Giant Shocks in the Fermi Bubbles and the Origin of the Microwave Haze

Abstract

Analysis of γ-ray data provided by the Fermi-LAT has revealed giant, hard-spectrum γ-ray lobes emanating from the Galactic nucleus (and extending to |b| ∼ 50◦). These ‘Fermi Bubbles’ have hard-spectrum, total-intensity microwave (∼ 20 − 40 GHz) counterparts in their lower reaches (the microwave ‘Haze’ extending to |b| ∼ 35◦) and, on large scales, are subsumed by steep spectrum, polarised radio (2-20 GHz) structures (the ‘S-PASS Lobes’ extending to |b| ∼ 60◦). Here we show that all three of these disparate, non-thermal phenomena may be explained if the Bubbles contain giant, internal (reverse) shocks at heights of ∼ kpc above the nucleus. At this height the ram pressure of the freely-expanding upstream flow from the nucleus equilibrates with the thermal pressure of the shocked plasma accumulated into the downstream bubbles. The shocks, for which we find signatures in both north and south bubbles, reheat and reaccelerate, respectively, the adiabatically cooling thermal plasma and cosmic ray content of the nuclear outflow. Synchrotron emission from shock-reaccelerated cosmic ray electrons, in particular, provides a compelling explanation for the spectrum, morphology, and vertical extent of both the microwave Haze and the S-PASS Lobes. Downstream of the shock, gas piles up in a shell before the contact discontinuity (corresponding to the Bubbles’ edges) and condenses into dense clumps and filaments that fountain back to the plane. Reaccelerated hadrons, escaping the shocks, are adiabatically compressed into this shell where they collide with the denser gas thereby accounting for most of the Bubbles’ γ-ray emissivity. We demonstrate explicitly how, accounting for the finite halo pressure and plasma cooling and drop-out, the Bubbles have a slowly growing total mass and radius and an internal pressure that has reached equilibrium with the halo gas. Altogether our model is strong evidence that the Bubbles are the signatures of a sustained nuclear outflow, energetically matched by Galactic center star formation. Subject headings: cosmic rays — gamma rays: diffuse background