High-velocity Type Ia Supernova Has a Unique Host Environment

Ejecta velocity of Type Ia supernovae (SNe Ia) is one powerful tool to differentiate between progenitor scenarios and explosion mechanisms. Here we revisit the relation between photospheric Si ii λ6355 velocities (vSi ii) and host-galaxy properties with ∼280 SNe Ia. A more stringent criterion on the phase of SN spectra is adopted to classify SNe Ia in terms of their photospheric velocities. We find a significant trend that SNe Ia with faster Si ii λ6355 (high-vSi ii SNe Ia) tend to explode in massive environments, whereas their slower counterparts can be found in both lower-mass and massive environments. This trend is further supported by the direct measurements on host gas-phase metallicities. We suggest this relation is likely caused by at least two populations of SNe Ia. Since stars of higher metallicity (at a given mass) generally form less massive white dwarfs, our results support some theoretical models that high-vSi ii SNe Ia may originate from sub-Chandrasekhar class of explosions. Previous observations also showed some evidence that high-vSi ii SNe Ia could be related to the single degenerate systems. However, we find high-vSi ii SNe Ia do not come from particularly young populations. We conclude metallicity is likely the dominant factor in forming high-vSi ii SNe Ia. This also implies their potential evolution with redshift and impact on the precision of SN Ia cosmology.