2014ApJ...791L..20C

Fermi Bubbles are enigmatic γ-ray features of the Galactic bulge. Both putative activity (within fewxMyr) connected to the Galactic center super-massive black hole and, alternatively, nuclear star formation have been claimed as the energizing source of the Bubbles. Likewise, both inverse-Compton emission by non-thermal electrons ("leptonic" models) and collisions between non-thermal protons and gas ("hadronic" models) have been advanced as the process supplying the Bubbles' γ-ray emission. An issue for any steady state hadronic model is that the very low density of the Bubbles' plasma seems to require that they accumulate protons over a multi-gigayear timescale, much longer than other natural timescales occurring in the problem. Here we present a mechanism wherein the timescale for generating the Bubbles' γ-ray emission via hadronic processes is ∼fewx10⁸ yr. Our model invokes the collapse of the Bubbles' thermally unstable plasma, leading to an accumulation of cosmic rays and magnetic field into localized, warm (∼10⁴ K), and likely filamentary condensations of higher-density gas. Under the condition that these filaments are supported by non-thermal pressure, the hadronic emission from the Bubbles is L_γ≃ 2x10³⁷ erg/s {dot}M_in/(0.1M_☉/yr)TFB_²/(3.5 x 10⁷K)² M_fil/M_pls, equal to their observed luminosity (normalizing to the star-formation-driven mass flux into the Bubbles and their measured plasma temperature and adopting the further result that the mass in the filaments, M_fil is approximately equal to the that of the Bubbles' plasma, M_pls).