Hard and soft X-ray selected active galactic nuclei: two distinct populations?

We report the results of a thorough analysis of the evolutionary properties of active galactic nuclei, exploiting data from both hard and soft X-ray surveys [counts, luminosity and redshift distributions, average X-ray spectral properties and P(D) distributions]. Strong indications are found that soft (< 3 keV) and hard X-ray surveys emphasize two distinct classes of AGN that have different X-ray spectra, luminosity functions and evolution. ‘Soft X-ray’ AGN have higher luminosities, steep source-count slopes and high evolution rates, similar to those of optically selected QSOs; their contribution to the 2–10 keV X-ray background (XRB) is probably < 20 per cent. ‘Hard X-ray’ AGN have relatively low luminosities and strong photoelectric absorption. The average spectra of AGN detected in deep soft X-ray surveys imply that hard X-ray AGN either undergo a very weak luminosity/density evolution or that such evolution is accompanied by a substantial increase with redshift of the absorbing column densities; in the latter case they can easily contribute the bulk of the XRB above 3 keV. The existence of a new AGN population, essentially invisible below 10 keV, and accounting for a large fraction of the hard XRB, is also discussed: constraints from the HEAO-1 A2 (and to some extent from A4) fluctuation data are pointed out. AGN can hardly account for the soft XRB intensity, especially if it is significantly higher than implied by a simple extrapolation of higher energy measurements; contributions from other classes of source (e.g. clusters of galaxies and active star-forming galaxies) are required.