Type Ia supernovae with bimodal explosions are common - Possible smoking gun for direct collisions of white dwarfs

We discover clear doubly peaked line profiles in 3 out of ~20 Type Ia supernovae (SNe Ia) with high-quality nebular-phase spectra. The profiles are consistently present in three separated Co/Fe emission features. The two peaks are respectively blueshifted and redshifted relative to the host galaxies and are separated by ~5000 km s^-1. The doubly peaked profiles directly reflect a bimodal velocity distribution of the radioactive ⁵⁶Ni in the ejecta that powers the emission of these SNe. Due to their random orientations, only a fraction of SNe with intrinsically bimodal velocity distributions will appear as doubly peaked spectra. Therefore, SNe with intrinsic bimodality are likely common, especially among the SNe in the low-luminosity part on the Phillips relation (Δm₁₅(B) ≳ 1.3; ~ 40 per cent of all SNe Ia). Such bimodality is naturally expected from direct collisions of white dwarfs (WDs) due to the detonation of both WDs and is demonstrated in a 3D 0.64-0.64M_⊙ WD collision simulation. In the future, with a large sample of nebular spectra and a comprehensive set of numerical simulations, the collision model can be unambiguously tested as the primary channel for SNe Ia, and the distribution of nebular line profiles will either be a smoking gun or rule it out.