Spectroscopic differences between supernovae of type IA in early-type and in late-type galaxies

Published data are used to derive a uniform set of blueshifts in the λ6355 feature of Si II for 33 supernovae of type Ia (SNe Ia). All seven SNe Ia with blueshifts at ten days past maximum <9500 km s^-1^ occurred in early-type (E, S0, Sa) galaxies. The observed dependence of Si II blueshift on parent-galaxy type indicates that differing SNe Ia blueshifts are due to global physical differences, rather than to asymmetries in their expansion that are viewed from different directions. It is speculated that SNe Ia with low expansion velocities might, in the mean, have older progenitors than those with high expansion velocities.