The Large Magellanic Cloud Heating the Milky Way’s Halo

In 2024, the X-ray observatory eROSITA detected that the southern half of the Milky Way's gaseous halo is up to 12 percent warmer than the northern half. The halo itself is vast, a thin sphere of gas extending far beyond the visible disk of stars, sitting at around 2 million kelvin, far hotter than the Sun's surface. No obvious mechanism initially explained the asymmetry. A team led by the University of Groningen traced the heating to a surprisingly indirect process. The Milky Way's cold disk is currently moving toward the Magellanic Clouds at about 40 kilometers per second due to the gravitational pull of the Large Magellanic Cloud. As the galaxy shifts, it effectively presses into the gas on its southern side, the researchers compare the effect to a piston in an internal combustion engine. Gas gets compressed, and temperature rises. In the words of lead researcher Filippo Fraternali, "It took a little longer before we realised 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."
 
The hydrodynamic simulations model three main components, a rotating disk of cold gas, a surrounding warmer region, and an extensive dark matter halo, and compute their motions under the gravitational pull of the Magellanic Clouds over roughly one billion years. The simulations predict compression heats the southern halo gas by about 13 to 20 percent, while observations report a difference of up to 12 percent between the two halves. The agreement is not perfect, but close enough to be convincing. The model suggests the temperature asymmetry developed within the last 100 million years, a relatively short period in cosmic terms, tied to the current passage of the Magellanic Clouds near the Milky Way.
 
The heating effect from the Milky Way's motion may also explain the uneven distribution of high-velocity clouds in the galaxy, shedding further light on how galaxies evolve over time. It's a striking case of gravitational dynamics producing thermodynamic effects across galactic scales, the LMC isn't heating the halo directly through radiation or shockwaves, but by dragging the entire Milky Way disk into its own surrounding gas.