THEFERMIBUBBLES. I. POSSIBLE EVIDENCE FOR RECENT AGN JET ACTIVITY IN THE GALAXY

Abstract

The Fermi Gamma-ray Space Telescope reveals two large gamma-ray bubbles in the Galaxy, which extend about 50• (∼10 kpc) above and below the Galactic center (GC) and are symmetric about the Galactic plane. Using axisymmetric hydrodynamic simulations with a self-consistent treatment of the dynamical cosmic ray (CR)-gas interaction, we show that the bubbles can be created with a recent active galactic nucleus (AGN) jet activity about 1-3 Myr ago, which was active for a duration of ∼0.1-0.5 Myr. The bipolar jets were ejected into the Galactic halo along the rotation axis of the Galaxy. Near the GC, the jets must be moderately light with a typical density contrast 0.001 η 0.1 relative to the ambient hot gas. The jets are energetically dominated by kinetic energy, and overpressured with either CR or thermal pressure which induces lateral jet expansion, creating fat CR bubbles as observed. The sharp edges of the bubbles imply that CR diffusion across the bubble surface is strongly suppressed. The jet activity induces a strong shock, which heats and compresses the ambient gas in the Galactic halo, potentially explaining the ROSAT X-ray shell features surrounding the bubbles. The Fermi bubbles provide plausible evidence for a recent powerful AGN jet activity in our Galaxy, providing new insights into the origin of the halo CR population and the channel through which massive black holes in disk galaxies release feedback energy during their growth.

Multiwavelength features of Fermi bubbles as signatures of a Galactic wind

“…9 is strictly valid only for an homogeneous and isotropic medium, and with isotropic energy injection, we expect it to be roughly valid, even with anisotropic AGN jets. Most AGN-based models consider a shorter age (∼ 1 Myr), which comes at the expense of a much larger mechanical power (up to 10 44 erg s −1 , Guo & Mathews 2012;Yang et al 2012). The velocity of the outer shock is given as V ≈ 3R/5t…”

“…The speed required to reach ∼ 10 kpc in 1 Myr is ∼ 10 4 km s −1 , achievable by relativistic jets slowed down by the hot circumgalactic medium (CGM). The power required for inflating young FBs is much higher and the outer shock is much stronger (Guo & Mathews 2012;Zubovas & Nayakshin 2012), and X-ray emissivity and temperature much higher than what is observed (Kataoka et al 2013). The SNe-driven models of FBs consider them to be long lived ( 10 Myr), and thus the injected power is smaller.…”

“…The high energy electrons or protons can either be accelerated in situ by internal shocks and turbulence (Mertsch & Sarkar 2011), or advected from the disc. Outflows triggered by star formation in the Galactic centre (GC) region (Crocker 2012;Lacki 2014) and by the black hole at the GC (Guo & Mathews 2012;Yang et al 2012;Mou et al 2014) have been proposed for the dynamical origin of the FBs.…”

“…The AGN-based models (both jet and wind driven) generally consider a shorter age ( few Myr) for the FBs because the inverse-Compton cooling time (due to up-scattering of starlight) for ∼ 100 GeV electrons responsible for gamma ray emission is a few Myr (Guo & Mathews 2012;Yang et al 2012). The speed required to reach ∼ 10 kpc in 1 Myr is ∼ 10 4 km s −1 , achievable by relativistic jets slowed down by the hot circumgalactic medium (CGM).…”

A possible origin of gamma rays from the Fermi Bubbles

“…Some models suggest that the cosmic rays might originate from activities at the Galactic center and transported into the bubbles by high-speed winds or jets [6,7,8], while others suggest they might be accelerated by shock waves or plasma turbulence present inside the bubbles [9,10,11].…”