Our Milky Way galaxy is surrounded by a clumpy halo of hot gases

The Milky Way galaxy is in the recycling business.


Using the data from HaloSat, one of a class of minisatellites designed and built at Iowa, astronomers at the University of Iowa have determined that a clumpy halo of hot gases surrounds the milky way galaxy. This heated halo, also known as the circumgalactic medium (CGM), is continually supplied with material ejected by birthing or dying stars.

According to scientists, the CGM has a disk-like geometry, based on the intensity of X-ray emissions.

Philip Kaaret, a professor in the Iowa Department of Physics and Astronomy and corresponding author on the study, published online in the journal Nature Astronomy, said, “Where the Milky Way is forming stars more vigorously 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.”

Each galaxy has a CGM. Understanding the CGM could reveal more about galaxy formation and its evolution. It could also offer details insights on 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.

HaloSat is observing the Milky Way’s CGM for evidence the leftover baryonic matter may reside there.

Scientists wanted to determine if the CGM is a huge, extended halo that is many times the size of our galaxy. But if the CGM 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.

Kaaret says“What we’ve done is 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 is this extended halo.”

“I got surprised by the CGM’s clumpiness, expecting its geometry to be more uniform. The denser areas are regions where stars are forming, and where the material is being traded between the Milky Way and the CGM.”

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

In the future, scientists will 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.

Study co-authors include 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; 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.

Journal Reference:
  1. Kaaret, P., Koutroumpa, D., Kuntz, K.D. et al. A disk-dominated and clumpy circumgalactic medium of the Milky Way seen in X-ray emission. Nat Astron (2020). DOI: 10.1038/s41550-020-01215-w