Milky Way's hot gas halo could solve "missing baryon" mystery

Astronomers have found evidence that the Milky Way is embedded in an enormous halo of hot gas extending for hundreds of thousands of light years (Image: NASA/CXC/M.Weiss)

An international team of astronomers has combined data from NASA's Chandra X-ray observatory, ESA's XMM-Newton space observatory and Japan's Suzaku satellite to suggest that our galaxy may be surrounded by a halo of hot gas extending in all directions for hundreds of thousands of light-years. The finding also offers clues as to why more than half of the ordinary matter in early galaxies has seemingly disappeared without leaving a trace.

Protons and neutrons are classified as "baryons," a type of subatomic particle that interacts strongly to form the nuclei of atoms. Taken together, baryons make up nearly all of the ordinary matter in our universe.

But if you were to tally the number of atoms in the universe, you'd find that something doesn't quite add up. Astronomers have observed that entire galaxies seem to lose over half of their atoms compared to when they first formed. All of this matter couldn't have simply disintegrated, so where has it gone? This decade-old question is known as the problem of the "missing baryons."

Now, a team of astronomers led by Dr. Anjali Gupta may have just found the answer, at least for our galaxy. The baryons, says Gupta, haven't disappeared from the Milky Way. Rather, a mass of up to sixty billion suns – well in excess of the matter contained in the entire galactic disk – is spread out over a halo of hot gas stretching all around us for hundreds of thousands of light-years, reaching temperatures in the millions of degrees, and with a density so low that, even if it were present in other galaxies, we would probably have no way of detecting it.

We already knew that the Milky Way and other galaxies are embedded in warm gas with temperatures reaching one million °C (1.8 million °F). More recently, scientists have discovered the presence of even hotter gases in their midst; but now, the scientists determined that these recently-discovered "hotspots" are much more massive than the warm gas we already knew about – with a mass so large, in fact, that could very well account for all of the missing baryons.

"With reasonable assumptions, our observations imply a huge reservoir of hot gas around the Milky Way," said co-author Smita Mathur of Ohio State University in Columbus. "It may extend for a few hundred thousand light-years around the Milky Way or it may extend farther into the surrounding local group of galaxies. Either way, its mass appears to be very large."

While there are still considerable margins of errors to be dealt with, this development is helping scientists understand more about how galaxies came to form, as well as offering a solution to a long-standing problem in astrophysics.

A paper detailing the study was published on The Astrophysical Journal.

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