Simulating the Formation of the Young “Fermi Bubbles” in the Circinus Galaxy

Abstract The Fermi and eROSITA bubbles in the Milky Way represent an archetypal case of galactic nucleus feedback, yet their origin remains highly debated. Here, we use hydrodynamic simulations to investigate the formation of the “Fermi bubbles” in the nearby Circinus galaxy, a pair of kiloparsec-scaled elliptical bubbles seen in both radio and X-ray observations. We find that a pair of active galactic nucleus (AGN) jets drive forward shocks in the circumgalactic medium, and after evolving for ∼0.95 Myr, the shock-delineated bubble pair roughly matches the observed Circinus bubbles in size and morphology. Our mock X-ray image and spectrum reproduce the observed edge-brightened X-ray surface-brightness distribution and spectrum quite well, and suggest that nonthermal emissions from the jet ejecta also contribute substantially to radio and X-ray emissions from the inner “hotspot” region. We further show that AGN winds tend to produce more spherical bubbles with a wider base near the galactic plane, inconsistent with observations. The hotspot emissions and the misalignment between the galaxy’s rotational axis and the bubble’s axis argue against a starburst wind origin. Our study thus corroborates the AGN jet-shock model for the origin of both the Circinus bubbles and the Fermi bubbles, and suggests that AGN jet feedback may be a common origin of extended gaseous bubbles in regular disk galaxies, potentially playing an important role in their evolution.