A Unified Model of the Fermi Bubbles, Microwave Haze, and Polarized Radio Lobes: Reverse Shocks in the Galactic Center's Giant Outflows

TL;DR

This paper presents a unified model explaining the Fermi Bubbles, microwave Haze, and polarized radio Lobes as results of reverse shocks from giant outflows in the Galactic Center, accounting for their spectra and morphology.

Contribution

It introduces a comprehensive model linking multiple non-thermal phenomena in the Galactic Center through reverse shocks in giant outflows, unifying their origin and observational features.

Findings

Reverse shocks are observed at ~1 kpc above the nucleus.

Synchrotron emission explains the microwave Haze and radio Lobes.

Hadronic collisions account for the gamma-ray emission.

Abstract

The Galactic Center's giant outflows are manifest in three different, non-thermal phenomena: i) the hard-spectrum, \gamma-ray `Fermi Bubbles' emanating from the nucleus and extending to |b| ~ 50 degrees; ii) the hard-spectrum, total-intensity microwave (~ 20-40 GHz) `Haze' extending to |b| ~ 35 degrees in the lower reaches of the Fermi Bubbles; and iii) the steep spectrum, polarized, `S-PASS' radio (~ 2-20 GHz) Lobes that envelop the Bubbles and extend to |b| ~ 60 degrees. We find that the nuclear outflows inflate a genuine bubble in each Galactic hemisphere that has the classical structure, working outwards, of reverse shock, contact discontinuity, and forward shock. Expanding into the finite pressure of the halo and given appreciable cooling and gravitational losses, the contact discontinuity 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 non-thermal 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 contact discontinuity account for most of the Bubbles' \gamma-ray emissivity.