Scientists explain why Milky Way's southern gas is glowing 12 percent hotter than the northern part
The Milky Way is surrounded by a halo of hot gas, which is warmer to the south than the north. This happens because an internal combustion engine-like effect compresses the southern part of the gas like a piston, a new study has discovered this cosmic phenomenon through computer simulations. The simulations, led by a team at the University of Groningen, reveal that the Large Magellanic Cloud, a satellite galaxy below or on the south side of our own, attracts the Milky Way, causing gas in the southern part to squeeze and heat up. This explains why the southern half of the halo is up to 12 percent hotter than the northern part, a discrepancy that was detected in 2024 by the X-ray observatory eROSITA on a German Russian space telescope. The findings of the study have been published in the Monthly Notices of the Royal Astronomical Society.
A vast sphere of thin and warm matter, also known as a halo of hot gas, envelopes many galaxies, including our own. The gaseous halo in the Milky Way has a mass that is equal to 100 billion solar masses. The matter in it outweighs that of the galactic disc. The halo’s temperature soars to about 2 million degrees kelvin, which is a few hundred times hotter than the surface of the Sun. The matter in the halo is the building material of a much more compact and cooler disc of gas and stars with the Sun at the center. In the simulations, the Milky Way has three 'components': the rotating disc with relatively cold gas, the much warmer gas around it and a large halo consisting of dark matter. The simulations then allowed the researchers to measure how the three components of our galaxy moved under the gravitational pull of the Magellanic Clouds. For over one billion years, these clouds have been passing by the Milky Way.
The analysis shows that the gravity of the Large Magellanic Cloud is pulling the cold disc of the Milky Way toward the satellite galaxies (the Large Magellanic Cloud and the Small Magellanic Cloud) at about 40 kilometers per second. This compresses the Milky Way’s gas at the bottom and heats it up to 13 to 20 per cent. The simulation also shows that the temperature gap between the southern and northern parts of the halo has increased in the last 100 million years. "We saw fairly quickly in the simulations that there was a warming effect," said co-author Filippo Fraternali, professor of gas dynamics and the evolution of galaxies at the University of Groningen, in a statement.
The simulations detected more asymmetries between the south and the north of the Milky Way. For instance, many more so-called high-velocity clouds are observed on the north side of the galaxy than on the south side. Gas in these regions is usually about 100 times cooler than the surrounding material and drifts around the galaxy at highly irregular speeds. "The lower pressure of the surrounding gas may make it easier for these clouds to form and survive there," Fraternali added. The researchers say that they initially looked for an explanation for other astrophysical phenomena, including gas movement around the Magellanic Clouds. It was in 2019 when they published their simulations. At that time, they did not know the temperature difference. "Typically, computer models are designed to explain certain observations. It is remarkable that these simulations already contained the temperature asymmetry before it was found. It makes this result extra robust," Fraternali noted.
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