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 - KS > 2) 2MASS active galactic nuclei (AGNs) using principle component analysis (PCA). The sample includes 44 low redshift AGNs with low or moderate obscuration (NH < 1023 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/LEdd ratio and strengthened by intrinsic absorption (both circumnuclear and galactic). Eigenvector 2 (18% of variance) correlates with optical/IR colors (B - KS, B - R, J - KS) 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, NH), 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/LEdd ratio is the most important factor, followed by host-galaxy emission.