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 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 ( K), but with significant uncertainties in the energetics, age, and thermal gas structure. The bubbles should contain thermal gas with temperatures between 10 6 and 10 8 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 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 km s −1 , Myr, and erg s −1 . These estimates are consistent with the bubbles forming from a Sgr A* accretion event rather than from nuclear star formation.