Dust-enshrouded AGN : Implications for cosmological backgrounds
The goal of this thesis is to develop an obscured AGN model for the X-ray background (XRB), and then to test the model via its predictions for the intensity of the cosmic background in the far infra-red, and by searching for obscured nuclei in individual X-ray sources in the near infra-red. A model invoking populations of obscured QSOs with a flat distribution of absorbing columns is found to give a good fit to the X-ray source counts and to the XRB spectrum. The model also accommodates the recent detection of a population of narrow-line X-ray galaxies, identifying them as low-luminosity obscured AGN. We investigate the selection effects present in optical QSO samples, and find that absorption in both the X-ray and optical wavebands combine to produce the tight observed X-ray/optical correlation, while still allowing a wide range of intrinsic column densities. X-ray sources with no obvious optical counterparts are predicted by this model, with a frequency in agreement with the numbers of these 'blank field' sources found in deep ROSAT surveys. We predict that in forthcoming AXAF surveys, ~ 50 per cent of the sources detected will be optically blank and lie at z (^>))(_~) 1. Obscured QSOs are estimated to outnumber unobscured QSOs by a factor of at least four to one. At longer wavelengths, we make a conservative prediction that obscured QSOs can account for approximately ten per cent of the sub-mm number counts and the far infra-red background, which is a modest, but non-negligible fraction. We then make observations to determine the extent of dust reddening in X-ray galaxies and QSOs, in the light of predictions of our obscured QSO model. We present K-band imaging of ROSAT and ASCA X-ray luminous galaxies and blank field sources. A trend is observed between the hardness of the X-ray spectra and the occurrence of red, unresolved counterparts, consistent with an obscured QSO origin for the X-ray emission. Near infra-red spectroscopy of X-ray luminous galaxies is then presented, searching for broad Pa(_a) emission lines. A broad line is possibly detected in one of six galaxies observed, which would indicate AGN activity. From far infra-red ISO photometry of X-ray galaxies and QSOs, we detect emission at ~ lOOμm from six narrow- line galaxies at low redshift, and from a Type 2 QSO at higher redshift. For at least two of the low redshift galaxies, there is clear proof of AGN activity on the grounds of X-ray variability and near infra-red spectroscopy. This, together with the 100μm detections, forms conclusive proof of the presence of dust-enshrouded AGN in at least some X-ray galaxies, supporting the obscured QSO explanation for the cosmic X-ray background.