Accuracy of environmental tracers and consequence for determining the Type Ia Supernovae magnitude step

Abstract

Type Ia Supernovae (SNe Ia) are standardizable candles that allow us to measure the recent expansion rate of the Universe. Due to uncertainties in progenitor physics, potential astrophysical dependencies may bias cosmological measurements if not properly accounted for. The dependency of the intrinsic luminosity of SNe Ia with their host-galaxy environment is often used to standardize SNe Ia luminosity and is commonly parameterized as a step function. This functional form implicitly assumes two-populations of SNe Ia. In the literature, multiple environmental indicators have been considered, finding different, sometimes incompatible, step function amplitudes. We compare these indicators in the context of a two-populations model, based on their ability to distinguish the two populations. We show that local Hα-based specific star formation rate (lsSFR) and global stellar mass are better tracers than, for instance, host galaxy morphology. We show that tracer accuracy can explain the discrepancy between the observed SNe Ia step amplitudes found in the literature. Using lsSFR or global mass to distinguish the two populations can explain all other observations, though lsSFR is favoured. As lsSFR is strongly connected to age, our results favour a prompt and delayed population model. In any case, there exists two populations that differ in standardized magnitude by at least 0.121 ± 0.010 mag.

Type Ia Supernova Magnitude Step from the local Dark Matter Environment

“…First, let us verify the existence of the stellar mass step in our data. Choosing two equal sized bins with high and low global stellar mass (with median 5.75 × 10 10 M as threshold), we find the luminosity step Δ 𝑀 𝐵 = 0.15 ± 0.11 mag (1𝜎 error bars), compatible with values from the literature (0.116±0.030, Briday et al 2021). Next, we divide our data into two equal bins with high and low local DM density environment, we find the luminosity step Δ 𝑀 𝐵 = 0.52±0.11 mag (1𝜎 error bars).…”

“…Next, we divide our data into two equal bins with high and low local DM density environment, we find the luminosity step Δ 𝑀 𝐵 = 0.52±0.11 mag (1𝜎 error bars). In comparison, with other magnitude steps from the literature, we find that the DM density step is much 1 http://leda.univ-lyon1.fr 2 http://simbad.u-strasbg.fr/simbad/ more pronounced (see Briday et al 2021, for a recent comparison of magnitude steps).…”

“…As one of the many unexplained observational aspects of SNe Ia, sub-luminous events (or fast decliners) occur more frequently in heavier host galaxies (Sullivan et al 2010;Lampeitl et al 2010;Kelly et al 2010;Gupta et al 2011;D'Andrea et al 2011;Hayden et al 2013;Childress et al 2014;Rose et al 2021;Briday et al 2021). The correlation is known as the magnitude step, or 'mass step', and is often used to standardize SNe Ia luminosity, mostly parametrized as a step-function.…”

“…The procedure is generally to divide a sample of SNe Ia into bins of high and low values of a certain property, say 𝑋, and then to compute the difference of the mean magnitudes at maximum luminosity in the 𝐵-band, Δ 𝑀 𝐵,max (the magnitude step is then informally called 𝑋-step). To exemplify this procedure with recent works, we cite the derivation of a global host stellar mass-step (0.116 ± 0.030 mag, Briday et al 2021), a metalicity-step (Childress et al 2014), a local stellar mass-step (0.059 ± 0.024 mag, Rigault et al 2020), a host morphology-step (0.058 ± 0.019 mag, Pruzhinskaya et al 2020), and local specific star formation rate step (0.127 ± 0.032 mag, Briday et al 2021). It is unclear if external factors like dust can (Brout & Scolnic 2021) or cannot Ponder et al (2020); Thorp et al (2021) account for the observed luminosity differences.…”