Supermassive Black Hole at Center of Our Galaxy Hiccups As It Feeds

Astronomers have discovered that the supermassive black hole at the center of our galaxy may have sprung a leak. This means Sagittarius A* (Sgr A*) at the heart of the Milky Way may not be as much of a sleeping giant as previously believed and could be "hiccuping" as it swallows gas clouds.

Using the Hubble Space Telescope, the team, including University of North Carolina in Chapel Hill physicist Gerald Cecil, found evidence of a blowtorch-like jet pushing weakly into clouds of hydrogen at the center of the Milky Way.

The researchers were unable to find direct evidence of the jet, which NASA says seems to occasionally "splatter" this gas like a weak hose directed into a mound of sand.

The discovery marks further evidence that our supermassive black hole, which has a mass 4.1 million times that of the sun, is not completely inactive, but springs to life "hiccupping" when dense gas clouds fall into it.

This occurs around other black holes when they are surrounded by accretion discs of material drawn to them as a result of their immense gravitational influence. While some of this material gradually falls to the surface of the black hole, some infalling material is snatched by jets created by powerful magnetic fields and is swept outwards.

These jets of material are accompanied by energetic and powerful radiation capable of stripping electrons away from atoms, a process called ionization.

The team's research, published in The Astrophysical Journal, suggests that when something sizable like a hefty gas cloud gets too close to our central supermassive black hole and is swallowed, it belches out a mini-jet.

Supermassive Black Hole Jet
A diagram of a jet of material launched from the supermassive black hole at the center of the Milky Way. Astronomers found evidence of a jet weekly pushing into hydrogen clouds at the heart of our galaxy. ESA, Gerald Cecil UNC-Chapel Hill), and Dani Player (STScI/NASA

Astronomers had already found evidence of a southern jet of material pushing gas around in the vicinity of Sgr A*. But what Cecil and his colleagues wanted to know was if there is a mirror northern jet doing the same on the opposite side.

Using data collected by the Atacama Large Millimeter/Submillimeter Array (ALMA) located in northern Chile, the astronomers found a narrow feature in massive gas bubbles proximate to Sgr A*. They traced this channel back 15 light-years to the central supermassive black hole.

These gas bubbles, known as the Fermi bubbles, themselves were created by an outburst from Sgr A* that occurred between 2 and 4 million years ago.

Using Hubble and observing the region in the infrared spectrum, Cecil found a glowing, inflating bubble of hot gas that aligns with the jet stretching back for a distance of at least 35 light-years from Sgr A*. Cecil and the team believe that the jet has plowed into the bubble of gas inflating it.

The astronomers say that these two residual effects of the fading jet are currently the only visual evidence of it impacting molecular gas.

As the channel carved by the jet from Sgr A* moves through the gas cloud, it splits from a single pencil-like channel to multiple thinner streams.

Alex Wagner, co-author and assistant professor at the University of Tsukuba, said: "The streams percolate out of the Milky Way's dense gas disk. The jet diverges from a pencil beam into tendrils, like that of an octopus."

As these tendrils pass out of the Milky Way's Fermi gas lobes, they create multiple expanding bubbles that extend as far as 500-light-years. This soap-bubble-like structure has been mapped by other telescopes in various wavelengths of light. The duo connected these observations to jet outflows by simulating the Milky Way using a supercomputer.

"Our central black hole clearly surged in luminosity at least 1 millionfold in the last million years. That sufficed for a jet to punch into the Galactic halo," Wagner said.

Black Hole Jet
A figure showing gas around the central region of the Milky Way as seen by various telescopes. The white fan shows where the proposed jet would extend into the Milky Way. ESA, and Gerald Cecil UNC-Chapel Hill); Image Processing: Joseph DePasquale (STScI/NASA

The astronomers also used observations of the galaxy NGC 1068, located 47 million light-years away from Earth, to help confirm their results. This active spiral galaxy has a chain of bubble-like features that branch off from an outflow from its central black hole, which is actively feeding on material surrounding it.

Cecil said that there are similarities between this structure and that at the center of the Milky Way. "A bow shock bubble at the top of the NGC 1068 outflow coincides with the scale of the Fermi bubble start in the Milky Way," he added. "NGC 1068 may be showing us what the Milky Way was doing during its major power surge several million years ago."

The researcher also suggests that the residual features he and his team observed are close enough to Sgr A* to become more prominent again in decades following a burst of activity. Cecil continued: "The black hole need only increase its luminosity by a hundredfold over that time to refill the jet channel with emitting particles. It would be cool to see how far the jet gets in that outburst.

"To reach into the Fermi gamma-ray bubbles would require that the jet sustains for hundreds of thousands of years because those bubbles are each 50,000 light-years across!"

Supermassive Black Hole
An artist's impression of a supermassive black hole feeding on material and belching out powerful jets of gas. New research has found evidence of such a jet extending from the northern hemisphere of the Milky Way's supermassive black hole. Nustar-Caltech/NASA