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 flat-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 II] and [Fe III] feature at ∼5300 Å, the [Co III] λ5891 feature, and the [Co III] and [Fe II] feature at ∼6600 Å. We identify nine bimodal SNe in our sample, and we find that these SNe have average peak M_V 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.