HaloSat reveals that the Milky Way is surrounded by a heated,...

The Milky Way Galaxie is in the recycling business.

University of Iowa Astronomers have found that our galaxy is surrounded by a lumpy halo of hot gases that is constantly fed with material emitted from giving birth or dying stars. This heated halo, known as the circumgalactic medium (CGM), was the incubator for the formation of the Milky Way about 10 billion years ago and could be where inexplicable primitives may have been since the birth of the universe.

The results come from observations made by HaloSat, a mini satellite in a class of mini satellites designed and built in Iowa. This was prepared for the examination of the X-rays emitted by the CGM. The researchers conclude that the CGM has a disk-like geometry based on the intensity of the X-ray emissions it emits. The HaloSat mini-satellite was launched from the International Space Station in May 2018 and is the first mini-satellite to be funded by NASADepartment of Astrophysics.

“Where the Milky Way stars more, there are more X-ray emissions from the circumgalactic medium,” says Philip Kaaret, professor at the Iowa Department of Physics and Astronomy and corresponding author of the study, which was published online in the journal Naturastronomie. “This suggests that the circumgalactic medium is related to star formation, and it is likely that we are seeing gas that previously fell into the Milky Way, contributed to star formation, and is now being returned to the circumgalactic medium.”

Every galaxy has a CGM, and these regions are crucial to understand not only how galaxies formed and evolved, but also how the universe changed from a core of helium and hydrogen to a cosmological expanse of stars, planets, comets and all other species evolved has heavenly components.

HaloSat was launched in 2018 to search for atomic debris known as baryonic matter, believed to have been absent since the universe was born nearly 14 billion years ago. The satellite observed the CGM of the Milky Way to find evidence that there may be remnants of baryonic matter there.

To this end, Kaaret and his team wanted to get a better grip on the configuration of the CGM.

In particular, the researchers wanted to find out whether the CGM is a huge, expanded halo that is many times the size of our galaxy. In that case, it could contain the total number of atoms to solve the missing baryon question. However, if the CGM was made primarily of recycled material, it would be a relatively thin, swollen layer of gas and an unlikely host of the missing baryonic matter.

“What we did is definitely show that there is a high-density portion of the CGM that is bright in x-rays and causes a lot of x-ray emissions,” says Kaaret. “But there could still be a really big, sprawling halo that is only faint in x-rays. And it might be more difficult to see this dark, extended halo because this bright emission disk is in the way.

“So it turns out that HaloSat alone can’t really tell whether this extended Halo really exists or not.”

Kaaret says he was surprised by the clumping of the CGM and expected its geometry to be more uniform. The denser areas are regions where stars form and where material is traded between the Milky Way and the CGM.

“It seems as if the Milky Way and other galaxies are not closed systems,” says Kaaret. “They actually interact, throwing material into the CGM and bringing material back as well.”

The next step is to combine the HaloSat data with data from other X-ray observatories to determine if the Milky Way is surrounded by an enlarged halo and to calculate its size. This in turn could solve the missing baryon puzzle.

“Those missing baryons are better off somewhere,” says Kaaret. “They are located in halos around individual galaxies such as our Milky Way or in filaments that extend between galaxies.”

Reference: October 19, 2020, Naturastronomie.

Co-authors on the study include Jesse Bluem, a PhD student in physics in Iowa; Hannah Gulick, PhD student in astronomy at the University of California, Berkeley who graduated from Iowa last May; Daniel LaRocca, who received his PhD in Iowa last July and is now a postdoctoral fellow at Pennsylvania State University; Rebecca Ringuette, a Kaaret postdoctoral fellow who joined NASA’s Goddard Space Flight Center this month; and Anna Zayczyk, a former Kaaret postdoctoral fellow and research fellow at NASA Goddard and the University of Maryland, Baltimore County.

NASA funded the research.