Ancient explosion in Milky Way's core lit up gas outside the galaxy

NASA's Hubble Space Telescope is further probing the remnants of a massive space explosion visible from Earth 3.5 million years ago.

Embedded in the center of our galaxy, the Milky Way, a supermassive black hole known as Sagittarius A* (Sgr A*) had unleashed a massive amount of energy. New research shows that the energy was so pervasive that it illuminated gas associated with two satellite galaxies of our own: the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC).

"The flash was so powerful that it lit up the [gas] stream like a Christmas tree. It was a cataclysmic event," Andrew Fox, the principal investigator of the study and an astronomer at the Space Telescope Science Institute (STScI) in Baltimore, said in a NASA statement.

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An artist's illustration of the massive explosion that rocked the center of the Milky Way galaxy 3.5 million years ago. Our distant hominid ancestors likely would have seen the resulting flare. (Image credit: J. DePasquale/STScI/NASA/ESA/G. Cecil (UNC, Chapel Hill))

"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," Fox added, referring to the massive, ribbon-like gas structure that trails the Magellanic Clouds. "We're learning about how the black hole impacts the galaxy and its environment."

The black hole's activity likely came from a large hydrogen cloud, about 100,000 times the mass of the sun, falling onto material circling near the black hole. Ultraviolet radiation from the subsequent explosion penetrated far above and below our galaxy's plane, stripping atoms of their electrons in the Magellanic Stream.

To learn more about the ancient explosion, the powerful ultraviolet eyes of Hubble peered at quasars (cores of active galaxies) in the background of the Magellanic Stream. This allowed the telescope's spectrograph to see the signatures of "excited" (ionized) atoms in ultraviolet light. Researchers also looked at 10 quasars behind the Leading Arm, which is a gaseous feature just ahead of the LMC and SMC in the galaxies' orbits around the Milky Way.

This illustration depicts the enormous outburst from the Milky Way's center, which formed Fermi Bubbles, or cones of ultraviolet radiation, above and below the plane of the galaxy. The radiation cone that blasted out of the Milky Way's south pole lit up the Magellanic Stream. (Image credit: L. Hustak/STScI/NASA/ESA)

"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 Elaine Frazer, a research and instrumentation specialist at STScI who participated in the research, in the same statement. "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."

The study showed differences in the gas between the Magellanic Stream and the Leading Arm. The ions in the Magellanic Stream were formed by the black hole explosion, even though the stream is roughly 200,000 light-years away from the black hole. The Leading Arm, however, doesn't seem to have been affected. Researchers suppose that is because the Leading Arm was well away from the direction of the explosion, which streamed past the south galactic pole of the Milky Way.

The Magellanic Stream isn't the only cosmic structure affected by the explosion of Sgr A*. A decade ago, researchers discovered that hot plasma from the explosion is still visible about 30,000 light-years above and below the Milky Way's plane. NASA's Fermi Gamma-ray Space Telescope spotted the plasma, which is called Fermi Bubbles, in 2010. Hubble then examined the bubbles in 2015 to measure how fast the bubbles are expanding, and to learn more about their composition.

"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, who also participated in the 2015 Hubble study. "Now we see that the same powerful flash from our galaxy's central black hole has played a major role in both."

A study based on the research has been accepted for publication in The Astrophysical Journal. The work was also presented virtually June 2 at the 236th meeting of the American Astronomical Society.

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Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel since 2022 specializing in Canadian space news. She was contributing writer for Space.com for 10 years before joining full-time. Elizabeth's reporting includes multiple exclusives with the White House, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, "Why Am I Taller?" (ECW Press, 2022) is co-written with astronaut Dave Williams.

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4 Comments Comment from the forums

rod

FYI, here is another report on the Fermi bubbles. First optical measurements of Milky Way's Fermi Bubbles probe their origin I note this in the phys.org report. "We combined those two measurements of emission and absorption to estimate the density, pressure and temperature of the ionized gas, and that lets us better understand where this gas is coming from,..But while the origin of the phenomenon has been inferred to date back several million years ago, the events that produced the bubbles remain a mystery."

My observation. Apparently an equation of state is being developed to better understand the Fermi bubbles. The precise cause for the origin of the bubbles is not solved yet or confirmed. What can be determined is that the Fermi bubbles are young in age compared to Milky Way H-R diagram ages for different star clusters like globular clusters or some open clusters like M67 as well as the time it takes for the Sun to revolve around the galaxy, i.e. the LSR and rotation period.
Reply
Torbjorn Larsson

rod said:
FYI, here is another report on the Fermi bubbles. First optical measurements of Milky Way's Fermi Bubbles probe their origin I note this in the phys.org report. "We combined those two measurements of emission and absorption to estimate the density, pressure and temperature of the ionized gas, and that lets us better understand where this gas is coming from,..But while the origin of the phenomenon has been inferred to date back several million years ago, the events that produced the bubbles remain a mystery."

