Merging White Dwarf Binaries Produce Type Ia Supernovae in Elliptical Galaxies

TL;DR

This study shows that Type Ia supernovae with bimodal nebular emission profiles predominantly occur in massive, low-star-formation galaxies, supporting the idea that they originate from white dwarf mergers or collisions, with implications for understanding their progenitors.

Contribution

It provides evidence linking bimodal SNe Ia to white dwarf mergers/collisions and quantifies their occurrence in different galaxy environments, highlighting multiple progenitor channels.

Findings

Bimodal SNe Ia are mostly in massive, quiescent galaxies.

A correlation exists between velocity separation and peak luminosity.

30-40% of all SNe Ia may originate from mergers or collisions.

Abstract

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.