A UNIFIED MODEL of the FERMI BUBBLES, MICROWAVE HAZE, and POLARIZED RADIO LOBES: REVERSE SHOCKS in the GALACTIC CENTER'S GIANT OUTFLOWS

Abstract

The Galactic center's giant outflows are manifest in three different, nonthermal phenomena: (1) the hard-spectrum, γ-ray "Fermi bubbles" emanating from the nucleus and extending to |b| ∼ 50°; (2) the hard-spectrum, total-intensity microwave (∼20-40 GHz) "haze" extending to |b| ∼ 35° in the lower reaches of the Fermi bubbles; and (3) the steep-spectrum, polarized, "S-PASS" radio (∼2-20 GHz) lobes that envelop the bubbles and extend to |b|∼ 60°. We find that the nuclear outflows inflate a genuine bubble in each Galactic hemisphere that has the classical structure, working outward, of reverse shock, contact discontinuity (CD), and forward shock. Expanding into the finite pressure of the halo and given appreciable cooling and gravitational losses, the CD of each bubble is now expanding only very slowly. We find observational signatures in both hemispheres of giant, reverse shocks at heights of ∼1 kpc above the nucleus; their presence ultimately explains all three of the nonthermal phenomena mentioned above. Synchrotron emission from shock-reaccelerated cosmic-ray electrons explains the spectrum, morphology, and vertical extent of the microwave haze and the polarized radio lobes. Collisions between shock-reaccelerated hadrons and denser gas in cooling condensations that form inside the CD account for most of the bubbles' γ-ray emissivity. Inverse Compton emission from primary electrons contributes at the 10%-30% level. Our model suggests that the bubbles are signatures of a comparatively weak but sustained nuclear outflow driven by Galactic center star formation over few × 108 yr.