The Milky Way's Fermi bubbles: Echoes of the last quasar outburst?

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 Sgr A*, the supermassive black hole (SMBH) of our Galaxy. Such an outflow would be a natural consequence of a short but bright accretion event on to Sgr A* if it happened concurrently with the well-known star formation event in the inner 0.5 pc of the Milky Way ∼6 Myr ago. We find that the hypothesized near-spherical outflow is focused 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 Sgr A* directly or accelerated in the shocks with the external medium. The Galaxy disc remains unaffected, agreeing with recent observational evidence that SMBHs do not correlate with galaxy disc properties. We estimate that an accreted mass ∼2 × 103 M⊙ is needed for the accretion event to power the observed 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 per cent of the gas was turned into stars, while the rest accreted on to Sgr A*. One interpretation of this is a reduced star formation efficiency inside the Sgr A* 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 ∼0.03 pc of the Galaxy.