When Supermassive Black Holes Were Growing: Clues from Deep X–ray Surveys

Abstract

Merging the Chandra and XMM-Newton deep surveys with the previously identified ROSAT sample, almost 1000 AGN-1 covering five orders of magnitude in 0.5-2 keV flux limit and six orders of magnitude in survey solid angle were identified with ~95% completeness. AGN-1 are by far the largest contributors to the soft X-ray samples. Their evolution is responsible for the break in the 0.5-2 keV source counts. Their soft X-ray luminosity function clearly changes shape as a function of redshift, confirming earlier reports of luminosity--dependent density evolution. The space density with redshift changes significantly for different luminosity classes, showing a strong positive evolution up to a certain redshift and then a flattening. The redshift, at which the evolution peaks, changes considerably with X-ray luminosity. The amount of density evolution also depends strongly on X-ray luminosity, more than a factor of 100 at high luminosities, but less than a factor of 10 for low luminosities. For the first time, a significant decline of the space density of X-ray selected AGN towards high redshift has been detected. No significant evolution of the X-ray to optical spectral index is found for AGN-1. The constraints from the AGN luminosity function and evolution in comparison with the mass function of black holes in local galaxies indicates, that the average supermassive black hole has built up its mass through efficient accretion and is likely rapidly spinning.