical bands.

Post-standardisation, the z-band Hubble diagram has RMS scatter of 0.195 mag.

The z-band mass step is consistent with optical estimates, unaffected by different dust extinction assumptions.

Abstract

Type Ia supernovae (SNe Ia) are standardisable candles: their peak magnitudes can be corrected for correlations between light curve properties and their luminosities to precisely estimate distances. Understanding SN Ia standardisation across wavelength improves methods for correcting SN Ia magnitudes. Using 150 SNe Ia from the Foundation Supernova Survey and Young Supernova Experiment, we present the first study focusing on SN Ia standardisation properties in the z band. Straddling the optical and near-infrared, SN Ia light in the z band is less sensitive to dust extinction and can be collected alongside the optical on CCDs. Pre-standardisation, SNe Ia exhibit less residual scatter in z-band peak magnitudes than in the g and r bands. SNe Ia peak z-band magnitudes still exhibit a significant dependence on light-curve shape. Post-standardisation, the z-band Hubble diagram has a total scatter of RMS $ = 0.195$ mag. We infer a z-band mass step of $\gamma_{z} = -0.105 \pm 0.031$ mag, which is consistent within $1\sigma$ of that estimated from gri data, assuming $R_{V} = 2.61$. When assuming different $R_{V}$ values for high and low mass host galaxies, the z-band and optical mass steps remain consistent within $1\sigma$. Based on current statistical precision, these results suggest dust reddening cannot fully explain the mass step. SNe Ia in the z band exhibit complementary standardisability properties to the optical that can improve distance estimates. Understanding these properties is important for the upcoming Vera Rubin Observatory and Nancy G. Roman Space Telescope, which will probe the rest-frame z band to redshifts 0.1 and 1.8.