The huge bubbles of plasma pouring out of the center of the Milky Way could contain debris from the entire galaxy and beyond.
A new look at the gas clouds in the galaxy’s Fermi bubbles shows that the clouds contain material from the galaxy’s starry disk and from some other mysterious source. The finding could shed light on how galaxies live and die in general, astronomers report July 18 in nature astronomy.
Fermi bubbles are giant blobs of plasma, tens of thousands of light-years tall, that straddle the Milky Way’s galactic disk. When the bubbles were discovered in 2010, astronomers thought they might have been formed by newborn stars (Serial number: 9/11/10). These days, many astronomers are convinced that the bubbles could have been expelled by a massive belch emitted long ago by the galaxy’s supermassive black hole.
In the years following the discovery, astronomers also observed clouds of relatively cool gas that seemed to flutter within bubbles, high above the starry disk. “We call them high-velocity clouds, because we’re not very good at naming things,” says astrophysicist Trisha Ashley of the Space Telescope Science Institute in Baltimore.
Scientists thought the clouds had been plucked from the Milky Way’s bright, starry disk and sent flying when Fermi bubbles formed. That assumption has been used to calculate things like the age of the bubbles, which could offer a clue to their origins.
“It made sense, it was a logical assumption,” says Ashley. “But no one had ever proven the origin of these clouds.”
Now, Ashley and her colleagues have made a first effort to find out where the clouds come from and found a surprising answer.
Using new and archived data from several telescopes, she and her team measured the metal content — the abundance of all elements heavier than helium — in 12 high-velocity clouds entrenched in Fermi bubbles. The researchers then compared the chemistry of the clouds with that of the stars in the Milky Way’s disk. If the clouds really did come from the disk, they should have metallic contents like the sun and other stars in the disk, says Ashley. If not, its metal content should be lower.
The team found a wide range of metals in the clouds, from less than a fifth of the sun to more than the sun. That means “these clouds have to originate from both the Milky Way’s disk and the Milky Way’s halo,” he says, referring to the chaotic cloud of gas and dust that surrounds the galaxy and provides it with fuel for new stars (Serial number: 12/7/18). “We have not discovered any other explanation.”
How those clouds got into the halo in the first place remains an open question, says Jessica Werk, an astronomer at the University of Washington in Seattle who was not involved in the study.
“There are various ways these clouds can be produced, various origins and various destinations,” he says. The clouds could have condensed within the halo on their own, or they could have been plucked from smaller galaxies cannibalized by the Milky Way, or a host of other origin stories (Serial Number: 7/24/02). “This cycle in general is a very messy process.”
That mess could help predict how the Milky Way’s star formation might change in the future. Clouds of cold gas like these are the fuel for future star formation. If these clouds were born in the Milky Way’s gaseous halo but are being propelled by Fermi bubbles instead of falling into the disk to form stars, that could eventually slow down the Milky Way’s star-forming factories.
But if the gas clouds end up forming new stars, that could mean the Milky Way is building new stars from a variety of cosmic sources.
“Ultimately, what people are interested in is how the Milky Way maintains its star formation for a long time.” Werk says. “This tells you it’s not just one thing.”
Studying these bubbles and clouds can also help astronomers understand other galaxies.
“We can see these things happening in other galaxies,” says Ashley. “But we have a front row seat to this one.”