Correlation between mass distribution of the Milky Way (Hunter et al. 2024) and total surface brightness, in the 0.1–10 keV band, of the 0.7 keV component within the sky area considered in this work (one data point for each sky tile). The surface brightness was computed extrapolating the best fit 0.7 keV component to the 0.1–10 keV band, and includes all emission, i.e. point sources and diffuse emission. We rebinned the points into N bins along the x-axis, evenly spaced in log_10, of the stellar mass surface density. In each bin, the dark blue dot shows the average (log₁₀) surface brightness, while the x and y error bars show the bin’s half-width and the standard dispersion of the (log₁₀) surface brightness in the bin, respectively. Each bin is also divided into other M sub-bins along the y-axis, with each sub-bin coloured according to the number of encompassed data points, as labelled in the colour bar on the right. The red line shows the relation between surface brightness and stellar mass surface density with L_x/M = 10.5 × 10²⁷ erg ${\text{s}}^{-1}{\text{M}}_{\odot }^{-1}$, as derived from the fit (Table 1).