Diverse reddening distributions in sight lines to type Ia supernovae

TL;DR

This paper interrogates the common assumption of a universal exponential prior for host-galaxy reddening in Type Ia supernova analyses. Using Monte Carlo simulations of dust and SN distributions across late-type, S0, and elliptical hosts, anchored to observed dust masses and disc/bulge scaling relations, it shows that reddening distributions differ substantially with morphology and deviate from a simple exponential form, with gamma-distributions (\gamma<1) providing better fits, especially for early types. The study finds mean reddening around $\langle E(B-V)\rangle\approx 0.15$ mag for late-type hosts, but far smaller values for early types, and demonstrates that a universal reddening prior biases extinction inferences and relative distances across host populations. By comparing simulated colour distributions to ZTF SN data, it discusses whether excess red colours in early types arise from intrinsic-colour variation or unusually dust-rich outliers, underscoring the need for host-type–dependent reddening priors to avoid systematic errors in cosmological inferences such as the Hubble constant.

Abstract

Precise cosmological constraints from type Ia supernovae require adequately accurate corrections for host-galaxy extinction. Modelling these corrections is challenged by the problem of disentangling supernova 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 which is applied across all types of supernova host galaxies. We test the robustness of the exponential model and its universality against predictions based on simulating dust and type Ia supernova distributions in host galaxies of different morphological types. We find substantial differences between predicted interstellar reddening in late- and early-type host galaxies, primarily driven by the stellar-to-dust mass ratios. The mean simulated reddening in late-type galaxies matches well those derived from type Ia supernova observations, but it is significantly lower for early-type host galaxies. The reddening distributions exhibit an excess of sight lines with vanishing reddening with respect to the exponential model, although the difference is quite mild for late-type galaxies. On the other hand, the distribution may peak at E(B-V)>0 when considering a population of young type Ia supernovae originating from lower heights within the dust disc. We demonstrate that assuming a universal reddening prior distribution for modeling peak magnitude-colour relation, which is currently a common practice, gives rise to a spurious scatter in the derived extinction properties. It may also bias relative distances between supernovae originating from different host-galaxy populations. The discrepancy between the simulated reddening in average early-type host galaxies and the observed occurrence of reddened supernovae suggests that reddening does not originate from interstellar dust expected in these galaxies.

Figures (7)

• Figure 1: Distributions of host-galaxy interstellar reddening $E(B-V)$ for type Ia supernovae originating from 9 specific locations within the dust disc of a disc-dominated ($B/T=0$) late-type galaxy. The locations are distributed on a grid with (cylindrical) radii $R$ running across the columns and disc heights $z$ across the rows. Galaxy-observer orientations are assumed to be random and parameters of the dust disc are listed in Table \ref{['table:disky_setups']}. Due to asymmetry between disc-supernova-observer and supernova-disc-observer orientations, the local distributions are bimodal for layers above the disc plane. The reddening distributions peak at $E(B-V)>0$ and vanish at $E(B-V)=0$ for $z\lesssim h_{\rm d}$.
• Figure 2: Distributions of host interstellar reddening $E(B-V)$ for type Ia supernovae originating from different layers in the dust disc of a late-type ($B/T=0$) galaxy, with vertical positions $z=\{0,h_{\rm d}, 2h_{\rm d}\}$. The distributions are computed for edge-on (left panel), face-on (middle panel) and random (right panel) orientations of the galaxy disc with respect to an observer. Parameters of the dust disc are summarised in Table \ref{['table:disky_setups']}. The results demonstrate that the distribution peak (the most probable reddening) depends strongly on supernova vertical position in the disc: the distributions are unimodal with maximum at $E(B-V)>0$ for $z\lesssim h_{\rm d}$ and monotonically decreasing, with maximum at $E(B-V)=0$ for $z>h_{\rm d}$.
• Figure 3: Distributions of host-galaxy interstellar reddening $E(B-V)$ for type Ia supernovae originating from host galaxies of four representative morphological types: late type with $B/T=0$ or $B/T=0.3$, S0 type and classical elliptical (for model parameters see Tables \ref{['table:disky_setups']}-\ref{['table:mprph_setups']}). The distributions are computed for edge-on (left panel), face-on (middle panel) and random (right panel) orientations (no difference for elliptical galaxies which are assumed to be spherically symmetric). The purple curve on the right panel shows the result of averaging type Ia supernova rates over host-galaxy morphological types. All distributions peak at $E(B-V)\approx 0$. The apparent differences between the late- and early-type host galaxies are primarily driven by different dust-to-stellar mass ratios measured from infrared observations. The distributions shown in Figure \ref{['fig:BT0_diffz_angleplot']} are strongly modified by averaging over supernova position in the host galaxy.
• Figure 4: Distributions of host-galaxy interstellar reddening $E(B-V)$ for type Ia supernovae at two different projected distance from the centre of a late-type ($B/T=0$ or $B/T=0.3$) host galaxy: $R_{\rm p} = 0$ and $R_{\rm p} = 0.5R_{25}$. The figure demonstrates a strong dependence of the conditional reddening on supernova location in the host galaxy.
• Figure 5: Comparison between the cumulative distribution of simulated host-galaxy reddening $E(B-V)$ (y-axis) and its approximation (x-axis) given by the best-fit exponential (exp-pdf) or $\gamma$-distribution ($\gamma$-pdf) model. Each panel in the upper half shows results for four morphological types of simulated host galaxies: late type (with $B/T=0$ and $B/T=0.3$), S0 type and early type. The columns show three cases of different galaxy-observer orientations: edge-on (left), face-on (middle) and random (right). The bottom panels show analogous comparison for supernovae originating from the $z=0$ plane of the dust disc in the late-type host-galaxy model ($B/T=0$). The symbols denote two characteristic scales of reddening with $E(B-V)=0.01$ mag ($\bullet$) and $E(B-V)=0.1$ mag ($\blacktriangle$), as given by the simulated distributions. The exponential model underestimates the probability of zero reddening, especially in the early-type host galaxies. The simulated data are better reproduced by the $\gamma$-distribution. For type Ia supernovae originating from the proximity of the $z=0$ disc plane, the exponential model overpredicts the supernova rate with $E(B-V)\approx 0$.