The Fermi Bubbles: Gamma-ray, Microwave, and Polarization Signatures of Leptonic AGN Jets

TL;DR

This study models the Fermi bubbles using leptonic cosmic rays from AGN jets, successfully explaining multi-wavelength emissions and polarization features, and providing insights into magnetic field structures and jet activity history.

Contribution

The paper presents detailed simulations linking leptonic cosmic rays and magnetic fields to observed gamma-ray, microwave, and polarization signatures of the Fermi bubbles, advancing understanding of their origin.

Findings

Leptons in AGN jets can explain bubble and haze emissions with ambient-like magnetic fields.

Centrally peaked microwave profile indicates recent CR replenishment from a second jet.

High polarization inside bubbles suggests ordered magnetic fields stretched by shocks.

Abstract

The origin of the Fermi bubbles and the microwave haze is yet to be determined. To disentangle different models requires detailed comparisons between theoretical predictions and multi-wavelength observations. Our previous simulations have demonstrated that the primary features of the Fermi bubbles could be successfully reproduced by recent jet activity from the central active galactic nucleus (AGN). In this work, we generate gamma-ray and microwave maps and spectra based on the simulated properties of cosmic rays (CRs) and magnetic fields in order to examine whether the observed bubble and haze emission could be explained by leptons contained in the AGN jets. We also investigate the model predictions of the polarization properties of the Fermi bubbles. We find that: (1) The same population of leptons can simultaneously explain the bubble and haze emission given that the magnetic fields within the bubbles are very close to the exponentially distributed ambient field, which can be explained by mixing in of the ambient field followed by turbulent field amplification; (2) The centrally peaked microwave profile suggests CR replenishment, which is consistent with the presence of a more recent second jet event; (3) The bubble interior exhibits a high degree of polarization because of ordered radial magnetic field lines stretched by elongated vortices behind the shocks; highly-polarized signals could also be observed inside the draping layer; (4) Enhancement of rotation measures could exist within the shock-compressed layer because of increased gas density and more amplified and ordered magnetic fields. We discuss the possibility that the deficient haze emission at b<-35 degrees is due to the suppression of magnetic fields, which is consistent with the existence of lower-energy CRs causing the polarized emission at 2.3 GHz. Possible AGN jet composition in the leptonic scenario is also discussed.