GIANT GAMMA-RAY BUBBLES FROM<i>FERMI</i>-LAT: ACTIVE GALACTIC NUCLEUS ACTIVITY OR BIPOLAR GALACTIC WIND?

TLDR

The gamma‑ray emission from the bubbles has a markedly harder spectrum (dN/dE ∼ E⁻²) than typical Galactic disk inverse Compton or pion‑decay emission. The study aims to determine the origin and evolution of these bubbles to better understand recent energetic events in the inner Galaxy and the high‑latitude cosmic‑ray population. Fermi‑LAT data reveal two symmetric, hard‑spectrum gamma‑ray bubbles extending 50° above and below the Galactic center, with no spectral or intensity variation, spatially coincident with the WMAP haze and ROSAT X‑ray features, indicating they were produced by a recent energetic event in the Galactic center rather than dark‑matter annihilation.

Abstract

Data from the Fermi-LAT reveal two large gamma-ray bubbles, extending 50° above and below the Galactic center (GC), with a width of about 40° in longitude. The gamma-ray emission associated with these bubbles has a significantly harder spectrum (dN/dE ∼ E−2) than the inverse Compton emission from electrons in the Galactic disk, or the gamma rays produced by the decay of pions from proton–interstellar medium collisions. There is no significant spatial variation in the spectrum or gamma-ray intensity within the bubbles, or between the north and south bubbles. The bubbles are spatially correlated with the hard-spectrum microwave excess known as the WMAP haze; the edges of the bubbles also line up with features in the ROSAT X-ray maps at 1.5–2 keV. We argue that these Galactic gamma-ray bubbles were most likely created by some large episode of energy injection in the GC, such as past accretion events onto the central massive black hole, or a nuclear starburst in the last ∼10 Myr. Dark matter annihilation/decay seems unlikely to generate all the features of the bubbles and the associated signals in WMAP and ROSAT; the bubbles must be understood in order to use measurements of the diffuse gamma-ray emission in the inner Galaxy as a probe of dark matter physics. Study of the origin and evolution of the bubbles also has the potential to improve our understanding of recent energetic events in the inner Galaxy and the high-latitude cosmic ray population.