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6 mins read 12 Oct 2019

The Milky Way’s Galactic Core Erupted 3.5 Million Years Ago

Researchers have found evidence of an astronomically recent event that occurred in our Galaxy – so powerful it was felt 200,000 light-years away. “Imagine darkness, and then someone switches on a lighthouse beacon for a brief period of time”.

Milky Way galaxy almost edge on and central showing purple conical beams radiating towards poles, with top beam impacting the trailing stream of gas from an orbiting galaxy.
Artist impression of the massive bursts of radiation emanating from the galactic core and disturbing the Magellanic stream. Credit: James Josephides/ASTRO 3D

Around the time our ancestral hominids – the Australopithecines – were starting to roam around Africa 3.5 million years ago, our galactic supermassive black hole set off two gargantuan beams of energy. Looking up at the sky at the time, much like our own primordial sense to do so both in awe and wonder today, the Australopithecines would have observed these enormous structures of light over thousands of years.

Unlike our ancient predecessors, we now understand that the heart of our galaxy has been active throughout human history. So powerful were these cataclysmic flares, each expanding towards the galactic poles – they disrupted a long trail of gas from the Milky Way’s own nearby dwarf satellite galaxies some 200,000 light-years away – and yet, we are unsure what could have caused this event to occur.

Using data from the Hubble Space Telescope, researchers have observed two enormous ‘ionisation cones’, known as a Seyfert Flare – which originate from a region near the supermassive black hole of our galaxy and sliced through the Milky Way just 3.5 million years ago – an extremely recent event in galactic timescales. The flare would have started with a relatively small diameter and expanded into two enormous cones, emanating into deep space over a period of 300,000 years.

Colour composite image showing large orange elipse (representing the sky) with the galactic centre in the core. There are three inserts of different observational wavelength highlight regions of the galactic core to emphasis the findings.
All-sky Mollweide projection aligned with the Galactic Centre showing the strong association between the 3-10 GeV gamma-ray emission, the 1.5 keV x-ray emission (blue inset), and the 21cm cold hydrogen emission (green inset with orange dots spaced 1 kpc apart at the distance of the Galactic Centre). On the right, the magnified region around the Galactic Centre as a colour composite with all three components overlaid.

The research, led by Professor Joss Bland-Hawthorn from ARC Centre of Excellence for All-Sky Astrophysics in 3 Dimensions (ASTRO 3D) and working as part of a global collaboration with the University of Sydney, Australian National University, and in the US the University of North Carolina, University of Colorado and the Space Telescope Science Institute in Baltimore, will be published in The Astrophysical Journal.

“These results dramatically change our understanding of the Milky Way,” says co-author Magda Guglielmo from the University of Sydney.

“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 research team believes that the huge, explosive event was triggered by nuclear activity caused by the supermassive black hole residing in the Milky Way’s galactic core.

What Is A Supermassive Black Hole?

Black holes were first derived as an outcome of Einstein’s General Relativity paper in 1915, a consequence of his science-changing formulation of our understanding of gravity. Generally speaking, a black hole is a large mass squeezed into a small radius, with a gravitational field so strong – that not even light can escape.

Black holes are normally associated with the death of massive stars (known as Stellar-mass black Holes) – some many times the mass of our Sun. As these huge stars reach the ends of their lives, they undergo a core-collapse which squeezes the infalling matter into a point known as the singularity – and thus a black hole is born.

However, at the heart of most galaxies lies an even bigger monster. These are galactic-sized blackholes, known as Supermassive Black Holes. Our Milky Way galaxy has one called Sgr A* (pronounced “Sagittarius-A-Star” as it is located in the constellation Sagittarius) and its mass is approx. 4.2 million times the mass of our Sun. Even still, this is considered on the lighter side as far as some supermassive black holes come.

In 2018, the first-ever image of a supermassive black hole was captured by linking numerous telescopes around the world – creating an ‘Earth-sized eye” to peer into a distant, massive galaxy known as M87.

Red ring of energy surrounding a black hole. The lowet half of the energy ring is brighter than the top half.
Using the Event Horizon Telescope, this is the first-ever image of a supermassive black hole - it shows photons of light and gas swirling around the event horizon - the dark sphere in the centre. Credit: Event Horizon Collaboration.

A New Light on a Recently Active Past

What makes these findings very interesting to scientists is that these events happened at such a recent epoch in history, especially relative to human history on Earth.

“This is a dramatic event that happened a few million years ago in the Milky Way’s history,” says Professor Lisa Kewley, Director of ASTRO 3D.  

“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!”

Ellipse showing Milky Way stretching across horizontal axis and highlighting the rotational angle of the Seyfert Flares through graphic overlaid. The flares are tilted towards the right of each North/South axis.
Rotated all-sky Aitoff projection (South Galactic Pole uppermost) aligned with the Galactic Centre showing the orientation of the ionization cones inferred from this work.

Galactic activity and collisions occur on the scales of hundreds if not billions of years, so this event occurred only an astronomical minute ago. Our own Milky Way is set to collide with a larger, neighbouring galaxy – Andromeda – around 4.5 billion years from now.

“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.” says Guglielmo.

Whilst this latest work points towards Sgr A* being the culprit, scientists have cautioned that further research would be required to pin down an actual event that triggered the Seyfert Flare.

Disturbing the Neighbours

Our Milky Way galaxy has a number of satellite galaxies that orbit it, all forming part of a larger collective known as the “Local Cluster’. Andromeda, the Milky Way, and the Triangulum galaxy are its largest members. Two prominent satellite galaxies, known as the Large and Small Magellanic Clouds (LMC and SMC) are visible to the naked eye for southern hemisphere sky-watchers.

As the LMC and SMC orbit the Milky Way, a trail of high-velocity gas that arcs across the southern sky chasing the orbiting satellite galaxies known as the ‘Magellanic Stream’. The stream is made of neutral hydrogen.

The extreme power radiated by the Seyfert Flare 3.5 million years ago was enough to disturb the Magellanic Stream as the cones of energy blasted against it.

Ellipse showing sky, with Milky Way as central bar (viewed edge on) and large scale structure coloured purple arching over the southern pole
The Magellanic Stream (purple) arching across the Milky Way. Credit: NASA

We can all but ponder what our ancient Australopithecine ancestors would have thought about when seeing these expanding cones of light as a dramatic feature of our galaxy, during their time on Earth. Picturing this event occurring in our modern times where the world has evolved to include powerful telescopes, amateur astronomy and advance cameras built into mobile devices – it would certainly be captured, studied and analysed in great detail for those awaiting their past, 3.5 million years from now.

The paper, titled ‘The Large-Scale Ionisation Cones In The Galaxy’ is currently available on arXiv