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Not long ago, the center of our Milky Way galaxy exploded

Edgewise galaxy with glowing purple blobs top and bottom, and a stream of material arcing over it.

Artist’s concept of the massive bursts of ionizing radiation exploding from the center of our Milky Way and impacting the Magellanic Stream. Image via James Josephides/ASTRO 3D.

Astronomers said today (October 6, 2019) that they’ve uncovered evidence for a titanic, expanding beam of energy that sprang from close to the supermassive black hole in the center of our Milky Way galaxy, just 3.5 million years ago. On Earth at that point, the asteroid that triggered the extinction of the dinosaurs was already 63 million years in the past, and humanity’s ancient ancestors, the australopithecines, were afoot in Africa. This explosion would have sent two enormous cone-shaped bursts of radiation through both poles of the galaxy and out into deep space. One burst must have been powerful enough to reach 200,000 light-years into space, so that its impact struck the Magellanic Stream, a long trail of gas extending from the nearby Large and Small Magellanic Clouds, dwarf galaxies orbiting our Milky Way.

Researchers estimate that the blast lasted for perhaps 300,000 years, a long time in human terms, but an extremely short time as measured on the scale of galaxies.

These new findings come from a team of scientists led by astronomer Joss Bland-Hawthorn from Australia’s ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D). They are soon to be published in the peer-reviewed Astrophysical Journal.

About 10% of all galaxies are known to have flares of this kind, which are called Seyfert flares. Our galaxy isn’t generally considered a Seyfert galaxy, or a particularly active galaxy at all. But the Milky Way is known to have a 4-million-solar-mass black hole at its heart, called Sagittarius A*, or Sgr A* (pronounced Sagittarius A-star). Even earlier this year, Sgr A* was caught having an unusually large meal of gas and dust.

So astronomers are learning that the Milky Way, too, can sometimes have a burst of activity.

The explosion 3.5 million years ago was too huge to have been triggered by anything other than nuclear activity associated with Sgr A*, said the team who studied it. Bland-Hawthorn commented:

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.

Lisa Kewley, director of ASTRO 3D, said:

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 center and into the surrounding material. This shows that the center of the Milky Way is a much more dynamic place than we had previously thought. It is lucky we’re not residing there!

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Animated graphic: cones of ionized radiation moving outward on both sides of the Milky Way's center.

A schematic diagram showing the ionizing radiation field over the south galactic hemisphere of the Milky Way, disrupted by the Seyfert flare event. Image via Bland-Hawthorne, et al./ASTRO 3D.

Bottom line: Researchers have found evidence of a cataclysmic flare that punched outward in both directions from our galaxy’s center, reaching so far into intergalactic space that its impact was felt 200,000 light-years away.

Source (to be published in ApJ): The Large-Scale Ionisation Cones In The Galaxy

Via ScienceinPublic

Deborah Byrd

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