Search for 511 keV Emission in Satellite Galaxies of the Milky Way with
  INTEGRAL/SPI
  The positron annihilation gamma-ray signal in the Milky Way (MW) shows a
puzzling morphology: a very bright bulge and a very low surface-brightness
disk. A coherent explanation of the positron origin, propagation through the
Galaxy and subsequent annihilation in the interstellar medium has not yet been
found. Tentative explanations involve positrons from radioactivity, X-ray
binaries, and dark matter (DM). Dwarf satellite galaxies (DSGs) are believed to
be DM-dominated and hence promising candidates in the search for 511 keV
emission as a result of DM annihilation into electron-positron pairs. The goal
of this study is to constrain possible 511 keV gamma-ray signals from 39 DSGs
of the MW and to test the annihilating DM scenario. We use the spectrometer SPI
on INTEGRAL to extract individual spectra for the studied objects. As the
diffuse galactic emission dominates the signal, the large scale morphology of
the MW has been modelled accordingly and was included in a maximum likelihood
analysis. Alternatively, a distance-weighted stacked spectrum has been
determined. Only Reticulum II (Ret II) shows a 3.1 sigma signal. Five other
sources show tentative 2 sigma signals. The mass-to-511-keV-luminosity-ratio
shows a marginal trend towards higher values for intrinsically brighter
objects, opposite to the V band mass-to-light-ratio, which is generally used to
uncover DM in DSGs. All derived flux values are above the level implied by a DM
interpretation of the MW bulge signal. The signal from Ret II is unlikely to be
related to a DM origin alone, otherwise, the MW bulge would be about 100 times
brighter than what is seen. Ret II is exceptional considering the DSG sample,
and rather points to enhanced recent star formation activity, if its origins
are similar to processes in the MW. Understanding this emission may provide
further clues regarding the origin of the annihilation emission in the MW.
