There’s a lot going on in the center of our galaxy. The core of the Milky Way is home to a supermassive black hole as massive as 4 million Suns named Sagittarius A *, and the surrounding environment is intense.
Blowing from this region is a nuclear galactic wind. It has two large gamma-ray beams cut out above and below the galactic plane, extending a total of 50,000 light-years into space. These Fermi bubbles are complex, and contain a mix of different gases and cosmic rays.
But astronomers have just spotted some news. Inside the Fermi bubbles are clumps of high-velocity cold molecular gas – the stuff of which stars are made. Even better, they are not sure how the galactic center that splashes gas “like bullets”, according to the researchers.
“The wind in the center of the Milky Way has been the subject of much debate since the discovery a decade ago of the so-called Fermi Bubbles,” said astrophysicist Naomi McClure-Griffiths of the Australian National University.
“We have found that not only hot gas comes from the center of our galaxy, but also cold and very dense gas. This cold gas is much heavier, so it moves around less easily.”
To find the clouds of dense, cold molecular gas, researchers examined previously identified clouds of atomic hydrogen in the bubbles, using the Atacama Pathfinder Experiment radio telescope to search for their spectral signature.
Sure enough, they found it in important quantities: two clouds with at least 380 and 375 solar masses’ of molecular gas, and moving 240 kilometers per second and 300 kilometers per second, respectively.
It seemed to mix in with the warmer medium, with the signature suggesting that the cold molecular gas could be in the process of disruption. This is fascinating because such extremely cold, extremely dense molecular gas clouds are just what is needed for star formation.
“Galaxies can be really good at shooting themselves in the foot,” McClure-Griffiths said.
“If you expel a lot of mass, you lose some material that could be used to form stars, and if you lose enough of it, the galaxy can no longer form stars. So, to see hints of the Milky Way loses this. star-forming gas is kind of exciting – it makes you wonder what happens next! “
As for the Fermi bubbles, it really is an open question. These bubbles have been a puzzle since their discovery because it is not clear what caused them. Whatever it was, it took place several million years ago, and there are two competing explanations.
The first is a burst of star formation in a cloud of molecular gas around Sgr A *, which would have produced a heap of Type II supernovae and generated powerful stellar winds. This is one model whereby the galactic center could have puffed two giant bubbles in space.
The second scenario – and one that seems to have more support – is that Sgr A * at one point had a snack a few miles ago, and swallowed a lot of material. Active action through a black hole can launch jets out of the pulse as material is channeled along the outside of the event horizon, or as wind from the rapidly rotating disk of material as it spirals toward the object.
We have observed similar processes in other galaxies, but they have not provided a solid answer. Distant galaxies often have larger, activated supermassive black holes, and higher star formation rates, so they can hook more material. They are also farther away so we can not really see their bubbles up close.
This new study also does not answer the question. In fact, it takes up a great deal – because neither star formation nor accretion of black hole at the levels we have observed in the galactic center are a viable resource.
It is possible that periodic star formation over the last 50 million years could partly explain the volume of gas that is expelled, but models suggest that such clouds have a rather short lifespan – and it is not clear how long they would survive at high acceleration , especially in a warm wind.
It is also possible that the rapidly moving cold gas could form directly in the outflow by mixing slow, cool clouds and rapid, hot winds. This would solve several problems, but current simulations are unable to replicate the whole process.
It’s a thorny problem, and one that the team continues to search for, hoping to make targeted observations of molecular gas tracers within the nuclear wind to gain a more detailed understanding of how they move, and interact with the hot gas around around them.
“We’re still looking for the smoking gun, but it’s getting more complicated the more we learn from it,” said astrophysicist and lead author Enrico Di Teodoro of Johns Hopkins University.
The study was published in Nature.
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