Giant Gamma-ray Bubbles from Fermi-LAT: AGN Activity or Bipolar Galactic Wind?

Abstract: Data from the Fermi-LAT reveal two large gamma-ray bubbles, extending 50 degrees above and below the Galactic center, with a width of about 40 degrees in longitude. The gamma-ray emission associated with these bubbles has a significantly harder spectrum (dN/dE ~ E^-2) than the IC emission from electrons in the Galactic disk, or the gamma-rays produced by decay of pions from proton-ISM 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 Galactic center, 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.

• The paper identifies giant gamma-ray bubbles with a hard E^-2 spectrum using Fermi-LAT observations, highlighting a distinct energetic electron population.
• The paper correlates bubble morphology with microwave and X-ray features, suggesting a common origin from significant central energy injections.
• The paper supports that symmetric, sharply defined bubble edges arise from AGN activity or starburst-driven winds rather than dark matter annihilation.

Overview of "Giant gamma-ray bubbles from Fermi-LAT: AGN activity or bipolar Galactic wind?"

The study conducted by Su, Slatyer, and Finkbeiner investigates the origins of two large gamma-ray bubbles identified using data from the Fermi-Large Area Telescope (Fermi-LAT). These bubbles emanate symmetrically from the Galactic center and extend up to 50 degrees above and below the Galactic plane, spanning about 40 degrees in longitude. The research explores various possible progenitors for these gamma-ray features, such as accretion events onto the supermassive black hole at the Galactic center or a nuclear starburst event in recent times.

Key Findings

• Gamma-ray Spectrum and Characteristics:

The Fermi bubbles have a hard gamma-ray spectrum consistent with $dN/dE \sim E^{-2}$, significantly harder than the gamma-ray spectrum typically observed from pion decay or inverse Compton (IC) scattering from the Galactic disk. This spectral hardness is uniformly observed across the bubbles and suggests a distinct population of energetic electrons.

• Morphology and Spatial Associations:

The gamma-ray bubbles align spatially with known microwave and X-ray features, specifically the WMAP microwave haze and a particular hard-spectrum X-ray excess in the $1.5-2$ keV band, suggesting a common physical origin of these multiwavelength phenomena.

• Energetic Origin and Bubbles' Dynamics:

Given that the observed bubbles are symmetrical and their edges are sharply defined, the bubbles are postulated to have been formed by substantial energy injection events, such as past activities of the Galactic center's supermassive black hole (Sgr A) or vigorous star formation episodes within the central few kiloparsecs. The minimal role of dark matter annihilation in the formation of these bubbles is noted due to the spatial and spectral characteristics observed.

Implications

1. Understanding Galactic Center Activity: This study enhances our understanding of the past energetic events at the Galactic center. The formation of such enormous structures implies significant episodes of energy output, either through AGN-like activity or starburst-driven winds, which can shed light on the dynamic processes occurring in our Galaxy.
2. Probes of Cosmic Ray Acceleration: The proposed correlation of these gamma-ray bubbles with cosmic-ray (CR) production and propagation mechanisms presents an invaluable opportunity to explore non-local CR acceleration processes and the global effects of energetic outflows from the Galactic center.
3. Constraints on Galactic Wind Models: By aligning observations with theoretical models, the study provides new constraints on possible past galactic wind activities and supports further analysis of thermal and non-thermal processes shaping the Galactic halo.

Future Prospects

The continuation of this research will likely involve more refined models of the star formation and black hole accretion histories in the Galactic center. New observational datasets from instruments such as the eROSITA X-ray observatory or the Planck microwave observatory could provide upper echelon data quality with which to verify these results and further constrain the processes shaping similar extragalactic and Galactic phenomena. Future studies could also further examine the diffusion scales and cooling times of CR electrons to validate existing models of CR transport and interaction.

The presence of such large-scale coherent gamma-ray bubbles presents an essential probe for the interaction between the central engine of galaxies and their environs. Understanding the nature of these gamma-ray bubbles could also have far-reaching implications on our perception of feedback processes in galaxy evolution.