The Milky Way's Fermi Bubbles: Echoes of the Last Quasar Outburst?
  {\it Fermi}-LAT has recently detected two gamma ray bubbles disposed
symmetrically with respect to the Galactic plane. The bubbles have been
suggested to be in a quasi-steady state, inflated by ongoing star formation
over the age of the Galaxy. Here we propose an alternative picture where the
bubbles are the remnants of a large-scale wide-angle outflow from \sgra, the
SMBH of our Galaxy. Such an outflow would be a natural consequence of a short
but bright accretion event on to \sgra\ if it happened concurrently with the
well known star formation event in the inner 0.5 pc of the Milky Way $\sim 6$
Myr ago. We find that the hypothesised near-spherical outflow is focussed into
a pair of symmetrical lobes by the greater gas pressure along the Galactic
plane. The outflow shocks against the interstellar gas in the Galaxy bulge.
Gamma--ray emission could be powered by cosmic rays created by either \sgra\
directly or accelerated in the shocks with the external medium. The Galaxy disc
remains unaffected, agreeing with recent observational evidence that
supermassive black holes do not correlate with galaxy disc properties. We
estimate that an accreted mass $\sim 2 \times 10^3\msun$ is needed for the
accretion event to power the observed {\it Fermi}--LAT lobes. Within a factor
of a few this agrees with the mass of the young stars born during the star
formation event. This estimate suggests that roughly 50% of the gas was turned
into stars, while the rest accreted onto \sgra. One interpretation of this is a
reduced star formation efficiency inside the \sgra\ accretion disc due to
stellar feedback, and the other a peculiar mass deposition geometry that
resulted in a significant amount of gas falling directly inside the inner $\sim
0.03$ pc of the Galaxy.
