The Milky Way's Hot Halo in Simulation express
Around our Galaxy, like an invisible crown, stretches a tenuous cloud of gas heated to millions of degrees. It is called the galactic halo. Seeing this glow directly is difficult: it emits soft X-rays that are absorbed by Earth's atmosphere. But space telescopes like eROSITA can capture this faint light and map the hot breath of the Milky Way.
In a new study, scientists took a sample of virtual galaxies similar to ours from the TNG50 cosmological simulation. This computer model simulates the universe's evolution from the Big Bang to the present day. For each simulated galaxy, they constructed artificial X-ray images and spectra, as if we were looking at them from inside (from the Sun's position) and from outside. It turned out that in terms of overall brightness and absorption of O VII oxygen ions, the model matches reality quite well.
But when the researchers compared the surface brightness profile at the outskirts, discrepancies emerged. In the simulation, the glow falls much more steeply with distance from the center than eROSITA observations of distant galaxies show. At distances beyond one hundred kiloparsecs (about three times the Milky Way's diameter), the model produces ten times less light. Additionally, the virtual halos lack O VIII absorption — a sign of gas with temperatures between one and a half and three million degrees. Real measurements in our Galaxy show lines twice as strong.
It seems that in the TNG50 simulation, energy from supernovae and active galactic nuclei is injected too close to the center and too powerfully. The gas heats up but doesn't have time to spread far, forming a compact cloud instead of an extended corona. This is an important lesson for future models: to get the correct hot halo, feedback must work more delicately, distributing heat over larger distances.
The temperature of the galactic halo reaches several million degrees, but the gas density is so low that if you were inside it, you would freeze faster than you would burn: there are too few particles to transfer heat.
In Stanislaw Lem's novel "Solaris," the planet's ocean creates mysterious structures reminiscent of extended gas coronas — a fitting analogy to the hot halo, which also follows its own, not fully understood laws.