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 SN Ia host galaxy stellar mass ($M_{\mathrm{stellar}}$) and SN Ia Hubble residual, and suggest that the cause of this relationship is dust. Using 675 photometrically-classified SNe Ia from the Dark Energy Survey 5-year sample, we study the differences in Hubble residual for a variety of global host galaxy and local environmental properties for SN Ia subsamples split by their colour. We find a $3\ddot{I}ƒ$ difference in the mass-step when comparing blue ($c<0$) and red ($c>0$) SNe. We observe the lowest r.m.s. scatter ($\sim 0.14$ mag) in the Hubble residual for blue SNe in low mass/blue environments, suggesting that this is the most homogeneous sample for cosmological analyses. By fitting for $c$-dependent relationships between Hubble residuals and $M_{\mathrm{stellar}}$, approximating existing dust models, we remove the mass-step from the data and find tentative $\sim 2\ddot{I}ƒ$ residual steps in rest-frame galaxy $U-R$ colour. This indicates that dust modelling based on $M_{\mathrm{stellar}}$ may not fully explain the remaining dispersion in SN Ia luminosity. Instead, accounting for a $c$-dependent relationship between Hubble residuals and global $U-R$, results in $\le 1\ddot{I}ƒ$ residual steps in $M_{\mathrm{stellar}}$ and local $U-R$, suggesting that $U-R$ provides different information about the environment of SNe Ia compared to $M_{\mathrm{stellar}}$, and motivating the inclusion of galaxy $U-R$ colour in SN Ia distance bias correction.

19 pages, 8 figures. Published in MNRAS