About 3.5 Million Years Ago, a Stream of Gas Outside the Milky Way Would Have Lit Up the Night Sky

It’s a truism to point out that modern humans have only been around for the blink of an eye, relative to the age of the Universe. But the Universe was an active place long before we were around to observe all of that activity. And about 3.5 million years ago, it’s possible—if only remotely—that our ancient ancestors noticed something change in the night sky.

Would it have stirred something inside them? Impossible to know.

But according to new research, about 3.5 mya there was an enormous flash, as Sgr. A*, the supermassive black hole at the center of the Milky Way, was rocked by a huge flare. And the light from that flare illuminated a stream of gas outside our galaxy called the Magellanic Stream. We’re only now learning more about this, thanks to the Hubble Space Telescope.

The new research is presented in a paper titled “Kinematics of the Magellanic Stream and Implications for its Ionization.” Lead author of the study is Andrew Fox of the Space Telescope Science Institute. The paper will be published in The Astrophysical Journal.  

About 3.5 million years ago, our Australopithecus ancestors had begun walking upright in Africa. Their brains were only about one-third the size of a modern human’s brain. Could they have been stargazers?

Who knows. But regardless of which of our ancestors may or may not have noticed, a portion of the night sky was lit up.

To understand the results of this new study, we need to take a look at the Milky Way and its neighbouring surroundings.

The Milky Way has a number of satellite galaxies. Two of them are visible to the naked eye—the Small Magellanic Cloud (SMC) and the Large Magellanic Cloud (LMC). Stretching out from the Magellanic Clouds is a huge filamentary stream of gas called the Magellanic Stream. There are really two parts of that stream: the Leading Arm, and the Magellanic Stream proper.

The Magellanic Stream is made up of clouds of gas moving at a high velocity. It was discovered in 1965, though it’s relationship to the LMC and the SMC was only brought to light in 1974. By a huge margin, the Magellanic Stream is the largest and most massive gaseous structure in the Milky Way’s halo.

“This shows us that different regions of the galaxy are linked—what happens in the galactic center makes a difference to what happens out in the Magellanic Stream.”

Andrew Fox, Lead Author, STScI
An llustration showing the Magellanic Clouds, the Stream and Leading Arm, the Milky Way, and the Fermi Bubbles. Image Credit: NASA, ESA, and L. Hustak (STScI)

The stream is massive: 600,000 light years long. When Sgr. A*, the SMBH at the center of the Milky Way emitted its bright flash, it left its imprint on the Magellanic Stream.

“The flash was so powerful that it lit up the stream like a Christmas tree—it was a cataclysmic event!” said principal investigator Andrew Fox of the Space Telescope Science Institute (STScI) in Baltimore, Maryland. “This shows us that different regions of the galaxy are linked—what happens in the galactic center makes a difference to what happens out in the Magellanic Stream. We’re learning about how the black hole impacts the galaxy and its environment.”

Fox and the other researchers behind this new study used the Hubble to hunt for the imprint of the ancient flash. They looked at the light from distant quasars as it passed through the Magellanic Stream, and examined it with Hubble’s Cosmic Origins Spectrograph. They found ions that were created by the 3.5 million year old flash.

This is a mosaic image of an edge-on view of the Milky Way galaxy, looking toward the central bulge. Superimposed on it are radio-telescope images, coloured pink, of the stretched, arc-shaped Magellanic Stream below the plane of the galaxy and the shredded, fragmented Leading Arm crossing the galaxy’s plane and extending above it. The ancient flash did not light up the Leading Arm, only the Magellanic Stream. By ESA/Hubble, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=68017420

“When the light from the quasar passes through the gas we’re interested in, some of the light at specific wavelengths gets absorbed by the atoms in the cloud,” said STScI’s Elaine Frazer, who analyzed the sightlines and discovered new trends in the data. “When we look at the quasar light spectrum at specific wavelengths, we see evidence of light absorption that we wouldn’t see if the light hadn’t passed through the cloud. From this, we can draw conclusions about the gas itself.”

An image from the study. The LMC and SMC are labelled toward the lower left. Each colored mark is one of the quasars, or active galactic nuclei, used in the study. The light from each quasar was analyzed with the Huble Telescope’s Cosmic Origins Spectrograph. The legend on the right shows the Local State of Rest velocity for absorption. The Leading Arm, up and to the left in this image, shows higher LSR velocities than the Magellanic Stream, to the lower right. The leading arm was not lit up, or ionized, by the flare from the black hole. Image Credit: Fox et al, 2020.

