At the center of our galaxy is a sleeping giant, a black hole more than four million times as massive as the sun. The Milky Way’s supermassive black hole is mostly quiet, nibbling on small objects at the galactic center and giving off only faint belches of radiation as it digests its prey. But in the past the sleeping giant may have been wide awake—and wreaking havoc.
Astrophysicists at the Harvard–Smithsonian Center for Astrophysics have uncovered evidence of two ghostly but enormous beams of gamma rays, each extending some 30,000 light-years from the galactic center, that seem to mark a violent episode of black hole consumption relatively recently in cosmic history.
Supermassive black holes in some other galaxies are much more active than Sagittarius A* (Sgr A*), the lurker at the center of the Milky Way. As black holes devour gas, dust or even fully formed stars, matter compresses and heats up in an accretion disk outside the event horizon, a black hole's point of no return. That messy accretion process can produce copious radiation or outflows of plasma. As a result, feeding black holes can glow brightly, even across billions of light-years, and some emit giant jets of particles that extend away from the black hole. But such jets had not been identified near our own, mostly dormant, supermassive black hole.
The jets appear as faint lines in maps of the galaxy made by NASA's Fermi Gamma-Ray Space Telescope. Harvard graduate student Meng Su and astrophysicist Douglas Finkbeiner identified the feature by subtracting known gamma-ray sources from Fermi's maps and teasing out the patterns in the diffuse emission left behind. Their finding appears in a study to be published in The Astrophysical Journal. The jets have not been detected at other wavelengths or with other telescopes, but next-generation x-ray space telescopes may be able to confirm or refute their existence.
If the jets emanating from Sgr A* prove to be real features and not just a trick of the eye, they would indicate that, not so long ago, the black hole was much feistier than it is now. One explanation for the jets is that during a feeding-driven outburst, material was funneled outward from the black hole along narrow, oppositely directed jets constrained by a corkscrewing magnetic field. High-energy electrons rushing along those jets collided with background photons, boosting them up to gamma-ray energies, which Fermi's Large Area Telescope can detect.
But just when the jets would have been produced is not simple to pin down—the gamma rays now reaching telescopes could be relics of a whole series of outbursts. Indeed, the hypothesized jets are not the first piece of indirect evidence that Sgr A* gorges itself from time to time. Radiation bouncing off of molecular clouds in the galaxy has been interpreted as an echo of earlier outbursts just hundreds of years ago. "We know that even on a few-century timescale the black hole flickers on and off," Finkbeiner says.
The newfound jets may be related to another feature that Su and Finkbeiner, along with their colleague Tracy Slatyer, now at the Institute for Advanced Study, recently spotted extending from the galactic center. In 2010 the researchers, again using the Fermi telescope, identified huge bubbles above and below the plane of the Milky Way, glowing with gamma rays. The bubbles could also have been produced by energetic electrons, ejected during a black hole feeding frenzy, colliding with mundane photons and raising them up to gamma-ray energies. The so-called Fermi bubbles and the jets may stem from the same outburst a million or so years ago—the jets would be the high-velocity black-hole output, and the bubbles the decelerated material spreading outward in a series of shocks and eddies. But whereas the jets and bubbles overlap in location and scale, the jets are tilted by about 15 degrees, and the researchers note that they could result from separate episodes.
If the two features are indeed related, the jets must have been much more robust at some point in the past to provide the energy that carved the Fermi bubbles out of intergalactic space. And, in turn, the black hole must have been much more voracious as well. "It didn't get to be four [million] or five million solar masses by never doing anything," Finkbeiner says. "At some time in the past it was consuming large amounts of material and making a mess of things."