Simulating the Fermi Bubbles as Forward Shocks Driven by AGN Jets

Abstract

The Fermi bubbles are two giant bubbles in gamma-rays lying above and below the Galactic Center (GC). Despite numerous studies on the bubbles, their origin and emission mechanism remain elusive. Here we use a suite of hydrodynamic simulations to study the scenario where the cosmic rays (CRs) in the bubbles are mainly accelerated at the forward shocks driven by a pair of opposing jets from Sgr A*. We find that an active galactic nucleus (AGN) jet event that happened 5–6 Myr ago can naturally reproduce the bilobular morphology of the bubbles, and the postshock gas temperature in the bubbles is heated to ∼0.4 keV, consistent with recent X-ray observations. The forward shocks compress the hot halo gas, and at low latitudes, the compressed gas shows an X-shaped structure, naturally explaining the biconical X-ray structure in the ROSAT 1.5 keV map in both morphology and X-ray surface brightness. CR acceleration is most efficient in the head regions of the bubbles during the first 2 Myr. The opposing jets release a total energy of ∼1055 erg with an Eddington ratio of ∼10−3, which falls well in the range of the hot accretion flow mode for black holes. Our simulations further show that the forward shocks driven by spherical winds at the GC typically produce bubbles with much wider bases than observed and could not reproduce the biconical X-ray structure at low latitudes. This suggests that starburst or AGN winds are unlikely the origin of the bubbles in the shock scenario.

Paradigm shift in understanding the Galactic eROSITA Bubbles

“…The age of the bubbles provide an important clue to differentiate the two feedback mechanisms. The AGN wind driven models require small age ∼ 3 − 12 Myr [22,23,24], while the stellar feedback models based on the star formation rate at the GC estimate the age of ∼ 25 Myr [25,26,27] to ∼ 200 Myr [13,28]. Our estimated age of 70-130 Myr support the stellar activity feedback models for the formation of the bubbles.…”

“…The estimated age of the bubbles may provide an important clue to differentiate between the two feedback models. The models requiring the explosive eruptions from the central AGN favor the age of the bubbles of the order of 3 − 12 Myr [22,23,24]. On the other hand, the star-formation driven outflow models estimate the age of the bubbles ranging from ∼ 25 Myr [25,26,27] in case of a bursty nuclear star-formation, to ∼ 200 Myr [28,13] for quasi steady-state star-formation.…”

New Constraints on Cosmic Particle Populations at the Galactic Center using X-ray Observations of the Molecular Cloud Sagittarius B2

“…Possible indication of the brilliance of Sgr A* in the more distant past, over 10 6 years ago, comes from observation of the Fermi Bubbles (Su et al 2010;Dobler et al 2010), a ∼ 15 kpc bi-lobed γ-ray structure extending out of the Galactic Plane, as well as from the even more extended X-ray eROSITA bubbles (Predehl et al 2020), the ∼ 100 pc X-ray Chimneys (Ponti et al 2019), and the ∼ 450 pc radio lobes (Heywood et al 2019). The origin of these structures is debated, but may be due to a past Active Galactic Nucleus (AGN) phase of Sgr A* (reviews in (Yang et al 2018;Kataoka et al 2018;Zhang & Guo 2020;Sofue & Kataoka 2021)).…”

Emission from hadronic and leptonic processes in galaxy bubbles

“…The physical properties are set by the initial conditions of our bubble simulations, which include the energy injected during the initial outburst and its duration (e.g. Guo & Mathews 2012;Zhang & Guo 2020), the effect of multiple outbursts, and the ambient conditions surrounding the bubble. This study is the first that explores the multi-wavelength non-thermal emission signatures of hadronic and leptonic CRs in galaxy bubbles, as well as their long term evolution.…”

“…Other similar approaches invoking Sgr A* activity, but where the CR composition is not specifically required to be leptonic, invoke a pair of jet-driven outflowing bubbles assuming constant AGN activity or continuous energy injection over Myr timescales (e.g. Zhang & Guo 2020). Notably, these models have been able to account for the bi-conical X-ray structures observed near the GC as part of the same phenomenon as the Fermi bubbles.…”