Steady-state hadronic gamma-ray emission from 100-MYR-old Fermi Bubbles

Abstract

Fermi Bubbles are enigmatic γ-ray features of the Galactic bulge. Both putative activity (within few × Myr) 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 ∼few × 108 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 ≃ 2 × 1037 erg s-1 Ṁin/(0.1 M⊙ yr-1) TFB2/3.5 × 107 K)2 Mfil/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, Mfil is approximately equal to the that of the Bubbles' plasma, Mpls). © 2014. The American Astronomical Society. All rights reserved.