A disk-dominated and clumpy circumgalactic medium of the Milky Way seen in X-ray emission

Abstract

The Milky Way galaxy is surrounded by a circumgalactic medium1 that may play a key role in galaxy evolution as the source of gas for star formation and a repository of metals and energy produced by star formation and nuclear activity2. The circumgalactic medium may also be a repository for baryons seen in the early universe, but undetected locally3. The circumgalactic medium has an ionized component at temperatures near 2 × 106 K studied primarily in the soft-X-ray band4,5. Here we report a survey of the southern Galactic sky with a soft-X-ray spectrometer optimized to study diffuse soft-X-ray emission6. The X-ray emission is best fitted with a disk-like model based on the radial profile of the surface density of molecular hydrogen, a tracer of star formation, suggesting that the X-ray emission is predominantly from hot plasma produced via stellar feedback. Strong variations in the X-ray emission on angular scales of ~10° indicate that the circumgalactic medium is clumpy. Addition of an extended, and possibly massive, halo component is needed to match the halo density inferred from other observations7–9.