Clouds in the Milky Way’s plasma bubbles came from the starry disk — and far beyond

Huge bubbles of plasma billowing out from the Milky Way’s center might contain scraps from all over the galaxy — and beyond.

A new look at gas clouds in the galaxy’s Fermi bubbles shows that the clouds contain stuff from the galaxy’s starry disk and from some mysterious other source. The finding could shed light on how galaxies in general live and die, astronomers report July 18 in Nature Astronomy.

The Fermi bubbles are giant blobs of plasma, tens of thousands of light-years tall, that extend on either side of the Milky Way’s galactic disk. When the bubbles were discovered in 2010, astronomers thought they could have been formed by newborn stars (SN: 11/9/10). These days, many astronomers are instead convinced the bubbles could have been blown by a massive, long-ago burp emitted from the galaxy’s supermassive black hole.

In the years that followed the discovery, astronomers also spotted clouds of relatively cool gas that seem to flit around within the 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 ripped from the Milky Way’s bright starry disk and sent flying when the 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,” Ashley says. “But no one had ever tested the origin of these clouds.”

Now Ashley and colleagues have made a first effort to figure 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 abundances of all the elements heavier than helium — in 12 high velocity clouds entrenched in the Fermi bubbles. Then the researchers compared the clouds’ chemistries to those of stars in the Milky Way’s disk. If the clouds really did come from the disk, they should have metal contents like the sun and other disk stars, Ashley says. If not, their metal contents should be lower.

The team found a wide range of metals in the clouds, from less than a fifth of the sun’s to more than the sun’s. That means “these clouds have to originate in both the disk of the Milky Way and the halo of the Milky Way,” she says, referring to the chaotic cloud of gas and dust that surrounds the galaxy and provides it with fuel for new stars (SN: 7/12/18). “We haven’t figured out any other explanation.”

How those clouds got into the halo in the first place is still an open question, says Jessica Werk, an astronomer at the University of Washington in Seattle who was not involved in the study.

“There’s a number of ways these clouds can be produced, a number of origins and a number of fates,” she says. The clouds could have condensed within the halo on their own, or they could have been ripped from smaller galaxies cannibalized by the Milky Way, or a number of other origin stories (SN: 7/24/02). “This cycle in general is a very messy process.”

That messiness could help predict how the Milky Way’s star formation could change in the future. Cold gas clouds like these are the fuel for future star formation. If these clouds were born in the Milky Way’s gaseous halo but are being buoyed up by the 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 do 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 does the Milky Way sustain its star formation for a long time?” Werk says. “This tells you it’s not just one thing.”

Studying these bubbles and clouds can help astronomers understand other galaxies, too.

“We can see these things going on in other galaxies,” Ashley says. “But we have a front row seat to this one.”