My observation. Apparently an equation of state is being developed to better understand the Fermi bubbles. The precise cause for the origin of the bubbles is not solved yet or confirmed. What can be determined is that the Fermi bubbles are young in age compared to Milky Way H-R diagram ages for different star clusters like globular clusters or some open clusters like M67 as well as the time it takes for the Sun to revolve around the galaxy, i.e. the LSR and rotation period.

Yes, that paper is interesting too, but can only equivocate on the mechanism since they don't see Milky Way's temporary Seyfert galaxy core activity as the ionization map team did. I also think this work gives a timeline that match the 3.5 million year age suggested by the hominin illustration, but I haven't read the paper.

I don't think the Fermi Bubble insides will have an equation of state since they are dynamical and not equilibrium phenomena. But I may be mistaken.
Reply
rod

FYI, the Fox study is found here with the arxiv report, Kinematics of the Magellanic Stream and Implications for its Ionization The arxiv report states "...the enhancement can be understood as fluorescence induced by a recent GC flare, in which the Milky Way's central supermassive black (SMBH) Sgr A* underwent an outburst several Myr ago (Bland-Hawthorn et al. 2013, 2019), releasing a burst of ionizing radiation and potentially creating the giant X-ray/ -ray Fermi Bubbles at the same time. This burst would have preferentially ionized the polar regions of the Stream since they lie in the ionization cone directly underneath the GC. The Stream would then recombine and produce the observed Halpha enhancement. In this scenario, the Magellanic Stream acts as a screen on which AGN-induced fluorescence occurs."

My note. The Fox, A. J. study proposes a Seyfert flare arising from the galaxy center to create what is observed today (including the Fermi bubbles), no indication that this is a repeating or cyclic event. After reading the arxiv copy, it seems to be a singular, catastrophic type of event proposed and not repeating. The space.com article stated "The black hole's activity likely came from a large hydrogen cloud, about 100,000 times the mass of the sun, falling onto material circling near the black hole. Ultraviolet radiation from the subsequent explosion penetrated far above and below our galaxy's plane, stripping atoms of their electrons in the Magellanic Stream."

Interesting, is this evidence for an abrupt, catastrophic, and rapid formation of the Milky Way galaxy's center? :) Other reports on the Fermi bubbles at the NASA ADS site suggest the origin of the Fermi bubbles remain elusive. Simulating the Fermi Bubbles as Forward Shocks Driven by AGN Jets, "The Fermi bubbles are two giant bubbles in gamma-rays lying above and below the Galactic Center (GC). Despite numerous studies on the bubbles, their origin and emission mechanism remain elusive...This suggests that starburst or AGN winds are unlikely the origin of the bubbles in the shock scenario."
Reply
Torbjorn Larsson

rod said:
FYI, the Fox study is found here with the arxiv report, Kinematics of the Magellanic Stream and Implications for its Ionization The arxiv report states "...the enhancement can be understood as fluorescence induced by a recent GC flare, in which the Milky Way's central supermassive black (SMBH) Sgr A* underwent an outburst several Myr ago (Bland-Hawthorn et al. 2013, 2019), releasing a burst of ionizing radiation and potentially creating the giant X-ray/ -ray Fermi Bubbles at the same time. This burst would have preferentially ionized the polar regions of the Stream since they lie in the ionization cone directly underneath the GC. The Stream would then recombine and produce the observed Halpha enhancement. In this scenario, the Magellanic Stream acts as a screen on which AGN-induced fluorescence occurs."

My note. The Fox, A. J. study proposes a Seyfert flare arising from the galaxy center to create what is observed today (including the Fermi bubbles), no indication that this is a repeating or cyclic event. After reading the arxiv copy, it seems to be a singular, catastrophic type of event proposed and not repeating. The space.com article stated "The black hole's activity likely came from a large hydrogen cloud, about 100,000 times the mass of the sun, falling onto material circling near the black hole. Ultraviolet radiation from the subsequent explosion penetrated far above and below our galaxy's plane, stripping atoms of their electrons in the Magellanic Stream."

Interesting, is this evidence for an abrupt, catastrophic, and rapid formation of the Milky Way galaxy's center? :) Other reports on the Fermi bubbles at the NASA ADS site suggest the origin of the Fermi bubbles remain elusive. Simulating the Fermi Bubbles as Forward Shocks Driven by AGN Jets, "The Fermi bubbles are two giant bubbles in gamma-rays lying above and below the Galactic Center (GC). Despite numerous studies on the bubbles, their origin and emission mechanism remain elusive...This suggests that starburst or AGN winds are unlikely the origin of the bubbles in the shock scenario."

More detail here: https://www.sciencealert.com/astonomers-pinpoint-the-origin-of-huge-bubbles-of-gas-flowing-our-of-the-milky-way . "We can make comparisons to other galaxies by making the same kind of measurements towards the center of the Milky Way."

If it is sporadic flares, maybe they result from satellite galaxy supermassive black holes accreting, with their more compact accretion disks relatively intact relative their star and gas stripping - c.f. the Sagittarius arm itself - to the Milky Way supermassive black hole.
Reply

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