On the patchy appearance of the circum-Galactic medium and the influence of foreground absorption

G. Ponti, J. S. Sanders, N. Locatelli, X. Zheng, Y. Zhang, M. Yeung, M. Freyberg, K. Dennerl, J. Comparat, A. Merloni, E. Di Teodoro, M. Sasaki, T. H. Reiprich

Submitted on 3 November 2022


Recent studies have demonstrated that the emission from the circumgalactic medium of the Milky Way displays a relatively high degree of patchiness on angular scales of 10. Taking advantage of the Spectrum Roentgen Gamma eROSITA Final Equatorial Depth Survey (eFEDS), we aim to constrain any variation in the soft X-ray surface brightness on scales going from sub-degree to a hundred square degrees. We observe modulations of about 60% on scales of several degrees and decreasing for higher energies. The observed patchiness is stable over a period of two years, therefore excluding that it is induced by Solar wind charge exchange. We also observe no correlation between such excess and the density of galaxies in the Local Universe, suggesting no strong contribution from the hot baryons in the filaments of the Cosmic web. Instead, the soft X-ray emission is anti-correlated with the column density of absorbing material. Indeed, we can reproduce the spectrum of the bright and dark regions by simply varying the column density of the matter absorbing the emission components located beyond the Local Hot Bubble, while no modulation of the intrinsic emission is required. At high Galactic latitudes, the eROSITA all sky map shows patchiness of the soft X-ray diffuse emission similar to the one observed in the eFEDS field, it is therefore likely that the same "absorption-modulation" is present over the entire sky. These results highlight the importance of an accurate treatment of the absorption effects, to determine the patchiness of the circumgalactic medium.


Comment: Accepted for publication in Astronomy and Astrophysics

Subjects: Astrophysics - High Energy Astrophysical Phenomena; Astrophysics - Cosmology and Nongalactic Astrophysics; Astrophysics - Astrophysics of Galaxies


Total X-ray emission observed by \erosita\ in the \efeds\ region, during e0. The contribution due to point sources has been removed through a wavelet filtering. With red, green and blue colors are shown the 0.3-0.45~keV; 0.45-0.7~keV and 0.7-2.3~keV bands, respectively. Variations of the surface brightness of the diffuse emission are clearly observed from a sub-degree to several degrees scale. The brightest patch appears slightly redder, consistent with the inference that the flux in the soft band is less absorbed. Equatorial coordinates are used.