I find that Type Ia supernovae (SNe Ia) with bimodal nebular emission profiles occur almost exclusively in massive ($M_\star \gtrsim 10^{11}~M_\odot$) galaxies with low star-formation rates (SFR~$\lesssim 0.5~M_\odot$/yr). The bimodal profiles are likely produced by two white dwarfs 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 white dwarf binaries synthesizing more $^{56}$Ni during the explosion. The quiescent hosts are consistent with the long delay times required to form double white dwarf binaries. The distributions of SNe Ia with and without bimodal nebular lines differ in host mass, SFR, and specific SFR with K-S test probabilities of $3.1\%$, $0.03\%$, and $0.02\%$, 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. $30-40\%$ of all SNe Ia originate from mergers or collisions depending on how cleanly host environment distinguishes progenitor scenarios. The bimodal SNe Ia share some characteristics with the underluminous 91bg-like SNe Ia that also prefer older populations, but there is no unambiguous connection between the two classifications. This may suggest separate processes or multiple axes of ejecta (a)symmetry. Existing models for white dwarf 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 white dwarfs in external galaxies.
Low-mass failed supernovae and the $10\,M_{\odot}$ peak in the merging black hole mass distribution
California Institute of Technology（加州理工学院）；LIGO Laboratory, California Institute of Technology（加州理工学院LIGO实验室）；Illinois Center for Advanced Studies of the Universe, Department of Physics, University of Illinois at Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校物理学院宇宙高级研究中心）
An imaging polarimetry survey of Type Ia supernovae: are peculiar extinction and polarization properties produced by circumstellar or interstellar matter?
