Detection of gamma-ray emission from the Sagittarius Dwarf Spheroidal
  galaxy
  The Fermi Bubbles are giant, gamma-ray emitting lobes emanating from the
nucleus of the Milky Way discovered in 1-100 GeV data collected by the Large
Area Telescope on board the Fermi Gamma-Ray Space Telescope. Previous work has
revealed substructure within the Fermi Bubbles that has been interpreted as a
signature of collimated outflows from the Galaxy's super-massive black hole.
Here we show that much of the gamma-ray emission associated to the brightest
region of substructure - the so-called cocoon - is actually due to the
Sagittarius dwarf spheroidal (Sgr dSph) galaxy. This large Milky Way satellite
is viewed through the Fermi Bubbles from the position of the Solar System. As a
tidally and ram-pressure stripped remnant, the Sgr dSph has no on-going star
formation, but we demonstrate that its gamma-ray signal is naturally explained
by inverse Compton scattering of cosmic microwave background photons by
high-energy electron-positron pairs injected by the dwarf's millisecond pulsar
(MSP) population, combined with these objects' magnetospheric emission. This
finding suggests that MSPs likely produce significant gamma-ray emission
amongst old stellar populations, potentially confounding indirect dark matter
searches in regions such as the Galactic Centre, the Andromeda galaxy, and
other massive Milky Way dwarf spheroidals.
