The Interaction of the Fermi Bubbles with the Milky Way’s Hot Gas Halo

Abstract

The Fermi bubbles are two lobes filled with non-thermal particles that emit gamma rays, extend ≈ 10 {{kpc}} vertically from the Galactic center, and formed from either nuclear star formation or accretion activity on Sgr A*. Simulations predict a range of shock strengths as the bubbles expand into the surrounding hot gas halo ({T}_{halo}≈ 2× {10}⁶ K), but with significant uncertainties in the energetics, age, and thermal gas structure. The bubbles should contain thermal gas with temperatures between 10⁶ and 10⁸ K, with potential X-ray signatures. In this work, we constrain the bubbles’ thermal gas structure by modeling O vii and O viii emission line strengths from archival XMM-Newton and Suzaku data. Our emission model includes a hot thermal volume-filled bubble component cospatial with the gamma-ray region, and a shell of compressed material. We find that a bubble/shell model with n≈ 1× {10}^-3 cm^-3 and with log(T) ≈ 6.60-6.70 is consistent with the observed line intensities. In the framework of a continuous Galactic outflow, we infer a bubble expansion rate, age, and energy injection rate of {490}_-77⁺²³⁰ km s^-1, {4.3}_-1.4^+0.8 Myr, and {2.3}_-0.9^+5.1× {10}⁴² erg s^-1. These estimates are consistent with the bubbles forming from a Sgr A* accretion event rather than from nuclear star formation.