  We present a measurement of the sky distribution of positronium (Ps)
annihilation continuum emission obtained with the SPI spectrometer on board
ESA's INTEGRAL observatory. The only sky region from which significant Ps
continuum emission is detected is the Galactic bulge. The Ps continuum emission
is circularly symmetric about the Galactic centre, with an extension of about 8
deg FWHM. Within measurement uncertainties, the sky distribution of the Ps
continuum emission is consistent with that found by us for the 511 keV
electron-positron annihilation line using SPI. Assuming that 511 keV line and
Ps continuum emission follow the same spatial distribution, we derive a Ps
fraction of 0.92 +/- 0.09. These results strengthen our conclusions regarding
the origin of positrons in our Galaxy based on observations of the 511 keV
line. In particular, they suggest that the main source of Galactic positrons is
associated with an old stellar population, such as Type Ia supernovae,
classical novae, or low-mass X-ray binaries. Light dark matter is a possible
alternative source of positrons.
