The Sources of the CDF - N : broadband spectral analyses , absorption measurements and the α OX − L UV relation

We present broadband properties of sources in the Chandra Deep Field-North (CDF-N) with known spectroscopic redshifts. The high luminosity and high redshift bins are dominated by typical quasars. The intermediate redshift (z ∼ 0.7− 1.5) and luminosity ranges (L0.5−8.0keV ∼ 10 42.5−43.5 erg s) show a mix of different source types, with the apparently absorbed objects in the majority. At the faint flux limit of the CDF-N, a substantial fraction of the detections can be identified as normal/star-forming galaxies at low redshift. The AGNs in the sample can be subdivided into four classes based upon their rest frame spectral energy distribution: luminous, apparently unabsorbed QSOs; objects with reddened optical spectra and no signs of X-ray absorption; apparently X-ray absorbed AGNs with no signs of reddening in their optical spectra; and optically reddened sources with X-ray spectra indicative of large amounts of obscuration. We argue that the AGNs of higher luminosity tend to have a lower X-ray absorbing column density, but the ratio of X-ray absorbed to unabsorbed AGNs remains constant with redshift. We find that the relations between UV and X-ray luminosities derived by Strateva et al. (2005) and Steffen et al. (2006) only hold for bright sources, and break down when faint objects in the sample are included. This is only partly owing to the fact that the majority of the faint sources are absorbed; several faint sources must also have intrinsically lower X-ray luminosity. A fit to the NIR-optical-UV broadband SEDs of AGNs allows us to constrain the absorption parameters independently from the X-ray analyses. We show that the NH values derived by the X-ray and optical methods are not correlated. This may be because the X-ray absorption and optical attenuation do not originate at the same location, and/or the dust properties responsible for the optical attenuation in AGNs are very different from the locally known dust properties.