The Dark Energy Survey Supernova Program results: type Ia supernova brightness correlates with host galaxy dust

Cosmological analyses with type Ia supernovae (SNe Ia) often assume a single empirical relation between colour and luminosity (beta) and do not account for varying host-galaxy dust properties. However, from studies of dust in large samples of galaxies, it is known that dust attenuation can vary significantly. Here, we take advantage of state-of-the-art modelling of galaxy properties to characterize dust parameters (dust attenuation A(V), and a parameter describing the dust law slope R-V) for 1100 Dark Energy Survey (DES) SN host galaxies. Utilizing optical and infrared data of the hosts alone, we find three key aspects of host dust that impact SN cosmology: (1) there exists a large range (similar to 1-6) of host R-V; (2) high-stellar mass hosts have R-V on average similar to 0.7 lower than that of low-mass hosts; (3) for a subsample of 81 spectroscopically classified SNe there is a significant (>3 sigma) correlation between the Hubble diagram residuals of red SNe Ia and the host R-V that when corrected for reduces scatter by similar to 13 per cent and the significance of the 'mass step' to similar to 1 sigma . These represent independent confirmations of recent predictions based on dust that attempted to explain the puzzling 'mass step' and intrinsic scatter (sigma(int)) in SN Ia analyses.