Is the Dust Obscuring Supernovae in Distant Galaxies the Same as Dust in the Milky Way?

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Abstract
Previous attempts to correct type Ia supernovae (SN Ia's) for host galaxy extinction have given strange results: increased dispersion on the Hubble diagram or impossibly low values of the reddening ratio for dust in distant galaxies. The cause is the incorrect assumption that SN Ia's have a uniform intrinsic luminosity and color at maximum light. Our multicolor light-curve shape (MLCS) method establishes the relation between intrinsic luminosity and color for SN Ia's using information in light-curve shapes. Here we estimate the B - V, V - R, and V - I color excess for 20 SN Ia's using MLCS and estimate the reddening ratios of dust in distant galaxies. The ratios of selective to total absorption from dust in distant galaxies hosting SN Ia's are consistent with the galactic extinction law. The SN Ia's in late-type galaxies are often obscured by dust, while those in early-type galaxies are dust free. This suggests that SN Ia extinction is caused by interstellar (not circumstellar) dust, with similar optical properties as the dust in the Milky Way.