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Register a new userThe X-ray Spectral Properties of SCUBA Galaxies
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D.M. Alexander
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Deep SCUBA surveys have uncovered a large population of massive submm galaxies (SMGs) at z>~1. Although it is generally believed that these galaxies host intense star-formation activity, there is growing evidence that a substantial fraction also harbor an AGN. We present here possibly the strongest evidence for this viewpoint to date: the combination of ultra-deep X-ray observations (the 2 Ms CDF-N) and deep Keck spectroscopic data. We find that the majority (~75%) of the spectroscopically identified SMGs host AGN activity; the other ~25% have X-ray properties consistent with star formation (X-ray derived SFRs of ~1300-2700 solars/yr). The AGNs have properties generally consistent with those of nearby luminous AGNs (L_X~1E43-3E44) and the majority (~80%) are heavily obscured. We construct composite rest-frame 2-20 keV spectra for three different obscuration classes. An ~1 keV equivalent width FeK-alpha emission line is seen in the composite X-ray spectrum of the most heavily obscured AGNs (N_H>5x10^{23}), suggesting Compton-thick or near Compton-thick absorption. Even taking into account the effects of absorption, we find that the average X-ray-FIR ratio of the AGN-classified SMGs is approximately one order of magnitude below that found for typical quasars. This result suggests that intense star-formation activity dominates the bolometric output; however, we also explore whether the X-ray-FIR ratio is intrinsically less than that found for typical quasars and postulate that some SMGs may be AGN dominated. We investigate the growth of massive black holes, discuss prospects for deeper observations, and explore the potential of the next generation of X-ray observatories. (ABRIDGED).
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Aims: The unification scheme of Seyfert galaxies predicts that the observed differences between type 1 and type 2 Seyfert galaxies are solely due to the differing orientations of the toroidal-shaped obscuring material around AGN. The observed X-ray spectra of Seyfert type 2s compared to type 1s are expected to be affected by higher absorbing column density due to the edge-on view of the obscuring torus. We study the 0.5 - 10 keV X-ray spectral properties of Seyfert type 1s and type 2s with the aim to test the predictions of Seyfert unification scheme in the X-ray regime. Methods: We use an optically selected Seyfert sample in which type 1s and type 2s have matched distributions in the orientation independent parameters of AGN and host galaxy. Results: The 0.5 - 10 keV XMM-Newton pn X-ray spectra of Seyfert galaxies are in general best fitted with a model consists of an absorbed power-law, a narrow Gaussian fitted to the Fe K{alpha} emission line and an often seen soft excess component characterized by either a thermal plasma model with temperature kT sim 0.1 - 1.0 keV and/or a steep power-law. The 2.0 - 10 keV hard X-ray continuum emission in several Seyfert type 2s is reflection dominated and suggests the Compton-thick obscuration. Results on the statistical comparison of the distributions of the observed X-ray luminosities in the soft (0.5 - 2.0 keV) and hard (2.0 - 10.0 keV) bands, the X-ray absorbing column densities, the equivalent widths of Fe K{alpha} line and the flux ratios of hard X-ray to [OIII] {lambda}5007{AA} for the two Seyfert subtypes are consistent with the obscuration and orientation based unification scheme.
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The amount and nature of the evolution of the X-ray properties of clusters of galaxies provides information on the formation of structure in the universe and on the properties of the universe itself. The cluster luminosity - temperature relation does not evolve strongly, suggesting that the hot X-ray gas had a more complicated thermodynamic history than simply collapsing into the cluster potential well. Cluster X-ray luminosities do evolve. The dependence of this evolution on redshift and luminosity is characterized using two large high redshift samples. Cluster X-ray temperatures also evolve. This evolution constrains the dark matter and dark energy content of the universe as well as other parameters of cosmological interest.
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