Astronomers Detect Large-Scale X-Ray Bubbles in Milky Way’s Halo

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Astronomers using the eROSITA X-ray telescope onboard the Spektrum-Roentgen-Gamma (SRG) observatory have detected X-ray-emitting bubbles that extend approximately 14,000 parsecs (45,662 light-years) above and below the central region of our Milky Way Galaxy.

This image from the eROSITA X-ray telescope shows the energetic Universe. Image credit: Jeremy Sanders, Hermann Brunner & the eSASS team / Max Planck Institute for Extraterrestrial Physics / Eugene Churazov & Marat Gilfanov, IKI.

This image from the eROSITA X-ray telescope shows the energetic Universe. Image credit: Jeremy Sanders, Hermann Brunner & the eSASS team / Max Planck Institute for Extraterrestrial Physics / Eugene Churazov & Marat Gilfanov, IKI.

Launched in July 2019, eROSITA is a large-collecting-area and wide-field-of-view X-ray telescope.

Over the course of six months, from December 2019 to June 2020, it has completed a survey of the whole sky.

A preliminary analysis of the all-sky survey map indicated that more than one million X-ray sources were detected by eROSITA. This is comparable to, and may exceed, the total number of X-ray sources known before this telescope launched.

Various large-scale structures are visible in the eROSITA map. The most obvious is a quasi-circular feature, which is part of the North Polar Spur and Loop I discovered in the early days of X-ray and radio astronomy, respectively.

Although less evident at first glance, close inspection of the image in the hemisphere below the plane of the Milky Way revealed an astonishing new feature — a huge circular structure of similar shape and scale to the structure seen in the north.

Together, they seem to form a pair of ‘bubbles’ that emerge from the Galactic center.

They are traceable at various levels of intensity throughout most of the sky, and should represent a very large object, akin to the Fermi bubbles, detected in 2010 with the Fermi-LAT (Fermi large-area telescope) gamma-ray instrument.

“Thanks to its sensitivity, spectral and angular resolution, eROSITA has been able to map the entire X-ray sky to unprecedented depth, revealing the southern bubble unambiguously,” said Dr. Michael Freyberg, an astronomer at the Max Planck Institute for Extraterrestrial Physics.

Schematic view of the eROSITA (yellow) and Fermi bubbles (purple). The Galactic disk is indicated with its spiral arms and the location of the Solar System is marked. The eROSITA bubbles are considerably larger than the Fermi bubbles, indicating that these structures are comparable in size to the whole Galaxy. Image credit: Max Planck Institute for Extraterrestrial Physics.

Schematic view of the eROSITA (yellow) and Fermi bubbles (purple). The Galactic disk is indicated with its spiral arms and the location of the Solar System is marked. The eROSITA bubbles are considerably larger than the Fermi bubbles, indicating that these structures are comparable in size to the whole Galaxy. Image credit: Max Planck Institute for Extraterrestrial Physics.

The large-scale X-ray emission observed by eROSITA in its medium energy band (0.6-1.0 keV) show that the bubbles are 14,000 parsecs across.

“The sharp boundaries of these bubbles most likely trace shocks caused by the massive injection of energy from the inner part of our Galaxy into its halo,” said Dr. Peter Predehl, also from the Max Planck Institute for Extraterrestrial Physics.

“Such an explanation has been previously suggested for the Fermi bubbles, and now with eROSITA their full extent and morphology has become evident.”

The bubbles detected by eROSITA trace disturbances in a hot gas envelope around the Milky Way, caused either by a burst of star formation or by an outburst from the supermassive black hole at the Galactic center.

While dormant now, the black hole could well have been active in the past, linking it to active galactic nuclei with rapidly growing black holes seen in distant galaxies.

In either case, the energy needed to power the formation of these huge bubbles must have been enormous at 1056 ergs, equivalent to the energy release of 100,000 supernovae, and similar to estimates of active galactic nucleus outbursts.

“The scars left by such outbursts take a very long time to heal in these haloes,” said Dr. Andrea Merloni, also from the Max Planck Institute for Extraterrestrial Physics.

“Scientists have been looking for the gigantic fingerprints of such past violent activity around many galaxies in the past.”

The study was published in the journal Nature.

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P. Predehl et al. 2020. Detection of large-scale X-ray bubbles in the Milky Way halo. Nature 588, 227-231; doi: 10.1038/s41586-020-2979-0

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