A galactic breeze origin for the Fermi bubbles emission

Abstract

The origin of the Fermi bubbles, which constitute two gamma-ray emitting lobes above and below the Galactic plane, remains unclear. The possibility that this Fermi bubbles gamma-ray emission originates from hadronic cosmic rays advected by a subsonic Galactic outflow, or breeze, is here explored. The simulation of a breeze solution and subsequent cosmic ray transport is carried out using the hydrodynamical code, PLUTO, in combination with a cosmic ray transport code. The Galactic outflow model obtained is found to be compatible with both inferences of the decelerating outflow velocity profile of the gas in the Fermi bubbles region, and evidence for the presence of a large amount of hot ionised gas out in the Galactic halo region. Although simple, this model is found to be able to reproduce the observed Fermi-LAT energy flux at high Galactic latitudes. Following these results a prediction concerning the gamma-ray emission for 1-3 TeV photons is made for future comparison with CTA/SWGO measurements.

Cosmic Ray Processes in Galactic Ecosystems

“…This outflow could serve to channel hadronic CRs from the ISM into the Galactic halo and CGM. This may also be achieved by subsonic breezes [457]. These breezes, although slower, can have a similar role in CR circulation through a combination of advection and diffusion.…”

“…These breezes, although slower, can have a similar role in CR circulation through a combination of advection and diffusion. They have been shown to be capable of supplying CRs several kpc into the CGM, where they can sustain CR energy densities as high as a few percent as that in the ISM [457,458]. Alternatively, the CRs could be supplied by past jet activity of the Sgr A*, where the CR electrons are rapidly transported to high Galactic latitudes before they cool [454,459,460].…”