eROSITA finds hot gas all around the Milky Way – closer than expected

December 14, 2023

A new all-sky map by the eROSITA telescope reveals X-rays emitted by million-degree hot plasma in and around the Milky Way. Analysing this data, the team at the Max Planck Institute for Extraterrestrial Physics found that the very hot, ionized gas shows a disk-like distribution similar to the stellar disk, possibly embedded in a much larger spherical halo. This discovery sheds light on the shape and size of a large portion of the Milky Way circumgalactic medium, providing a large reservoir of gas to fuel future star formation.

Stars are formed from gas in an endless process that feeds on both pristine cosmic material and recycled gas from previous generations of stars. Spiral galaxies like the Milky Way, however, have just too many stars and not enough visible gas to sustain their current level of star formation for very long. Therefore, the astronomers assume that there is a large reservoir of gas distributed all around the Galaxy, with a size possibly as large as ten times the diameter of the stellar disk.

Details on the shape, size and amount of matter in this so-called circumgalactic medium, however, are still being debated – they are poorly constrained observationally. What is clear: so far, it has escaped detection with optical, IR, or radio telescopes; therefore most of the gas in the circumgalactic medium has to be very hot (about a million degrees) and at very low density (less than 1000 particles per cubic metres). Due to these high temperatures, the gas should emit X-rays; but the emission has to be faint because of its low density – fainter than what could be observed so far. A distinct feature confirming the existence of such a thin, hot gas are emission lines of highly ionized oxygen atoms (for example the O VIII atomic line), observable in X-rays.

The eROSITA telescope, built entirely at the Max Planck Institute for Extraterrestrial Physics (MPE), has now made the first ever scan of the sky looking for soft X-ray emission. The resulting map of the entire Western Galactic hemisphere has been generated and validated at MPE. “The map not only reveals the ubiquitous presence of hot gas all around us, but also provides enough details to explore its geometry to an unprecedented level of detail,” says Xueying Zheng from MPE, whose work provides the basis for the analysis of the hot plasma distribution.

“We see the O VIII emission from all directions in the soft X-ray sky,” points out Nicola Locatelli, who led the eROSITA data analysis at MPE. “This confirms the diffuse nature of the hot gas, and we can now even probe its distribution around us.”

In particular, the team at MPE found that the gas geometry can be described by two components: a very large, more or less spherical halo and a nearby component resembling the stellar disk. The hot halo is about four times larger (up to ~100 kiloparsec) than the optical size of the Milky Way and the nearby component extends to the size of the thick disk (about 7 kpc with a height of 1 kpc). Due to its enormous volume, the hot halo includes most of the mass – but the closer disk-like component is producing most of the photons observed by eROSITA, being about 10 times brighter than the halo.

In principle, the high temperature of the gas can be explained by the energy injected into the circumgalactic medium by supernovae explosions from the star-forming disk of the Milky Way. In an alternative scenario, pristine accretion from regions even farther out, the so-called intergalactic medium, provides the raw material, which gets heated during the in-fall and thus forms the spherical halo.

An important aspect of this new study is the distance to which most of the emission is observed, measured as a few kiloparsecs from the Sun. This relative proximity favours the supernovae explosion scenario for the origin of the hot gas. This also corroborates galaxy evolution theories, where the gas is being recycled to and from the stellar disk itself. Soon, state-of-the-art X-ray spectrographs will be able to infer the radial velocity of the same gas, complementing this mapping of the overall geometry of the hot gas and further probing the models for galaxy formation and evolution. MPE will remain a crucial player to tackle this challenge thanks to upcoming the Athena instrument.

This movie shows the distribution of the hot gas in the Milky Way (see figure above) from different angles.

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