Study Finds Clumpy Halo of Hot Gases Around the Milky Way Galaxy

The Milky Way galaxy is in the process of recycling.

Astronomers at the University of Iowa have determined our galaxy is surrounded by a clumpy halo of hot gases that is continually being supplied with material ejected by birthing or dying stars. The halo also may be where matter unaccounted for since the birth of the universe may reside. Image Credit: Christien Nielsen/Unsplash.

Astronomers from the University of Iowa have found that a clumpy halo of hot gases surrounds the Milky Way galaxy and is constantly being provided with material emitted by dying or birthing stars.

Dubbed the circumgalactic medium (CGM), this heated halo was the breeding ground for the formation of the Milky Way about 10 billion years ago and could hold basic matter unaccounted for from the time universe was born.

The results of the study arise from observations by HaloSat, one of a kind of minisatellites engineered and developed at Iowa—this one equipped to analyze the X-rays emitted by the CGM. According to the researchers, the CGM features a disk-like geometry, based on the intensity of X-ray emissions from it.

Launched from the International Space Station in May 2018, the HaloSat minisatellite is the first minisatellite funded by NASA’s Astrophysics Division.

Where the Milky Way is forming stars more vigorously, there are more X-ray emissions from the circumgalactic medium. That suggests the circumgalactic medium is related to star formation, and it is likely we are seeing gas that previously fell into the Milky Way, helped make stars, and now is being recycled into the circumgalactic medium.

Philip Kaaret, Professor, Department of Physics and Astronomy, University of Iowa

Kaaret is the corresponding author of the study that was published online in the Nature Astronomy journal.

Each galaxy features a CGM and it is vital to gain insights into not just how galaxies were born and evolved but also how the universe advanced from being a core of hydrogen and helium to a cosmological expanse abounding in stars, comets, planets, and celestial constituents of all other forms.

HaloSat was launched to look for atomic remnants known as baryonic matter, which are considered to be missing from the time the universe was born, which is almost 14 billion years ago. The satellite has been surveying the Milky Way’s CGM for proof that the leftover baryonic matter might be found there.

Kaaret and his colleagues intended to get a better understanding of the CGM’s configuration to achieve their aim.

In particular, the team wished to determine whether the CGM is a large, extended halo several times the size of the Milky Way—in such a case, it could hold the total number of atoms to find a solution to the missing baryon problem.

However, if the CGM predominantly consists of recycled material, it would be a comparatively thin, puffy gas layer and an improbable host of the missing baryonic matter.

What we’ve done is definitely show that there’s a high-density part of the CGM that’s bright in X-rays, that makes lots of X-ray emissions. 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.

Philip Kaaret, Professor, Department of Physics and Astronomy, University of Iowa

Kaaret noted that he was amazed by the clumpiness of the CGM, predicting its geometry to be more uniform. The denser areas denote regions in which star formation is underway, and where material is being transferred between the CGM and the Milky Way.

It seems as if the Milky Way and other galaxies are not closed systems,” added Kaaret. “They’re actually interacting, throwing material out to the CGM and bringing back material as well.”

For the team, the next goal is to merge the HaloSat data with data obtained from other X-ray observatories to find if there is an extended halo around the Milky Way and, in case it exists, to calculate its size. This, in turn, could offer a solution to the missing baryon problem.

Those missing baryons better be somewhere. They’re in halos around individual galaxies like our Milky Way or they’re located in filaments that stretch between galaxies.

Philip Kaaret, Professor, Department of Physics and Astronomy, University of Iowa

Co-authors of the study are Jesse Bluem, a graduate student in physics at Iowa; Hannah Gulick, a graduate student in astronomy at the University of California, Berkeley who graduated from Iowa last May; Daniel LaRocca, who earned his doctorate at Iowa last July and is now a postdoctoral researcher at Pennsylvania State University.

Other co-authors of the study include Rebecca Ringuette, a postdoctoral researcher with Kaaret who joined NASA’s Goddard Space Flight Center this month; and Anna Zayczyk, a former postdoctoral researcher with Kaaret and a research scientist at both NASA Goddard and the University of Maryland, Baltimore County.

This study was funded by NASA.

Journal Reference:

Kaaret, P., et al. (2020) A disk-dominated and clumpy circumgalactic medium of the Milky Way seen in X-ray emission. Nature Astronomy.


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