Concerning Colour: The Effect of Environment on Type Ia Supernova Colour in the Dark Energy Survey

Abstract

Recent analyses have found intriguing correlations between the colour (𝑐) of type Ia supernovae (SNe Ia) and the size of their mass-step, the relationship between host galaxy stellar mass and Hubble residual. These analyses suggest that the underlying cause of this relationship is dust. Using a sample of 675 photometrically-classified SNe Ia from the Dark Energy Survey 5-year sample, we study the differences in Hubble residual for a variety of host and local properties for subsamples split by their colour (𝑐). We find a 3𝜎 difference for the size of the mass-step when comparing blue (𝑐 < 0) and red (𝑐 > 0) SNe. We observe the lowest r.m.s. scatter (∼ 0.14) in Hubble residual for blue SNe in low mass or blue environments, suggesting that these objects provide the most homogeneous sample for cosmological analyses. By fitting for 𝑐-dependent relationships between Hubble residuals and 𝑀 stellar , approximating existing dust models, we remove the mass-step from the data but find significant remaining steps in rest-frame 𝑈 − 𝑅, indicating that current dust modelling based on 𝑀 stellar may not fully explain the remaining dispersion in SN luminosity. The most dispersion is removed by instead accounting for a 𝑐-dependent relationship between Hubble residuals and global 𝑈 − 𝑅, resulting in ≤ 1𝜎 remaining steps in other environmental properties, suggesting that 𝑈 − 𝑅 provides different information about the environment of SNe Ia to 𝑀 stellar . This 𝑐-dependent 𝑈 − 𝑅 relation implies that 𝑈 − 𝑅 may be more closely linked to dust, motivating the future inclusion of galaxy 𝑈 − 𝑅 colour in the correction for SN distance biases.