The Milky Way is enveloped in a vast halo of hot diffuse gas — a circumgalactic medium with temperatures reaching millions of degrees, extending far beyond the visible disc. This invisible reservoir of matter and energy governs how the galaxy breathes: drawing in fresh gas from outside, venting outflows from within, and preserving a record of billions of years of evolution.
Recently, the eROSITA X-ray telescope captured something unexpected: the southern hemisphere of this hot halo is on average roughly 12% warmer than the northern one. A modest figure in absolute terms — but for a structure of this scale, it is an anomaly that demands an explanation.
A new study points to the culprit: the Magellanic Clouds. These two satellite galaxies, on a long inward journey toward the Milky Way, exert enough gravitational pull to shift the galactic disc relative to the surrounding hot halo at velocities of up to 40 km/s. This relative motion compresses the circumgalactic gas in the southern hemisphere — and compressed gas heats up. Hydrodynamical simulations reproduce the observed asymmetry well, yielding a south-to-north temperature difference of 13–20%, consistent with eROSITA measurements.
Notably, this asymmetry is a recent development: the authors estimate it began forming only around 100 million years ago — a fleeting moment on galactic timescales. The Magellanic Clouds have not yet merged with the Milky Way, but they are already reshaping its thermal portrait, quietly heating half the sky.