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.
Original abstract
as written by the authors arXiv:2604.24494
The origin and structure of the hot ($$T\gtrsim10^6$$K) gaseous halo around Milky Way (MW)-mass galaxies provide a critical test for galaxy formation models. We perform a comprehensive comparison for a sample of MW analogues from the TNG50 cosmological simulation by generating synthetic soft X-ray emission and O VII/O VIII absorption lines, viewed from both internal (Solar) and external perspectives. The simulated halos successfully reproduce the observed global soft X-ray luminosity, inner-halo X-ray surface brightness, emission measure, and O VII absorption strength. However, two interconnected discrepancies are identified. First, the azimuthally averaged X-ray surface brightness profile from external viewpoints declines too steeply with radius compared to the extended emission detected in eROSITA stacking of SDSS galaxies, falling below the observations by up to $$\sim 1$$ dex at $$R \gtrsim 100$$ kpc. Second, the halos systematically underproduce O VIII absorption, with a median equivalent width $$\sim 65\%$$ lower than that observed in the Galactic halo, pointing to a deficit of hotter-phase gas at $$T\sim(1.6-3.2)\times10^6$$ K. These findings indicate that the simulated hot halos are too spatially compact and lack a hotter gas phase, suggesting that the TNG50 feedback model, while generating hot gas, deposits energy too centrally and too vigorously to sustain a gently extended, multi-phase corona.
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.