Signatures of bimodality in nebular phase Type Ia supernova spectra

One observational prediction for Type Ia supernovae (SNe Ia) explosions produced through white dwarf-white dwarf collisions is the presence of bimodal velocity distributions for the 56Ni decay products, although this signature can also be produced by an off-centre ignition in a delayed detonation explosion. These bimodal velocity distributions can manifest as double peaked or slat-topped spectral features in late -time spectroscopic observations for favourable viewing angles. We present nebular -phase spectroscopic observations of 17 SNe Ia obtained with the Large Binocular Telescope. Combining these observations with an extensive search of publicly available archival data, we collect a total sample of 48 SNe Ia and classify them based on whether they show compelling evidence for bimodal velocity profiles in three features associated with 56Ni decay products: the [Fe and [Fe fill feature at '-5300 A, the [Co [fi]),5891 feature, and the [Co In] and [Fell] feature at -6600 A. We identify nine bimodal SNe in our sample, and we find that these SNe have average peak Mt about 0.3 mag fainter than those that do not. This is consistent with theoretical predictions for explosions created by nearly head-on collisions of white dwarfs due to viewing angle effects and 56Ni yields.