Scientists Seek Origin of Mysterious Gigantic Bubble Structures In Our Galaxy

For more than a decade, astronomers have been perplexed by the existence of two gigantic “bubbles'' that extend for tens of thousands of light years on either side of our galaxy, the Milky Way. New observations now reveal that these enormous bubbles are likely remnants of a powerful explosion that occurred some 2.6 million years ago when the supermassive black hole at the Milky Way’s center gobbled up a huge clump of matter.

This glimpse into the pyrotechnic past of our galaxy was captured by eROSITA, a sophisticated X-ray instrument developed by the Max Planck Institute for Extraterrestrial Physics in Germany. This breakthrough from eROSITA, which is part of the Russian–German Spektr-RG space observatory, has a special poignancy to it considering that the instrument was indefinitely switched off on February 26 in response to Russia’s invasion of Ukraine.

A team led by Hsiang-Yi Yang, an assistant professor of astronomy at National Tsing Hua University in Taiwan, shares new insights about these giant X-ray bubbles, which stretch across a total distance of about 50,000 light years above and below the Milky Way’s center, perpendicular to the galactic disk. 

These X-ray structures were first reported in 2020 and bear a striking resemblance to the so-called Fermi bubbles, which are gamma-ray structures spotted by NASA’s Fermi Gamma-ray Space Telescope in 2010. This likeness suggests that past jet activity from the Milky Way’s black hole “could be the common origin of these fascinating structures,” according to a study published on Monday in Nature Astronomy. In other words, the Fermi and eROSITA bubbles may be the fallout of the same galactic eruption, though they shine in gamma ray and X-ray light, respectively.

In an email to Motherboard, Yang called the eROSITA image “quite stunning” and noted that “it shows two gigantic bubbles nearly symmetric about the Galactic plane, which can only be explained by a Galactic-center event.”

“The Fermi/eROSITA bubbles are unique because they are just in the backyard, which allows us to study them in detail,” she continued. “The natural next question we'd like to know is how this event fits into the broader picture of galaxy evolution.”

After the launch of Spektr-RG in 2019, Yang and her colleagues started searching for clues about a bizarre unidentified arc structure called the North Polar Spur that shines brightly at X-ray wavelengths. These observations led to the discovery of the enormous eROSITA bubbles, which seem to be the X-ray counterparts of the Fermi bubbles.

The team used observations of the X-ray structures, as well as complex computer simulations, to reconstruct what is likely the shared backstory of the bubbles, the North Polar Spur, and other emissions captured in the microwave and radio wavelengths that hint at a recent eruption from the Milky Way’s core.

Sometimes in the life of a galaxy, a huge clump of gassy or stellar material gets too close to a supermassive black hole that inhabits the galactic center. An enormous amount of energy is released as the black hole rips up and consumes these cosmic meals, often resulting in explosive twin jets, made of super-hot radiation and particles, that shoot out in opposite directions relative to the galactic center. When this occurs, the galactic core is known as an active galactic nuclei (AGN); astronomers have seen the extraordinary light shows put on by numerous AGN out in the universe.

While the Milky Way’s core is currently quiet, the Fermi and eROSITA bubbles suggest that it recently was going through an active phase. In the new study, Yang and her colleagues estimate that the Milky Way’s black hole, called Sagittarius A*, spent 100,000 years consuming something that was anywhere between 1,000 to 10,000 times more massive than the Sun, about 2.6 million years ago, which is a very short amount of time given the multi-billion-year lifespans of galaxies.

“It is a bit difficult to really tell what was eaten by the black hole a few million years ago, but the Galactic center is a very dense environment full of stars and gas, so in terms of matter supply there should be no problem,” Yang said. “That said, I think consumption of many individual stars is less likely, as the stars would get tidally disrupted when they get too close to the black hole. During the process, some of the materials would be flung away and only a fraction would be finally accreted.” 

The estimated mass of several thousands Suns “is comparable to a giant molecular cloud,” she continued. “If there were some kind of instabilities that caused the cloud to flow to the black hole, that could be a good source of fuel for the event. However, note all of the above is only my personal conjecture.”

Regardless of the nature of the black hole’s recent snack, the event seems to have switched the Milky Way into an AGN state. Today, the glowing remains of that explosion continue to expand into deep space, perpendicular to the galactic disk, and the after-shocks may eventually cause some havoc for our solar system.

“It is true that the radiation and outflow from the AGN jets are mainly directed vertically to the Galactic plane, so they are not going to directly impact the solar system,” said Yang. “The shock is currently propagating outward from the Galactic center and eventually would pass the solar system though.” 

“It might generate turbulence within the Galactic plane and might have some influence on the solar system,” she added. “Fortunately it will take another few million years at least for that to happen.”

While previous studies have suggested that the Fermi bubbles were caused by a recent AGN flare-up, the discovery of the eROSITA bubbles will help refine the exact mechanism behind the explosion. Yang and her colleagues think that the “combination of the gamma-ray, X-ray and microwave images and spectra strongly suggest that past jet activity of the galactic center black hole is the likely culprit,” according to the study.

Given that eROSITA has only just opened up this powerful new window into the X-ray sky and the secrets hidden there, it is deeply disappointing that Russia’s invasion of Ukraine has forced the instrument’s team to suspend its operations.  

“This is indeed very sad news,” Yang said. “Not mentioning all the negative impacts of the war worldwide, it is really unfortunate that it is also hurting scientific progress. In particular, astronomy and space science are unique fields in science, because a lot of the achievements need to involve huge investments and require international collaborations. Important breakthroughs—like discovering the eRosita bubbles—can only happen when people are willing to put in resources and work together.” 

“I sincerely hope that the war will cease soon, eROSITA will come back online, and the current situation will not hurt the trust among the nations and future scientific collaboration,” she said.

In the meantime, the team plans to follow up on their observations with other telescopes, as there are many mysteries yet to be unraveled about the gigantic bubbles at the center of the Milky Way.

“Important questions include whether there are Fermi/eROSITA bubble-like structures in other galaxies as well, whether they could be observed given the sensitivity limits of current/upcoming telescopes, what this event tells us about how supermassive black holes impact their host galaxies, and how supermassive black holes grow in general,” Yang said.

“In the future, we will continue to use this tool to understand the ample multi-wavelength data from the current/upcoming observations, as well as to apply this technique to a broader population of galaxies to study the impact of supermassive-black-hole jets to their surrounding environments,” she concluded.