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

Abstract

Cosmological analyses with type Ia supernovae (SNe Ia) often assume a single empirical relation between color and luminosity (β) 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 modeling of galaxy properties to characterize dust parameters (dust attenuation AV , and a parameter describing the dust law slope RV ) for the Dark Energy Survey (DES) SN Ia host galaxies using the publicly available BAGPIPES code. Utilizing optical and infrared data of the hosts alone, we find three key aspects of host dust that impact SN Ia cosmology: 1) there exists a large range (∼ 1 − 6) of host RV 2) high stellar mass hosts have RV on average ∼ 0.7 lower than that of low-mass hosts 3) there is a significant (> 3σ) correlation between the Hubble diagram residuals of red SNe Ia that when corrected for reduces scatter by ∼ 13% and the significance of the “mass step” to ∼ 1σ. These represent independent confirmations of recent predictions based on dust that attempted to explain the puzzling “mass step” and intrinsic scatter (σint) in SN Ia analyses. We also find that red-sequence galaxies have both lower and more peaked dust law slope distributions on average in comparison to non red-sequence galaxies. We find that the SN Ia β and σint both differ by > 3σ when determined separately for red-sequence galaxy and all other galaxy hosts. The agreement between fitted host-RV and SN Ia β & σint suggests that host dust properties play a major role in SN Ia color-luminosity standardization and supports the claim that SN Ia intrinsic scatter is driven by RV variation