What are Fermi Bubbles?

Fermi Bubbles are giant gamma ray structures emanating from the center of the Milky Way Galaxy, extending tens of thousands of light-years north and south of the galactic plane. They were discovered using the Fermi Gamma-ray Space Telescope during a survey for dark matter.  In 2010, gamma-ray observations by Fermi revealed previously unknown features in our galaxy that stretch halfway across the sky. Now called the Fermi Bubbles, these mysterious structures emerge above and below the center of our galaxy, spanning a total length of about 50,000 light-years.

The plane of our galaxy glows brightly in gamma rays, which result when high-energy particles called cosmic rays interact with gas and dust. The Fermi Bubbles emit higher-energy gamma rays than the rest of the galaxy’s disk.  The bubbles may be related to the release of vast amounts of energy emitted from the supermassive black hole at the center of our Milky Way galaxy. We know that in other galaxies, supermassive black holes that ingest large amounts of matter can power high-energy jets. It’s possible the Milky Way’s central black hole went through such a phase in the past, producing jets responsible for the Fermi Bubbles we see today.

What are Gamma Rays?

A gamma ray, or gamma radiation {\displaystyle \gamma }, is a penetrating electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves and so imparts the highest photon energy.  Natural sources of gamma rays originating on Earth are mostly as a result of radioactive decay and secondary radiation from atmospheric interactions with cosmic ray particles. However, there are other rare natural sources, such as terrestrial gamma-ray flashes, which produce gamma rays from electron action upon the nucleus. Notable artificial sources of gamma rays include fission, such as that which occurs in nuclear reactors, and high energy physics experiments, such as neutral pion decay and nuclear fusion.

Gamma rays and X-rays are both electromagnetic radiation, and since they overlap in the electromagnetic spectrum, the terminology varies between scientific disciplines. A large fraction of astronomical gamma rays are screened by Earth’s atmosphere. Gamma rays are ionizing radiation and are thus biologically hazardous. Due to their high penetration power, they can damage bone marrow and internal organs. Unlike alpha and beta rays, they pass easily through the body and thus pose a formidable radiation protection challenge, requiring shielding made from dense materials such as lead or concrete.

Latest developments

• For the first time, scientists have observed visible light from the Fermi bubbles, enormous blobs of gas that sandwich the plane of the Milky Way galaxy. The newly spotted glow was emitted by hydrogen gas that was electrically charged, or ionized, within the bubbles. Originally observed in 2010, the bubbles spew high-energy light known as gamma rays. The towering structures are thought to be relics of an ancient outburst of gas from the galaxy’s center. But scientists don’t know the source. The outflow could have been the result of the black hole at the center of the galaxy messily gobbling up matter, or emissions caused by bursts of stars forming. Within the bubbles, gas is expanding outward, its motion altering the apparent wavelength of its light. Material closer to the solar system is traveling toward it, appearing bluer and more distant gas is moving away, appearing redder.  The wavelength shift allowed the researchers to pinpoint the gas’s velocity at one location within the bubbles. Using the Wisconsin H-Alpha Mapper telescope, or WHAM, the researchers determined that the gas flowed outward at about 220 kilometers per second. The estimate agreed with an earlier measurement made using ultraviolet light. By taking measurements in other locations, the researchers hope to more fully map out the velocity of the gas.
•  In 2010, astronomers working with the Fermi Gamma-ray Space Telescope announced the discovery of two giant blobs. These blobs were centered on the core of the Milky Way galaxy, but they extended above and below the plane of our galactic home. Their origins are still a mystery, but however they got there, they are emitting copious amounts of high-energy radiation.  More recently, the IceCube array in Antarctica has reported 10 super-duper-high-energy neutrinos sourced from the bubbles, leading some astrophysicists to speculate that some crazy subatomic interactions are afoot. Recently, a team of researchers pored through the available data, even adding results from the newly operational High Altitude Water Cherenkov detector and combined that information with various theoretical models for the Bubbles, searching for just the right combo.  In one possible scenario, protons inside the Bubbles occasionally slam into each other and produce pions, which are exotic particles that quickly decay into gamma rays. In another one, the flood of high-energy electrons in the Bubbles interacts with the ever-present radiation of the cosmic microwave background, boosting some lucky photons into the gamma regime. In a third, shock waves at the outer edges of the Bubbles use magnetic fields to drive local but lethargic particles to high velocities, which then begin emitting cosmic rays. But try as they might, the authors of this study couldn’t find any of the scenarios (or any combination of these scenarios) to fit all the data. In short, we still don’t know what drives the gamma ray emission from the Bubbles, whether the Bubbles also produce neutrinos, or what made the Bubbles in the first place.
• A pair of gigantic gamma-ray bubbles centered on the core of the Milky Way galaxy was discovered by the Fermi Gamma-ray Space Telescope 10 years ago. But how these so-called “Fermi bubbles” arose was a mystery. Recently, however, researchers at the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences have presented a new model that, for the first time, simultaneously explains the origins of both the Fermi bubbles and the galactic center biconical X-ray structure, which was discovered in 2003. According to this model, the two structures are essentially the same phenomenon and was caused by the forward shock driven by a pair of jets emanating from Sagittarius A* (commonly called: Sgr A*) – the supermassive black hole lurking at the galactic center—about five million years ago. Although they cannot be seen with the naked eye, they are very bright in diffuse gamma-ray emissions. In gamma rays, the Fermi bubbles have very sharp edges and the edges coincide well with an X-ray structure called the galactic center biconical X-ray structure. Seeing the very similar edges of Fermi bubbles and the galactic center biconical X-ray structure, the SHAO researchers realized these structures might share the same origin. Furthermore, the biconical X-ray structure could be naturally explained by the shock-compressed thin shell of hot thermal gas driven by a past energy outburst from the galactic center. In previous theoretical models and computer simulations of the Fermi bubbles, two major competing energy sources were proposed, i.e., star formation at the galactic center and Sgr A*. However, in both models, the Fermi bubbles are explained as ‘ejecta’ bubbles, while the forward shock is always located much further away from the edge of the Fermi bubbles. In other words, these models could not explain the Fermi bubbles and the galactic center biconical X-ray structure simultaneously. In contrast, the theoretical model in this study, proposed by Guo Fulai and his graduate student Zhang Ruiyu from SHAO, used computer simulations to demonstrate for the first time that the Fermi bubbles and the galactic center biconical X-ray structure are the same phenomenon. In this model, the edge of the Fermi bubbles is the forward shock driven by a pair of jets emanating from Sgr A* about five million years ago. The age of the bubbles inferred in this study is also consistent with that derived from recent ultraviolet observations of some high velocity clouds along many sightlines towards the bubble region. The new model indicates that the total energy injected during the Fermi bubble event by the supermassive black hole is close to that released by about 20,000 supernovae. The total matter consumed by Sgr A* during this event is about 100 solar masses. Near the galactic center, the biconical X-ray structure has a very narrow base, while the forward shock produced by star formation or black hole winds can easily propagate to large distances, leading to a base much wider than observed. In contrast, collimated jets deposit most of the energy quickly to large distances along the jet direction, naturally leading to a narrow base for the shock front near the galactic plane.