Signatures of Bimodality in Nebular Phase Type Ia Supernova Spectra: Indications of White Dwarf Collision Progenitors

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. 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 47 SNe Ia and classify them based on whether they show compelling evidence for bimodal velocity profiles. We identify 8 bimodal SNe in our sample, and we find that these SNe have average peak $M_V$ about 0.4 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.