Astronomers have observed cold, dense and high-velocity molecular gas outflowing from the center of our Milky Way Galaxy. The presence of this gas is puzzling, because neither Sagittarius A*, a supermassive black hole in the Milky Way’s center, at its current level of activity nor star formation in the inner Galaxy seems to be a viable source for this cold material.
Supermassive black holes at the cores of galaxies blast radiation and ultra-fast winds outward, as illustrated in this artist’s conception. Image credit: NASA / JPL-Caltech.
“Galaxies can be really good at shooting themselves in the foot,” said Professor Naomi McClure-Griffiths, an astronomer in the Research School of Astronomy and Astrophysics at the Australian National University.
“When you drive out a lot of mass, you’re losing some of the material that could be used to form stars, and if you lose enough of it, the galaxy can’t form stars at all anymore.”
“So, to be able to see hints of the Milky Way losing this star forming gas is kind of exciting — it makes you wonder what’s going to happen next!”
The new observations raise questions about what’s happening in the Galactic center right now.
“The wind at the center of the Milky Way has been the topic of plenty of debate since the discovery a decade ago of the so-called Fermi Bubbles, two giant orbs filled with hot gas and cosmic rays,” Professor McClure-Griffiths said.
“We’ve observed there’s not only hot gas coming from the center of our Galaxy, but also cold and very dense gas.”
“This cold gas is much heavier, so moves around less easily.”
Atomic hydrogen gas outflowing from the Galactic center: the color scale shows the column density of anomalous hydrogen clouds in the Milky Way’s nuclear wind; the green dashed line is the boundary of a volume-filled model for the Fermi bubbles; MW-C1 and MW-C2, the two clouds observed by Di Teodoro et al, are marked by red boxes. Image credit: Di Teodoro et al, doi: 10.1038/s41586-020-2595-z.
It’s unclear whether Sagittarius A* has expelled this gas, or whether it was blown by the thousands of massive stars at the Galactic center.
“We don’t know how either the black hole or the star formation can produce this phenomenon,” said Dr. Enrico Di Teodoro, an astronomer in the Department of Physics and Astronomy at Johns Hopkins University and the Research School of Astronomy and Astrophysics at the Australian National University.
“We’re still looking for the smoking gun, but it gets more complicated the more we learn about it.”
“This is the first time something like this has been observed in our Galaxy.”
Astronomers see these kind of processes happening in other galaxies. But, with external galaxies you get much more massive black holes, star formation activity is higher, it makes it easier for the galaxy to expel material.
“And these other galaxies are obviously a long way away, we can’t see them in a lot of detail,” Professor McClure-Griffiths said.
“Our own Galaxy is almost like a laboratory that we can actually get into and try to understand how things work by looking at them up close.”
The findings were published in the journal Nature.
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E.M. Di Teodoro et al. 2020. Cold gas in the Milky Way’s nuclear wind. Nature 584, 364-367; doi: 10.1038/s41586-020-2595-z
