Using X-ray data from NASA’s HaloSat minisatellite, astronomers have found that our Milky Way Galaxy is surrounded by a clumpy halo of hot gases that is continually being supplied with material ejected by birthing or dying stars, and that this halo, also known as the circumgalactic medium, has a disk-like geometry.
Our Milky Way Galaxy is surrounded by an enormous halo of plasma (seen in blue in this artists’ rendition). Image credit: NASA / CXC / M.Weiss / Ohio State / A. Gupta et al.
Each galaxy has a circumgalactic medium, and these regions are crucial to understanding not only how galaxies formed and evolved but also how the Universe progressed from a kernel of helium and hydrogen to a cosmological expanse teeming with stars, planets, comets, and all other sorts of celestial constituents.
Launched into space in 2018, NASA’s HaloSat X-ray observatory searches for baryonic matter — that is, the same kind of particles that compose the visible world — believed to be missing since the Universe’s birth nearly 14 billion years ago.
This minisatellite has been observing the Milky Way’s circumgalactic medium for evidence that the missing baryonic matter may reside there.
Baryonic matter is distinct from dark matter, which is invisible and does not interact through any force except gravity. Scientists can only account for about two-thirds of the baryonic matter that should be present in the Universe.
To do look for the missing matter, University of Iowa’s Professor Philip Kaaret and colleagues wanted to get a better handle on the circumgalactic medium’s configuration.
More specifically, the astronomers wanted to find out if the circumgalactic medium is a huge, extended halo that is many times the size of our Galaxy — in which case, it could house the total number of atoms to solve the missing baryon question.
But if the circumgalactic medium is mostly comprised of recycled material, it would be a relatively thin, puffy layer of gas and an unlikely host of the missing baryonic matter.
“What we’ve done is definitely show that there’s a high-density part of the circumgalactic medium that’s bright in X-rays, that makes lots of X-ray emissions,” Professor Kaaret said.
“But there still could be a really big, extended halo that is just dim in X-rays. And it might be harder to see that dim, extended halo because there’s this bright emission disc in the way.”
“So it turns out with HaloSat alone, we really can’t say whether or not there really is this extended halo.”
The team was surprised by the clumpiness of the circumgalactic medium, expecting its geometry to be more uniform.
The denser areas are regions where stars are forming, and where material is being traded between the Milky Way and the circumgalactic medium.
“It seems as if the Milky Way and other galaxies are not closed systems,” Professor Kaaret said.
“They’re actually interacting, throwing material out to the circumgalactic medium and bringing back material as well.”
The next step is to combine the HaloSat data with data from other X-ray observatories to determine whether there’s an extended halo surrounding the Milky Way, and if it’s there, to calculate its size. That, in turn, could solve the missing baryon puzzle.
“Those missing baryons better be somewhere,” Professor Kaaret said.
“They’re in halos around individual galaxies like our Milky Way or they’re located in filaments that stretch between galaxies.”
The findings were published in the journal Nature Astronomy.
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P. Kaaret et al. A disk-dominated and clumpy circumgalactic medium of the Milky Way seen in X-ray emission. Nat Astron, published online October 19, 2020; doi: 10.1038/s41550-020-01215-w
This article is based on press-releases provided by the University of Iowa and the National Aeronautics and Space Administration.
