Astrophysics Seminar

Fifteen years after the discovery of the Fermi Bubbles, their origin remains unclear. Several models have been proposed, involving either leptonic or hadronic emission processes. The most widely discussed scenarios are the AGN jet model, the AGN wind model, and the nuclear star-formation-driven wind model, all of which predict supersonic outflows.

However, observations of emission and absorption lines from clouds entrained by the outflow within the Fermi Bubbles reveal a continuously decreasing velocity profile, reaching approximately 300 km s⁻¹ at a distance of ~1 kpc and ~100 km s⁻¹ at ~10 kpc. This behaviour differs from the predictions of standard jet and wind models.

An alternative scenario has therefore been explored, in which the outflow is thermally driven but remains subsonic throughout its evolution. This model, known as a Galactic breeze, naturally reproduces the observed continuous deceleration. Combined with anisotropic cosmic-ray diffusion induced by the magnetic-field structure, it can also account for the sharp edges observed in the gamma-ray emission of the Fermi Bubbles.