Evidence for sub-Chandrasekhar Type Ia supernovae from the last major
  merger
  We investigate the contribution of sub-Chandrasekhar mass Type Ia supernovae
to the chemical enrichment of the Gaia Sausage galaxy, the progenitor of a
significant merger event in the early life of the Milky Way. Using a
combination of data from Nissen & Schuster (2010), the 3rd GALAH data release
(with 1D NLTE abundance corrections) and APOGEE data release 16, we fit
analytic chemical evolution models to a 9-dimensional chemical abundance space
(Fe, Mg, Si, Ca, Cr, Mn, Ni, Cu, Zn) in particular focusing on the iron-peak
elements, Mn and Ni. We find that low [Mn/Fe] $\sim-0.15\,\mathrm{dex}$ and low
[Ni/Fe] $\sim-0.3\,\mathrm{dex}$ Type Ia yields are required to explain the
observed trends beyond the [$\alpha$/Fe] knee of the Gaia Sausage
(approximately at [Fe/H] $=-1.4\,\mathrm{dex}$). Comparison to theoretical
yield calculations indicate a significant contribution from sub-Chandrasekhar
mass Type Ia supernovae in this system (from $\sim60$% to $100$% depending on
the theoretical model with an additional $\pm10$% systematic from NLTE
corrections). We compare to results from other Local Group environments
including dwarf spheroidal galaxies, the Magellanic Clouds and the Milky Way's
bulge, finding the Type Ia [Mn/Fe] yield must be metallicity-dependent. Our
results suggest that sub-Chandrasekhar mass channels are a significant, perhaps
even dominant, contribution to Type Ia supernovae in metal-poor systems, whilst
more metal-rich systems could be explained by metallicity-dependent
sub-Chandrasekhar mass yields, possibly with additional progenitor mass
variation related to star formation history, or an increased contribution from
Chandrasekhar mass channels at higher metallicity.
