2025MNRAS.538L...1T

I find that Type Ia supernovae (SNe Ia) with bimodal nebular emission profiles occur almost exclusively in massive (${\rm M_\star } \gtrsim 10^{11}~{\rm M_\odot }$) galaxies with low star formation rates (SFR $\lesssim 0.5~{\rm M_\odot }$ yr^–1). The bimodal profiles are likely produced by two white dwarfs (WDs) that exploded during a merger or collision, supported by a correlation between the peak-to-peak velocity separation ($v_{\rm sep}$) and the SN Ia peak luminosity ($M_V$) which arises naturally from more massive WD binaries synthesizing more $^{56}$Ni during the explosion. The distributions of SNe Ia with and without bimodal nebular lines differ in host mass, SFR, and specific SFR with Kolmogorov-Smirnov test probabilities of $3.1{{\ \rm per\ cent}}$, $0.03{{\ \rm per\ cent}}$, and $0.02{{\ \rm per\ cent}}$, respectively. Viewing angle effects can fully explain the SNe Ia in quiescent hosts without bimodal emission profiles and the dearth of merger/collision driven SNe Ia in star-forming hosts requires at least two distinct progenitor channels for normal SNe Ia. $30\!-\!40{{\ \rm per\ cent}}$ of all SNe Ia originate from mergers or collisions depending on how cleanly host environment distinguishes progenitor scenarios. Existing models for WD mergers and collisions broadly reproduce the $v_{\rm sep}$-$M_V$ correlation and future analyses may be able to infer the masses/mass-ratios of merging WDs in external galaxies.