The day the Milky Way erupted

Researchers have found evidence of a cataclysmic flare that erupted from near the black hole at the centre of our galaxy — so powerful that its effect was felt 200,000 light-years away.

Just 3.5 million years ago, a short space of time on a cosmic scale, a tremendously powerful jet of energy punched out from the close to the supermassive black hole sitting at the centre of the Milky Way. This cone-shaped burst of radiation erupted from the two poles of our galaxy out into the depths of space.

These cones of ionising energy — known as a Seyfert flare — began with a relatively small diameter, cutting their way through the Milky Way, vastly expanding as they did so.

The magnitude of power that the flare possessed was so great that after it exited our galaxy it was able to strike the Magellanic stream — a long trail of gas extending from dwarf galaxies approximately 2.0 x 10⁵ light-years from the Milky Way.

An artist’s impression of the massive bursts of ionizing radiation exploding from the centre of the Milky Way and impacting the Magellanic Stream. (James Josephides/ASTRO 3D)

The burst was “too huge” to be caused by anything but nuclear activity associated with the black hole — known as Sagittarius A (Sgr A*) — say the team of scientists from the ARC Centre of Excellence for All-Sky Astrophysics in 3 Dimensions (ASTRO 3D)in Australia who made the finding.

The team’s discovery shows that the centre of our galaxy may be a far more turbulent place than we have previously believed. Professor Lisa Kewley, Director of ASTRO 3D, explains: “This is a dramatic event that happened a few million years ago in the Milky Way’s history.

“A massive blast of energy and radiation came right out of the galactic centre and into the surrounding material. This shows that the centre of the Milky Way is a much more dynamic place than we had previously thought.

“It is lucky we’re not residing there!”

Professor Joss Bland-Hawthorn led the team, their research is published in The Astrophysical Journal. He says: “The flare must have been a bit like a lighthouse beam.

“Imagine darkness, and then someone switches on a lighthouse beacon for a brief period of time.”

The team was able to determine the era in which the flare erupted by employing data provided by the Hubble Space Telescope — dating it as occurring a little more than 3.5 million years ago — and lasting for around 3.0 x 10⁵ years. In cosmic terms, both of these time scales are incredibly short.

To put it into perspective, the asteroid that is believed to have triggered the extinction event that wiped out the dinosaurs hit the Earth 63 million years before this flare. In fact, as it occurred humanities ancestors — the Australopithecines — were populating Africa.

The research builds upon Professor Bland-Hawthorn’s previous study published in 2013, which considered evidence of a massive explosive event beginning in the centre of the Milky Way. This study ruled out a nuclear starburst as the cause and tentatively tied it to activity around SgrA*.

There goes the neighbourhood. SgrA* maybe a rowdy tenant

Co-author Magda Guglielmo from the University of Sydney highlights how the study may redefine our knowledge of what is occurring at the centre of our galaxy: “These results dramatically change our understanding of the Milky Way.

“We always thought about our Galaxy as an inactive galaxy, with a not so bright centre. These new results instead open the possibility of a complete reinterpretation of its evolution and nature.

“The flare event that occurred three million years ago was so powerful that it had consequences on the surrounding of our Galaxy.

“We are the witness to the awakening of the sleeping beauty.”

But let’s not call the neighbourhood watch committee just yet. The researchers are quick to add that there is much more work to be done before we can conclusively say that SgrA* is the culprit for this burst.

How black holes evolve and thus influence their surrounding galaxies is an outstanding problem in astrophysics, they concede. But, it may be a good idea if we keep our eyes on SgrA*.