Average soft X-ray luminosity (L_{0.3−1 keV}) of ULX populations as a function of the host galaxy’s metallicity (solar value indicated with a vertical magenta line), assuming CO masses of 20 M_⊙ (top, intense lines) and 1.4 M_⊙ (bottom, pale lines) and spectrum described by the P07 model with e_w = 0.3. Black lines correspond to the intrinsic (unabsorbed) X-ray luminosity, while blue and orange lines account for the absorption in the galaxy’s ISM, with n_H = 10²¹ and 10²² cm², respectively. Dashed, dotted, and solid lines indicate the blackbody component, disk component, and their sum, respectively. (For 20 M_⊙, the black dashed and dotted lines coincide, indicating equal component contributions.) The gray band denotes the L_{X < 2 keV} 68% confidence interval constrained from the cosmic 21-cm signal for galaxies at z ∼ 8 (Abdurashidova et al. 2022).