Signatures of Bi-Modality in Nebular Phase Type Ia Supernova Spectra: Indications of White Dwarf Collision Progenitors

We present spectroscopic observations of 16 nebular phase SNe Ia, and after an exhaustive search of available archival data for comparable events, include an additional ~70 SNe in our sample. We identify compelling evidence for bi-modal velocity distributions among 56Ni decay products in ~10 of these spectra, indicative of explosions generated through the collision (not merger!) of two white dwarfs. We find that the peak M_V of events with bi-modal velocity components are systematically lower than events without these signatures by approximately -0.3 mag. This provides enticing evidence that the WD-WD collision progenitor channel may be responsible for producing a considerable fraction of the observed low-luminosity SNe Ia population, and merits further theoretical and observational work.