The Sources of the CDF-N : broadband spectral analyses, absorption measurements and the alpha_ox - L_UV relation

Abstract

We present broadband properties of sources in the Chandra Deep Field-North with spectroscopic redshifts. The high luminosity and redshift bins are dominated by typical quasars. The intermediate redshift (z = 0.7-1.5) and luminosity ranges (L(0.5-8.0keV) = 10^(42.5-43.5) erg/s) show a mix of different source types, with the absorbed objects in the majority. At the faint flux limit of the CDF-N, a substantial fraction of the sources are identified as star-forming galaxies. The AGN in the sample can be divided into four classes based on their rest frame SEDs: luminous, unabsorbed QSOs; objects with reddened optical spectra and X-ray absorption; X-ray absorbed AGN with no signs of reddening in the optical spectra; and optically reddened sources with X-ray spectra indicative of obscuration. We argue that the AGN of higher luminosity tend to have a lower X-ray absorbing column density, but the ratio of X-ray absorbed to unabsorbed AGN 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 the AGN allows us to constrain the absorption parameters independently from the X-ray analyses. We show that the N_H 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. (abridged)