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Gamma ray emission of 511 keV lines arising from electron-positronannihilation has been detected from the Galaxy since the 70s. Spatiallyresolved observations using the INTEGRAL satellite have shown its full skydistribution to be strongly concentrated in the Galactic bulge, with a smallercontribution from the disk, unlike the situation in any other wavelength. Thepuzzling distribution of the positrons gave rise to various suggestions,including stellar nucleosynthesis in core-collapse (CC) and type Iathermonuclear supernovae (SNe), accreting compact objects, and more "exotic"explanations of annihilation of dark-matter particles. However, such modelsencounter difficulties in reproducing the total Galactic 511 keV flux as wellas its peculiar bulge-centered distribution. Theoretical models of SNe fromthermonuclear Helium detonations on white dwarfs (WDs) were suggested aspotential additional sources of positrons, contributing to the GalacticGamma-ray emission. Here, we show that the recently discovered class of faint,calcium-rich type Ib SNe (with the prototype SN 2005E), thought to arise fromsuch explosions, also called ".Ia" SNe, can explain both the 511 keV flux andits distribution, and can eliminate the need for non-astrophysical (e.g., darkmatter annihilation) sources. Such SNe comprise a fraction of only ~2% of allSNe, but they currently inject hundreds of times more positrons (from $^{44}$Tidecay) to the ISM than injected by CC SNe, enough to reproduce the totalGalactic 511 keV line emission. They exclusively explode in old environments,and their contribution to the 511 keV emission therefore follows the old (>10Gyrs) Galactic stellar population, dominated by the bulge, thereby reproducingthe observed large bulge-to-disk ratio.

Origin of the Galactic 511 keV emission from positrons produced in irregular supernovae