During the past two years, two strange events have occurred at our Milky Way’s violent center. In October of 2019, Milky Way’s central black hole, SgrA* (literally “Sagittarius A-star”) which until May, 2019, when it suddenly brightened, appeared like a massive, dormant volcano, a sleeping monster, a slumbering region of spacetime where gravity is so strong that “what goes into them does not come out”. A “one-way door out of our universe,” said Event Horizon Telescope director and astronomer Shep Doeleman of the Harvard Smithsonian Center for Astrophysics about discoveries of the enigmatic objects stranger than science fiction that led to three 2020 Noble Prize awards in physics.
A Cataclysmic Event
In June of 2020, astronomers with the Hubble Space Telescope reported that Hubble’s infrared eye revealed an enormous flash from Milky Way’s black hole about 3.5 million years ago –the floodlight reached so far into space it illuminated a vast train of gas trailing the Milky Way’s two prominent satellite galaxies: the Large Magellanic Cloud (LMC), and its companion, the Small Magellanic Cloud (SMC).The flash was undoubtedly witnessed by the early hominids on the African savannas as the eerie glow spread high overhead in what was later named the constellation Sagittarius.
The flash was so powerful that it lit up the Magellanic 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.
Eons later, astronomers are using NASA’s Hubble Space Telescope’s unique capabilities to uncover even more clues about this cataclysmic explosion, found that the black hole’s outburst was probably caused by a large hydrogen cloud up to 100,000 times the Sun’s mass falling onto the disk of material swirling near the central black hole sending vast cones of ultraviolet radiation above and below the plane of the galaxy and deep into space.
The eROSITA X-ray Telescope Discovery
Now in December 2020, Andrea Merloni at the Max Planck Institute for Extraterrestrial Physics and his colleagues using the eROSITA X-ray telescope aboard the Spektr-RG space observatory, report finding massive balloons –two colossal blobs filled with hot plasma, cosmic rays and magnetic fields. The energy needed to power the formation of these huge bubbles must have been enormous at 10^56 ergs, equivalent to the energy release of 100,000 supernovae, and similar to estimates of AGN outbursts.
Although they cannot be seen with the naked eye, they are very bright in diffuse gamma-ray emissions. In gamma rays, the Fermi bubbles have very sharp edges and the edges coincide well with an X-ray structure called the Galactic center biconical X-ray structure. Astronomers suggest that may unveil secrets about the structure and history of our galaxy that explain why the mass of the black hole in the center of the Milky Way is so small relative to other supermassive black holes. The bubbles provide a missing link for both how the black hole grew and how the energy injection from the black hole accretion process impacted the Milky Way as a whole.
Astronomers detected the remarkable new feature in the first all-sky survey map produced by the eROSITA X-ray telescope on SRG: a huge circular structure of hot gas below the plane of the Milky Way occupying most of the southern sky. A similar structure in the Northern sky, the “North polar spur,” has been known for a long time and had been thought to be the trace of an old supernova explosion. Taken together, the northern and the southern structures instead are reminiscent of a single hourglass-shaped set of bubbles emerging from the galactic center.
“Thanks to its sensitivity, spectral and angular resolution, eROSITA has been able to map the entire X-ray sky to unprecedented depth, revealing the southern bubble unambiguously,” explains Michael Freyberg, a senior scientist working on eROSITA at the Max Planck Institute for Extraterrestrial Physics (MPE). eROSITA scans the entire sky every six months and the data allows the scientists to look for structures that cover a significant portion of the whole sky.
Outbursts Take a Very Long Time to Heal
“The scars left by such outbursts take a very long time to heal in these haloes,” adds Merloni, eROSITA Principal Investigator. “Scientists have been looking for the gigantic fingerprints of such past violent activity around many galaxies in the past.” The eROSITA bubbles now provide strong support for large-scale interactions between the galaxy core and the halo around it, which are energetic enough to perturb the structure, energy content and chemical enrichment of the circumgalactic medium of the Milky Way.
Pictures of the entire sky from eROSITA revealed giant spheres of gas extending more than 45,000 light years above and below the disc of the galaxy, combine they roughly equal the size of the Milky Way which is about 105,000 light years across.
A Black Hole Relic?
Astronomers had already spotted a bubble above the galaxy in 2010 when the Fermi Gamma-Ray Space Telescope revealed a previously unseen structure centered in the Milky Way that spans 50,000 light-years that was hypothesized to be the remnant of an eruption from Sgr A*. The bubble emissions, much more energetic than the gamma-ray fog seen elsewhere in the Milky Way, spans more than half of the visible sky, a region roughly as large as the galaxy itself, and it may be millions of years old, its origin, until now, an unsolved mystery.
Hints of the Fermi bubbles’ edges shown below were first observed in X-rays by ROSAT, which operated in the 1990s. The gamma rays mapped by the Fermi Gamma-ray Space Telescope extend much farther from the galaxy’s plane.
But in 2010, we didn’t have any X-ray telescopes sensitive enough to see the more tenuous southern one, says Merloni. “Because it was only one-sided, people had a lot of trouble figuring out what they were and where they come from,” says Merloni about new data showing that the northern bubble originated at the center of the galaxy. “Now we finally see the southern bubble, so a lot of the controversy about the northern bubble is resolved.”
The invisible X-ray bubbles, reports Merloni, were probably caused by the same event as the Fermi bubbles, which are similar but slightly smaller structures that emit gamma rays. The enormous size of the eROSITA bubbles gives us some insight into what kind of event could have caused both sets of spheres point to their formation when matter fell into our galaxy’s central supermassive black hole, releasing an extraordinary blast of energy.
“It would be no problem to have a little bit of gas falling onto the black hole and releasing the energy required to inflate these bubbles,” says Merloni. The eROSITA mission is expected to last at least six more years, so astronomers should be able to learn more from its later observations. The bubbles are not thought to be unique phenomena observed only in the Milky Way –similar structures can be detected in other galactic systems with active nuclei.
Unknown Subatomic Interactions?
The IceCube Observatory, situated in Antarctica at the Amundsen-Scott South Pole Station, has detected 10 ghostly cosmic neutrino messengers coming from roughly the direction of the two Fermi Bubbles, leading some astrophysicists to conjecture that some unknown subatomic interactions are occurring there. On Dec. 4, 2012, IceCube detected an event now known as Big Bird, the highest-energy neutrino ever detected, with an energy exceeding 2 quadrillion electron volts (PeV) or more than a million million times greater than the energy of a dental X-ray packed into a single particle thought to possess less than a millionth the mass of an electron.
“Like a Crime Scene Investigation”
“It’s like a crime scene investigation”, says Matthias Kadler, a professor of astrophysics at the University of Würzburg in Germany, “The case involves an explosion, a suspect, and various pieces of circumstantial evidence.”
In the eROSITA bubbles shown in the false-color map at the top of the page. the extended emission at energies of 0.6-1.0 keV is highlighted. The contribution of the point sources was removed and the scaling adjusted to enhance large-scale structures in our galaxy. (University of Tübingen)
The Daily Galaxy, Max Goldberg, via University of Tübingen–eROSITA finds large-scale bubbles in the halo of the Milky Way, Nature, NASA, New Scientist.