Giant gamma-ray bubbles in the Milky Way

At the center of our galaxy lurks a black hole 4 million times as massive as the Sun. As with some 90% of the supermassive black holes in the universe, the one in our galaxy is currently quiet, meaning the rate of accretion of matter onto it is low. But gamma rays recorded by the Large Area Telescope (LAT) aboard NASA’s Fermi Gamma-Ray Space Telescope reveal a faint glow from two giant structures—the so-called Fermi bubbles—that seem to emerge from the galactic center. It is possible to explain the physical origin of the bubbles without recourse to the galaxy’s central black hole. If, however, the Fermi bubbles are related to the black hole, the scientific consensus is that they reveal a signature of past eruptions and may provide clues to help us better understand the energy output of that enigmatic, massive object at the center of the Milky Way.

Standing out from the gamma glow

Gamma rays, the highest-energy photons, are a prime tool for exposing high-energy processes in the cosmos—specifically, interactions of relativistic charged particles. Because cosmic gamma rays are absorbed by Earth’s atmosphere, they are notoriously hard to detect. To observe them, detectors have to be carried into space by satellites.

The LAT is sensitive to gamma rays in the energy range of a few tens of MeV to a few hundred GeV. It has a field of view of about 20% of the sky at any given moment, and it continuously scans the whole sky with unprecedented precision. (For more on Fermi and the LAT, see the article by David Thompson, Seth Digel, and Judith Racusin, Physics Today, November 2012, page 39 .) In late 2010, about two years after the LAT began taking measurements, Meng Su, Tracy Slatyer, and Douglas Finkbeiner’s analysis of Fermi data uncovered the Fermi bubbles. For their surprising discovery, the three researchers were awarded the 2014 Rossi Prize from the American Astronomical Society.

Our galaxy shines in gamma rays because energetic particles called cosmic rays, which are most likely accelerated in the expanding shells of supernova explosions, interact with the gas and light present between stars. The Milky Way also hosts many bright gamma-ray point sources spotted by Fermi; the most common of those are pulsars and supernova remnants. Beyond the galaxy, the LAT has detected numerous flares from large black holes. And distant, faint sources in the universe, too weak to be detected individually, collectively give off a uniform gamma-ray glow across the night sky.

Panel a of the figure presents the sky as recorded by the LAT at energies above 6.4 GeV. Clearly visible are the galactic interstellar emission and point sources within and beyond the galaxy. Less evident, but still visible, is an additional component of gamma-ray emission above and below the galactic center, extending perpendicular to the galactic plane. After careful modeling and subtraction of the known galactic and extragalactic sources of emission, the Fermi bubbles stand out as the symmetrical structures shown in panel b.

Several ways to make a Fermi bubble

The properties of the Fermi bubbles hold important clues about their origin. The bubbles extend 39 000 light-years above and below the galactic center, have well-defined edges, and the brightness of their radiation does not vary strongly across them. Typically, the gamma-ray photons they emit are significantly more energetic than most of the others born in the Milky Way. At the highest energies recorded by the LAT, and at a few thousand light-years away from the galactic plane, the bubbles become as bright as other sources of gamma rays in the galaxy.

Before we can address what is causing the gamma-ray emission from the Fermi bubbles, we need to consider in greater detail how high-energy gammas are produced. The two main scenarios involve processes well known to particle physicists. In one, electrons with velocities close to the speed of light collide with low-energy photons—for example, IR, optical, or UV starlight—and boost them to gamma-ray energies; the process is called inverse Compton scattering. In the second scenario, high-energy protons interact with nucleons in interstellar gas to produce neutral particles called pions that immediately decay into two gamma rays. Thus the key to the bubble emission lies in gamma-ray production mechanisms involving relativistic electrons or protons. Scientists are searching for spectral changes within the bubbles as a function of distance from the galactic center; such variations would signal evolution or aging of the particles responsible for the radiation as they voyage through galactic space. For now, though, the exact origin of the emission remains an open question.

What processes could accelerate particles to the necessary energies? That the bubbles are symmetric about the galactic plane and galactic center strongly suggests their origin was in our galaxy’s complex central region. If so, whatever generated the Fermi bubbles had the power to accelerate particles out to enormous distances. Today the supermassive black hole at the center of our galaxy may be quiet, but it was not always thus. X-ray light echoing off interstellar gas clouds reveals episodes of intense eruptions that occurred a century ago. From observations of distant galaxies, astronomers know that central black holes have active states during which matter spirals toward them to form an accretion disk. Gravitational and frictional forces compress and raise the temperature of the material in that disk, a process accompanied by lots of radiated energy and often by a pair of huge jets of plasma accelerated by winding magnetic fields. Both protons and electrons can be accelerated to relativistic speeds in the plasma jets.

Scientists using the LAT have already detected examples of jets glowing in gamma rays—for example, in the nearby Centaurus A galaxy. The Fermi bubbles bear a remarkable resemblance to the giant lobes seen in other galaxies and may be the remnant of jets that were thrust by a past eruption of the central black hole in our own galaxy.

A rather different scenario also invokes the active state of the black hole. Driven by the strong heating of the accretion flow onto the black hole, a hot spherical wind forms and flows outward. The wind gets shaped into the observed bubble or hourglass shape because it is free to buoy up away from the disk, but the dense gas in the galactic disk slows down the wind’s lateral expansion.

Alternative proposals do not assume past activity of the black hole. Instead, they posit a burst of star formation in dense gas clouds in the vicinity of the galactic center. Very massive stars quickly expend their nuclear fuel and die in supernova explosions that can accelerate particles to high energies. An ensemble of supernova explosions that took place in the compact stellar clusters in the central galactic region could power a strong plasma wind and transport energetic particles into the bubbles.

Companion microwave bubbles?

To get a deeper understanding of the origin of the Fermi bubbles, astrophysicists have searched for counterparts at other wavelengths. Microwave data recorded by the Planck and Wilkinson Microwave Anisotropy Probe satellites show a haze of radiation that may relate to the bubbles. But the extent of that haze perpendicular to the galactic plane is only about 60% of the Fermi bubbles’ height. Given the general similarity of the microwave and gamma-ray data, it is natural to postulate that a single population of electrons is responsible for both the microwave haze and the gamma-ray emission. The low-energy microwave photons would be synchrotron radiation produced by acceleration of the relativistic charged particles in a magnetic field of a few microgauss, and the gamma-ray emission would be generated by inverse Compton scattering. The smaller height of the microwave haze could be explained by a drop in the strength of the magnetic field.

The ongoing study of the enigmatic Fermi bubbles will include tracing their emission as close as possible to the galactic center, a task that is complicated by the bright gamma-ray foreground in that region. And continued observations of the bubbles at other than gamma-ray wavelengths should help scientists home in on the nature of the energetic event that formed the Fermi bubbles—an event that possibly involved the supermassive black hole in the center of our galaxy.