Southern eROSITA bubble as a forward shock and the low-metallicity CGM. South-east side story

Abstract

Unlike the complicated X-ray and radio structure observed in the North Polar Spur area, the South-Eastern part of the eROSITA bubbles can be reasonably well described as a propagating forward shock, plausibly created by the transient energy release at the Galactic Center. In this model, the physical radius of the bubble is $R_{\mathrm{b}}\sim 7-8\mathrm{kpc}$ and the age of the outburst is $t_{\mathrm{age}}\sim 5-8\mathrm{Myr}$. The latter quantity is plausibly a lower limit on the true age. The visible segment of the shock front (located at a distance of $\sim 10-12\mathrm{kpc}$ above the Galactic Disk and at a similar distance from the Sun) is currently expanding with the velocity $\sim 700\mathrm{km}{\mathrm{s}}^{-1}$ through the gas with density $n_e\sim 3\times 10^{-4}\mathrm{c}{\mathrm{m}}^{-3}$, and the abundance of heavy elements in this gas is low $Z\sim 0.1-0.2\times Z_{\odot }$ (depending on the adopted reference Solar abundances). Unlike constraints derived from the line-of-sight-integrated quantities, these are effectively in situ measurements of the circumgalactic medium (CGM) properties. Given the simplifying assumptions used in deriving the density and abundance, we assign a factor of 2 systematic uncertainty to the final estimates. An eventual decisive test for the shock properties can be provided by the velocity measurements of the X-ray-emitting gas with soft X-ray bolometers. The extended forward shock propagating through low-metallicity gas is a favorable site to accelerate very high-energy cosmic rays, which might contribute to the recently discovered proton-rich Galactic cosmic ray component at PeV energies impinging on the Earth's atmosphere.

A&A, accepted version