Evidence for Two Distinct Populations of Type Ia Supernovae
  Type Ia supernovae (SNe Ia) have been used as excellent standardizable
candles for measuring cosmic expansion, but their progenitors are still
elusive. Here we report that the spectral diversity of SNe Ia is tied to their
birthplace environments. We find that those with high-velocity ejecta are
substantially more concentrated in the inner and brighter regions of their host
galaxies than are normal-velocity SNe Ia. Furthermore, the former tend to
inhabit larger and more-luminous hosts. These results suggest that
high-velocity SNe Ia likely originate from relatively younger and more
metal-rich progenitors than normal-velocity SNe Ia, and are restricted to
galaxies with substantial chemical evolution.
