When Our Galaxy's Dark Heart Exploded

 Voracious, bizarre, and bewitching, supermassive black holes are thought to haunt the mysterious hearts of perhaps every large galaxy in the observable Universe--including our own Milky Way. Our Galaxy's resident dark heart is named Sagittarius A*--Sgr A*, for short (pronounced saj-a-star), and it is dormant now, but still manages to awaken from its slumber now and then to dine on an unlucky star or cloud of gas that has wandered too close to its gravitational embrace. Sgr A*, despite its currently quiet nature, still has enough "life" left to surprise astronomers with a sudden and dramatic episode of turbulent activity. In October 2019, a team of astronomers announced that they had found evidence of a recent cataclysmic flare that exploded so far out of the Milky Way that its destructive influence was felt 200,000 light-years from its origins.

This enormous, expanding beam of energy erupted close to Sgr A* only a "mere" 3.5 million years ago--which is only a blink of the eye in cosmic time. The flare shot out a cone-shaped burst of radiation through both poles of our Galaxy, that then rushed screaming into deep space.

This new finding is based on research conducted by a team of scientists led by Dr. Joss Bland-Hawthorn from Australia's ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ARSTRO 3D). Their work is published in The Astrophysical Journal.

Our Milky Way's Bewitching Heart

Sgr A* is a brilliant and compact astronomical radio source. It is situated at the very heart of our large spiral Galaxy, close to the border of the constellations Sagittarius and Scorpius. As supermassive black holes go, Sgr A* is relatively light in weight. Similar dark hearts inhabiting other galaxies sport millions to billions of times more than our Sun. Sgr A* sports "only" millions of solar-masses.

Observations of many stars circling around Sgr A* have been used to provide important evidence for the presence of, as well as providing data about, our Milky Way's supermassive black hole. These observations have caused some astronomers to conclude that Sgr A* is a black hole beyond a shadow of a doubt.

The shredded remains of what were once stars, as well as clouds of unfortunate and disrupted gas, create the wreckage that eventually tumbles down into the waiting maw of a supermassive gravitational beast like Sgr A*. This terrible banquet of swirling material creates a massive disk encircling the black hole. The disk itself, termed an accretion disk, grows hotter and hotter with the passage of time--particularly when it is pulled into the bizarre vortex close to the dreaded point of no return, called the event horizon. Nothing can return to the outside once it has entered this point. Not even light can return once it has been swallowed. The event horizon is situated at the innermost region of the brilliant accretion disk.

Supermassive black holes, along with their surrounding glaring accretion disks, can be as large as our Solar System--at least. These mysterious entities are described by their greedy appetites, large masses, and messy table manners. When Sgr A* was a young black hole, billions of years ago, it tripped the light fantastic as a searing-hot young, dazzling quasar in the ancient Universe. However, it has grown quiet in its old age, with only a shadow of the appetite it once had when it was in its flaming youth. Quasars lit up the ancient Universe, but like Sgr A* many of them have also lost their sparkle in their old age.

All black holes are dense, compact regions of space. Sgr A* sports the compressed mass of approximately 4.5 million suns, and this hefty mass is squeezed into a relatively small area of space.

In a paper published on October 31, 2018, astronomers announced their discovery of conclusive evidence that Sgr A* is a black hole. Using the GRAVITY interferometer and a quartet of telescopes of the Very Large Telescope (VLT) to form a virtual telescope 130 meters in diameters, the scientists spotted clumps of gas traveling at approximately 30% the speed of light. Emission from highly energetic electrons very close to Sgr A* was visible as a trio of brilliant flares. The flares precisely matched theoretical predictions for hot spots circling close to a black hole of about four milliion times solar mass. The flares are believed to originate from magnetic interactions in the extremely hot gas circling close to our Galaxy's resident dark heart.

What has been seen, so far, of Sgr A*, is not the black hole itself. However, the observations are consistent only if there really is a black hole lurking near Sgr A*. In the case of such a black hole, the observed radio and infrared energy emanates from the gas and dust that is heated to millions of degrees while falling to its doom into the maw of the black hole. But the dark-hearted beast itself is believed to emit only Hawking radiation at a negligible temperature.

Sgr A* is invisible to the eyes of observers. Like all others of its kind, it sends no energy out into space, and it is completely dark. Our Galaxy's elderly, quiet resident supermassive black hole shows very little of the greedy appetite of its youth, when it was still in its brilliant quasar stage. Indeed, at least in the case of Sgr A*, it's been a very long time between dinners. It is thought to have feasted on its last big buffet about six million years ago, when it dined on an unfortunate large cloud of messed up gas. Afterwards, the sloppy black hole, its hunger now quenched, dispatched a huge bubble of gas into interstellar space that was equivalent to millions of solar masses. This gas bubble now bounces both below and above Sgr A*. These after-dinner bubbles are termed Fermi Bubbles, and they were first discovered by NASA's Fermi Gamma-ray Space Telescope back in 2010.