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 $R_V$ will lead to a lower average value ($\bar{R}_V$) for SNIa observed at higher redshift.   Two relations in SNIa observations indicate evolution of the average $R_V$: the relation of observed $R_V$ with inclination of the host galaxy at low redshift and the matching of the distribution of extinction values ($A_V$) for SNIa in different redshift intervals. The inclination effect does point to a halo with lower $R_V$ values. In contrast, the distributions of $A_V$ values match best for a $\bar{R}_V(z)$ evolution that mimics the relation of SNIa dimming with redshift attributed to the cosmological constant. However, even in the worse case scenario, the evolution $\bar{R}_V$ 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 $A_V$ and $R_V$ for each SNIa . Solving for evolution of $\bar{R}_V$ (and $A_V$) 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. 
B.W. Holwerda