This is some astonishing scientific sleuthing. The Magellanic Stream is about 200,000 light years from the galactic center, where the source of the flash, the SMBH Sgr. A*, lies. And it all happened about 3.5 million years ago. And using the light from 31 distant quasars to figure it all out is also very impressive.

What Caused the Flash?

The huge outburst from Sgr. A*, called a Seyfert Flare, was likely caused by a massive hydrogen cloud. That enormous cloud was about 100,000 times more massive than the Sun. It would have orbited the black hole in its accretion disc before being drawn in. As it was consumed, the black hole would have heated the gas, and emitted an enormous burst of energy, leaving its fingerprints on the Magellanic Stream.

In their paper, the authors write “In the Seyfert-flare model, the flare photoionized the Stream… but not the Leading Arm, which lies outside of the ionization cone.”

The flash wasn’t the only result of the consumption of the hydrogen cloud. There was also a massive outpouring of plasma, in the form of Fermi Bubbles. The Fermi Gamma-Ray Space Telescope found them in 2010. They’re enormous bubbles of plasma that protrude from the flat plane of the Milky Way, about 25,000 light years in each direction. These bubbles are millions of times more massive than the Sun, and they emit gamma radiation at a higher energy level than the Milky Way.

An illustration of the Fermi Bubbles that extend beyond the plane of the Milky Way. Discovered in 2010, they are the result of an outburst of energy from Sgr. A*, the supermassive black hole at the heart of the Milky Way. Image Credit: NASA/GSFC

In their paper the authors say that “the enhancement <in the Magellanic Stream> can be understood as fluorescence induced by a recent GC flare, in which the Milky Way’s central supermassive black hole (SMBH) Sgr A? underwent an outburst several Myr ago, releasing a burst of ionizing radiation and potentially creating the giant X-ray/?-ray Fermi Bubbles at the same time.”

Andrew Fox, the lead author of this study, was also a part of the 2015 research that measured the expansion velocity and the composition of the Fermi Bubbles.

The discovery of the ancient flash has tied up some “loose ends” for scientists studying the Milky Way.

“We always thought that the Fermi Bubbles and the Magellanic Stream were separate and unrelated to each other and doing their own things in different parts of the galaxy’s halo,” said Fox in a press release. “Now we see that the same powerful flash from our galaxy’s central black hole has played a major role in both.”

The Hubble Space Telescope deserves some credit for all of this work. Ultraviolet observations were key in uncovering the relationships between the Magellanic Stream, the Fermi Bubbles, and the black hole flash. We can’t study ultraviolet from the ground because Earth’s atmosphere gets in the way. But the Hubble is in space, with nothing between it and the ultraviolet light moving through the Universe.

“It’s a very rich region of the electromagnetic spectrum—there’s a lot of features that can be measured in the ultraviolet,” explained Fox. “If you work in the optical and infrared, you can’t see them. That’s why we have to go to space to do this. For this type of work, Hubble is the only game in town.”

The venerable Hubble Space Telescope. After 30 years, it’s still a productive scientific workhorse. Image Credit: NASA/ESA

These results are not only filling in holes in our knowledge of the Milky Way. They likely apply to at least some other galaxies as well. In their paper the authors write “The Milky Way provides an unmatched opportunity to dissect the gas flows around a star-forming spiral galaxy.” Later they add “This additional knowledge makes the Galactic halo an ideal location for studying gas flows and their role in galaxy evolution.”

As for our ancient ancestors, the flash changed the night sky. Not only the initial outburst, but for long after. Researchers think the glow in the Magellanic Stream could have lasted for one million years. By the end of that time, Australopithecus would’ve been sharing the landscape with Homo Habilis, another of our ancient ancestors. (Some recent evidence suggests they’re not our direct ancestors, but that discussion is beyond the scope of this article.)

A scientific reconstruction of Homo Habilis, our ancient ancestor. Would they have noticed the glowing Magellanic Stream? Image Credit: By Reconstruction by W. Schnaubelt & N. Kieser (Atelier WILD LIFE ART)Photographed by User:Lillyundfreya – Photographed at Westfälisches Museum für Archäologie, Herne, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1843470

The brain of tool-using Homo Habilis was larger than Australopithecus’ by about 45%. Would that have been enough to make them stargazers? Would they have noticed the glow in the sky?

We’ll never know.

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Evan Gough

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