Fermi bubbles

Fermi bubbles
2MASS/G. Kopan, R. Hurt · Public domain · source
Name	Fermi bubbles
Type	Galactic halo structures
Epoch	J2000
Constellation	Sagittarius (constellation), Centaurus (constellation)
Distance	~Milky Way center
Dimensions	~8 kpc each
Discovered	2010
Discoverers	Fermi Gamma-ray Space Telescope team

Fermi bubbles are large, bilobed lobes of gamma-ray emission extending above and below the Galactic Center of the Milky Way. They span roughly 50 degrees (~8 kiloparsecs) in latitude and are prominent in high-energy surveys by the Fermi Gamma-ray Space Telescope, with counterparts detected in X-ray and microwave bands. Their morphology, spectrum, and correlation with structures around the Galactic nucleus link them to energetic processes associated with the Supermassive black hole at the center of the Milky Way and with past episodes of nuclear activity.

Overview

The structures appear as two symmetric lobes perpendicular to the plane of the Milky Way, centered near the Sagittarius A* region and aligned with the Galactic bulge. Observationally, they are defined by hard-spectrum gamma-ray emission detected by the Large Area Telescope (LAT), and are associated with features seen by the ROSAT X-ray survey and the Wilkinson Microwave Anisotropy Probe microwave maps. Their discovery prompted comparisons with extragalactic phenomena such as radio bubbles in Centaurus A, active galactic nucleus outflows in M87, and giant lobes seen in Seyfert galaxies and quasars.

Discovery and observations

The bubbles were reported in 2010 by analyses of data from the Fermi Gamma-ray Space Telescope's Large Area Telescope team, building on earlier high-energy surveys by instruments like EGRET and follow-up studies using ROSAT, Chandra X-ray Observatory, XMM-Newton, and microwave observations from WMAP and Planck. Subsequent mapping used radio facilities such as the Parkes Observatory and the Green Bank Telescope as well as optical surveys from the Sloan Digital Sky Survey to constrain foregrounds. International collaborations including teams affiliated with NASA, European Space Agency, Kavli Institute for Particle Astrophysics and Cosmology, and various university groups combined multi-instrument datasets to characterize morphology, spectrum, and edge sharpness.

Physical characteristics

Each lobe extends approximately 8 kiloparsecs above and below the Galactic plane, with a roughly circular cross-section and sharp edges. The gamma-ray spectrum is hard and spatially uniform, consistent with a power-law index near ~2, with a cutoff at tens of GeV in some analyses. Thermal and nonthermal plasma components are inferred from soft X-ray emission, while polarized microwave and radio signals suggest synchrotron radiation produced by relativistic electrons in magnetic fields of order a few microgauss. The total energy content is estimated to be 10^54–10^55 ergs, comparable to energies involved in supernova feedback and low-luminosity episodes of active galactic nucleus activity in other galaxies such as M82 and NGC 3079.

Origin and formation theories

Proposed origins include past jet or outflow activity from Sagittarius A*, starburst-driven winds from a central episode of rapid star formation akin to the starburst in M82, and cumulative effects of multiple supernovae and stellar winds from the Central Molecular Zone. Models invoke mechanisms such as shock acceleration at bubble edges, magnetic reconnection, and injection of cosmic-ray protons or electrons; competing scenarios emphasize either hadronic interactions (cosmic-ray protons colliding with ambient gas producing neutral pions) or leptonic processes (inverse Compton scattering by cosmic-ray electrons on photon fields like the Cosmic Microwave Background and starlight). Numerical simulations draw on techniques developed for modeling jet feedback in Active Galactic Nucleus hosts like Centaurus A and the hydrodynamics of galactic winds applied in studies of NGC 253 and NGC 4945.

Multiwavelength studies

X-ray observations by ROSAT, Chandra X-ray Observatory, and XMM-Newton reveal counterpart features including soft X-ray arcs and hot plasma that trace shock-heated gas. Microwave evidence from WMAP and Planck highlights a microwave haze with spectral similarities to the bubbles, while radio polarization studies using facilities like the Very Large Array and Parkes Observatory probe synchrotron-emitting populations. Infrared surveys by Spitzer Space Telescope and optical absorption-line studies using instruments on the Very Large Telescope and Keck Observatory constrain the distribution of cooler gas and kinematics, and ultraviolet absorption measurements with the Hubble Space Telescope trace high-velocity clouds possibly entrained by the outflow. Gamma-ray spectral analyses use datasets from Fermi LAT alongside ground-based Cherenkov arrays such as H.E.S.S. and VERITAS to search for spectral cutoffs and to test hadronic versus leptonic emission models.

Impact on Galactic environment

The bubbles influence the Milky Way's circumgalactic medium by injecting energy, momentum, and cosmic rays into the halo, affecting gas cooling, magnetic-field topology, and the circulation of metals from the Galactic Center to larger radii. They serve as a nearby laboratory for feedback processes central to galaxy evolution theories that involve Active Galactic Nucleus feedback and starburst-driven winds, analogous to phenomena observed in elliptical galaxies and galaxy clusters such as those in the Perseus Cluster. Studies of the bubbles inform models of cosmic-ray propagation developed for interpreting observations by the Alpha Magnetic Spectrometer and influence searches for dark matter signatures in the inner Milky Way performed by experiments like Fermi LAT and ground-based observatories.

Category:Milky Way Category:Gamma-ray astronomy Category:Galactic halo