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Signatures of Bimodality in Nebular Phase Type Ia Supernova Spectra

One observational prediction for Type Ia supernova (SNe Ia) explosions produced through white dwarf-white dwarf collisions is the presence of bimodal velocity distributions for the $^{56}$Ni decay products, although this signature can also be produced by an off-center 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 favorable viewing angles. We present nebular-phase spectroscopic observations of 17 SNe Ia obtained with the Large Binocular Telescope (LBT). 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 $^{56}$Ni decay products: the [Fe II] and [Fe III] feature at $\sim5300$ Å, the [Co III] $\lambda5891$ feature, and the [Co III] and [Fe II] feature at $\sim6600$ Å. We identify 9 bimodal SNe in our sample, and we find that these SNe have average peak $M_V$ about 0.3 mag fainter than those which 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 $^{56}$Ni yields.