Diverse reddening distributions in sight lines for type Ia supernovae

Context. Accurate cosmological constraints from type Ia supernovae (SNe) require sufficiently accurate corrections for host-galaxy extinction. Modelling these corrections is challenged by the problem of disentangling the SN’s intrinsic colours from host-galaxy interstellar reddening. The latter is commonly modelled in a probabilistic way assuming an exponential distribution exp(− E ( B − V )/ τ ) as a universal prior, applied across all types of SN host galaxies. Aims. We tested the robustness of the exponential model of host-galaxy reddening and its universality against predictions based on simulating dust and type Ia SN distributions in host galaxies of different morphological types. Methods. Our simulations incorporated up-to-date observational constraints on dust masses across host-galaxy morphological types, scaling relations between the dust and stellar disc parameters, and the SN distribution. Results. We find substantial differences between predicted interstellar reddening in late- (LT) and early-type (ET) host galaxies, primarily driven by the stellar-to-dust mass ratios. The mean simulated reddening in LT galaxies matches those derived from type Ia SN observations well, but it is significantly lower for ET host galaxies. The obtained reddening distributions exhibit an excess of sight lines with zero reddening with respect to the commonly used exponential model, although the difference is quite mild for LT galaxies. On the other hand, the distribution could peak at E ( B − V ) > 0 when considering a population of young type Ia SNe originating from lower heights within the dust disc of spiral galaxies. Conclusions. The reddening distribution strongly depends on the SN host-galaxy morphological type. Assuming a universal reddening prior distribution for modelling the peak magnitude-colour relation, which is currently a common practice, gives rise to a spurious scatter in the derived extinction properties. It could also bias relative distances between SNe originating from different host-galaxy populations. The discrepancy between the simulated reddening in average ET host galaxies and the observed occurrence of reddened SNe in these galaxies suggests that reddening does not originate from the interstellar dust expected in these hosts.