Left: distribution of rest frame, absorption-corrected 2–10 keV luminosity vs. redshift for the XMM–Newton (blue squares) and Chandra (red crosses) detected sources that are also Herschel detected (X-FIR sample). The dotted (dashed) line marks the sensitivity limit of the XMM–Newton (Chandra) surveys. The redshift bins adopted in the text are marked by vertical gray dashed lines. Right: distribution of intrinsic column density vs. redshift for the sample. Symbols as in left panel. Arrows show upper limits for unobscured sources. The average 1σ error bars on L_X and N_H are shown at the top left of both panels.