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Published February 20, 2009 | Published
Journal Article Open

Principal Component Analysis of the Spectral Energy Distribution and Emission Line Properties of Red 2MASS Active Galactic Nuclei

Abstract

We analyze the spectral energy distributions (SEDs) and emission-line properties of the red (J – K_S >2) 2MASS active galactic nuclei (AGNs) using principle component analysis (PCA). The sample includes 44 low redshift AGNs with low or moderate obscuration (N_H <10^(23) cm^(–2)) as indicated by X-rays and SED modeling. The obscuration of the AGNs allows us to see weaker emission components (host-galaxy emission, AGN scattered light) which are usually outshone by the AGN. The first four eigenvectors explain 70% of the variance in the sample. Eigenvector 1 (33% variance in the sample) correlates with the ratios of the intrinsic X-ray flux to the observed optical/IR fluxes and the F(2-10 keV)/F([O III]) ratio. We suggest that it is primarily driven by the L/L_(Edd) ratio and strengthened by intrinsic absorption (both circumnuclear and galactic). Eigenvector 2 (18% of variance) correlates with optical/IR colors (B – K_S , B – R, J – K_S ) and optical spectral type and depends on the contribution of the host galaxy relative to the observed AGN emission. Eigenvector 3 (12% of variance) correlates with reddening indicators obtained from the X-rays (hardness ratio, spectral index, N_H ), and the narrow Hα/Hβ emission-line ratio. Their relation suggests a common absorber for the optical/X-rays lying outside the narrow-line region possibly in a moderately inclined host galaxy. Eigenvector 4 (8% of variance) correlates with the degree of polarization and the broad Hα/Hβ ratio, indicating that dust, which scatters the nuclear emission (continuum and the broad-line region emission), also reddens the broad lines. Our analysis shows that, although as suggested by unification schemes, the inclination dependent obscuration (circumnuclear and the host galaxy) is important in determining the AGN SEDs, the L/L_(Edd) ratio is the most important factor, followed by host-galaxy emission.

Additional Information

© 2009. The American Astronomical Society. Received 2008 October 27; accepted 2008 November 5; published 2009 February 23. We thank the referee, Gordon Richards, for comments that helped improve the paper and Martin Gaskell for useful discussions. B.J.W. and J.K. gratefully acknowledge the financial support of NASA Chandra grants: GO1-2112A, GO3- 4138A and NASA XMM-Newton grants: NNG04GD27G, NNG05GM24G, which supported various aspects of this work. We also gratefully acknowledge the financial support of grants: NAS8-39073, GO-09161.05-A (HST). P.S.S. acknowledges support from NASA/JPL contract 1256424. This publication makes use of data products from the TwoMicronAll Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.We thank F. Murtagh for making his PCA program available on the web.

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