Solving the Milky Way’s Halo Temperature Mystery

A team of scientists led by the University of Groningen found that the hot gas halo of the Milky Way is warmer in the “south” than in the “north” due to an internal combustion engine-like mechanism that compresses the gas in a piston-like manner. Their findings were published in Monthly Notices of the Royal Astronomical Society.

Computer simulations show that the Large Magellanic Cloud, a satellite galaxy located on the southern side of the Milky Way,  pulls on the Milky Way, causing gas in the southern portion of the halo to compress and heat up.

This helps explain why the southern half of the halo is up to 12% warmer than the northern region above the Milky Way’s disk, a difference measured in 2024 by the eROSITA X-ray observatory aboard a German-Russian space telescope.

Many galaxies, including the Milky Way, are surrounded by a vast sphere of diffuse, warm material, or halos of hot gas.

Researchers estimate that the Milky Way’s gaseous halo has a mass of around 100 billion solar masses, suggesting that more matter resides in the halo than in the galactic disk. With a temperature of about 2 million kelvin (several hundred times hotter than the Sun’s surface), the halo acts as the “building material” for the much more compact and cooler disk of gas and stars at its center, including the Sun.

In computer simulations, the Milky Way is typically modeled with three main components: a rotating disk containing relatively cold gas, a surrounding region of much warmer gas, and an extensive halo composed of dark matter.

The so-called hydrodynamic simulation computes the motions of these three components driven by the gravitational pull of the Magellanic Clouds, which pass near the Milky Way, over a period of about one billion years.

The results indicate that the Milky Way’s cold disk is currently moving toward the satellite galaxies at about 40 kilometers per second due to the gravitational pull of the Large Magellanic Cloud. During this process, the Milky Way compresses the gas at the bottom, and the material heats up by 13 to 20 %, according to the calculations.

The simulation also indicates that the temperature difference between the northern and southern regions of the halo has developed over the past 100 million years.

We saw fairly quickly in the simulations that there was a warming effect. It took a little longer before we realized what is going on here – namely the compression of gas like in the piston of an internal combustion engine, which then heats up to make the southern side of our Milky Way's halo warmer.

Filippo Fraternali, Professor, Gas Dynamics and the Evolution of Galaxies, University of Groningen

The simulations may also help explain additional asymmetries around the Milky Way, according to the researchers. For example, significantly more so-called high-velocity clouds are observed on the northern side of the galaxy than on the southern side. These regions of gas, typically about 100 times cooler than the surrounding material, move through the galaxy at unusually high speeds.

The lower pressure of the surrounding gas may make it easier for these clouds to form and survive there.

Filippo Fraternali, Professor, Gas Dynamics and the Evolution of Galaxies, University of Groningen

Initially, the researchers were not searching for what they ultimately found. The simulations had already been published in 2019 as part of an effort to explain gas moving around the Magellanic Clouds, among other aspects. At that time, the temperature difference had not yet been identified.

Typically, computer models are designed to explain certain observations. It is remarkable these simulations already contained the temperature asymmetry before it was found. It makes this result extra robust.

Filippo Fraternali, Professor, Gas Dynamics and the Evolution of Galaxies, University of Groningen

“Our explanation for the temperature asymmetry measured by eROSITA is based on simple and well-understood physical processes, as we also find them in, for example, combustion engines. That gives the result extra elegance,” adds Co-author Else Starkenburg, associate professor at the University of Groningen.