Giant X-ray Bubbles in the Milky Way halo
Main takeaway
A team of scientists from German, Russia and Italy report the discovery of two giant X-ray bubbles that extend approximately 14 kpc above and below the Galactic center with the X-ray telescope eROSITA aboard the Spektr-RG mission. The bubbles with sharp boundaries are not a remnant of a local supernova but part of a vast Galaxy-scale structure which could be powered by star-formation or supermassive black hole activities in the past.
Details
- The giant X-ray bubble (eROSITA bubble) in the north of Galactic plane has been detected as part of the North Polar Spur and Loop I discovered in the early days of X-ray and radio astronomy.
- A new structure below the Galactic plane is found for this time with the similar shape and scale to the structure seen in the north owing to the higher sensitivity and harder energy response of eROSITA.
- The previous study of absorption in X-ray and radio bands places a lower limit of 300 pc on the distance to the structure, which rules out a nearby supernova remnant.
- eROSITA bubbles show remarkable morphological similarity with Fermi bubble, kpc scale bubbles above and below the Galactic center with hard, non-thermal spectrum at energies more than 1 GeV, which is discovered by Fermi-LAT in 2010. Their emission is probably due to inverse Compton scattering of cosmic-ray electrons on the cosmic microwave background and other radiation fields.
- The 0.6-1 keV map (Fig. 3) shows that each bubble is close to spherical with a radius of about 6-7 kpc, while the Fermi bubbles are roughly elliptical.
- One-dimensional surface-brightness profiles along the Galactic longitude (Fig. 2) are qualitatively consistent with the projection of a 2 kpc thick spherical shell with an outer diameter of 14 kpc.
- The total thermal energy of the eROSITA bubbles is almost 10 times larger than that of the Fermi bubbles, \(10^{56}\) erg and \(10^{55}\) erg respectively.
- AGN feedback (the typical luminosity of Syfert-like activity is \(10^{41}\) erg) & stellar feedback (the typical kinetic energy of type II supernova ejecta is \(10^{51}\) erg)
- The temperature increases from about 0.2 keV outside the bubbles to around 0.3 keV inside, which indicates the presence of a feedback mechanism in quiescent galaxies,
- The hot plasma outside the bubbles is shock-heated during the collapse of the dark-matter halo.
- The cooling time of the hot plasma in the halo is comparable to the Hubble time, so the process of growing a galaxy is assumed to be steady (apart from mergers) and slow.
Citation
Detection of large-scale X-ray bubbles in the Milky Way halo
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