Host Galaxy Extinction of SNIa: Co-evolution of ISM Structure and Extinction Law with Star-Formation

This paper presents a mechanism that may modify the extinction law for SNIa observed at higher redshift. Starting from the observations that (1) SNIa occur predominantly in spiral galaxies, (2) star-formation ejects ISM out of the plane of spirals, (3) star-formation alters the extinction properties of the dust in the ISM, and (4) there is substantially more star-formation at higher redshift, I propose that spiral galaxies have a dustier halo in the past than they do now. The ejected material’s lower value of RV will lead to a lower average value (R̄V ) for SNIa observed at higher redshift. Two relations in SNIa observations indicate evolution of the average RV : the relation of observed RV with inclination of the host galaxy at low redshift and the matching of the distribution of extinction values (AV ) for SNIa in different redshift intervals. The inclination effect does point to a halo with lower RV values. In contrast, the distributions of AV values match best for a R̄V (z) evolution that mimics the relation of SNIa dimming with redshift attributed to the cosmological constant. However, even in the worse case scenario, the evolution R̄V can not fully explain the dimming of SNIa: host galaxy extinction law evolution is not a viable alternative to account for the dimming of SNIa. Future observations of SNIa –multi-color lightcurves and spectra– will solve separately for values of AV and RV for each SNIa . Solving for evolution of R̄V (and AV ) with redshift will be important for the coming generation of cosmological SNIa measurements and has the bonus science of insight into the distribution of dust-rich ISM in the host galaxies in the distant past.