Seyfert Galaxies and the Hard X-Ray Background: Artificial Chandra Observations of Active Galaxies

Deep X-ray surveys have resolved much of the X-ray background radiation below 2 keV into discrete sources, but the background above 8 keV remains largely unresolved. The obscured (type 2) active galactic nuclei (AGNs) that are expected to dominate the hard X-ray background have not yet been detected in sufficient numbers to account for the observed background flux. However, deep X-ray surveys have revealed large numbers of faint quiescent and starburst galaxies at moderate redshifts. In hopes of recovering the missing AGN population, it has been suggested that the defining optical spectral features of low-luminosity Seyfert nuclei at large distances may be overwhelmed by their host galaxies, causing them to appear optically quiescent in deep surveys. We test this possibility by artificially redshifting a sample of 23 nearby, well-studied active galaxies to z = 0.3, testing them for X-ray AGN signatures, and comparing them to the objects detected in deep X-ray surveys. We find that these redshifted galaxies have properties consistent with the deep-field normal and optically bright, X-ray-faint galaxy (OBXF) populations, supporting the hypothesis that the numbers of AGNs in deep X-ray surveys are being underestimated and suggesting that OBXFs should not be ruled out as candidate AGN hosts that could contribute to the hard X-ray background source population.