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Late-time spectra of Type Ia supernovae (SNe Ia) are important in clarifyingthe physics of their explosions, as they provide key clues to the innerstructure of the exploding white dwarfs. We examined late-time optical spectraof 36 SNe Ia, including five from our own project (SNe 2019np, 2019ein,2021hpr, 2021wuf, and 2022hrs), with phase coverage of $\sim 200$ to $\sim 400$days after maximum light. At this late phase, the outer ejecta have becometransparent and the features of inner iron-group elements emerge in thespectra. Based on multicomponent Gaussian fits and reasonable choices for thepseudocontinuum around Ni and Fe emission features, we get reliable estimatesof the Ni to Fe ratio, which is sensitive to the explosion models of SNe Ia.Our results show that the majority (about 67%) of our SNe Ia are moreconsistent with the sub-Chandrasekhar-mass (i.e., double-detonation) model,although they could be affected by evolutionary or ionisation effects.Moreover, we find that the Si II $\lambda$6355 velocity measured around thetime of maximum light tends to increase with the Ni to Fe ratio for thesubsample with either redshifted or blueshifted nebular velocities, suggestingthat progenitor metallicity might play an important role in accounting for theobserved velocity diversity of SNe Ia.

Implications for the Explosion Mechanism of Type Ia Supernovae from their Late-time Spectra