Revisiting the relationship between 6 μm and 2–10 keV continuum luminosities of AGN

• Published in: Monthly notices of the Royal Astronomical Society Vol. 449; no. 2; pp. 1422 - 1440
• Main Authors: Mateos, S., Carrera, F. J., Alonso-Herrero, A., Rovilos, E., Hernán-Caballero, A., Barcons, X., Blain, A., Caccianiga, A., Della Ceca, R., Severgnini, P.
• Format: Journal Article
• Language: English
• Published: Oxford University Press 11.05.2015
• Subjects: Bolometers; Constants; Continuums; Emission; Luminosity; Spectral energy distribution; Symbols; X-rays; XMM (spacecraft); quasars: supermassive black holes; infrared: galaxies; galaxies: active; X-rays: galaxies
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stv299

We have determined the relation between the AGN luminosities at rest-frame 6 μm associated with the dusty torus emission and at 2–10 keV energies using a complete, X-ray-flux-limited sample of 232 AGN drawn from the Bright Ultra-hard XMM-Newton Survey. The objects have intrinsic X-ray luminosities between 1042 and 1046 erg s−1 and redshifts from 0.05 to 2.8. The rest-frame 6 μm luminosities were computed using data from the Wide-field Infrared Survey Explorer and are based on a spectral energy distribution decomposition into AGN and galaxy emission. The best-fitting relationship for the full sample is consistent with being linear, L 6 μm ∝  $L_{\rm 2{\rm -}10\,keV}^{0.99\pm 0.03}$ , with intrinsic scatter, Δ log L 6 μm ∼ 0.35 dex. The $L_{\rm 6\,\mu m}/L_{\rm 2{\rm -}10\,keV}$ luminosity ratio is largely independent of the line-of-sight X-ray absorption. Assuming a constant X-ray bolometric correction, the fraction of AGN bolometric luminosity reprocessed in the mid-IR decreases weakly, if at all, with the AGN luminosity, a finding at odds with simple receding torus models. Type 2 AGN have redder mid-IR continua at rest-frame wavelengths <12 μm and are overall ∼1.3–2 times fainter at 6 μm than type 1 AGN at a given X-ray luminosity. Regardless of whether type 1 and type 2 AGN have the same or different nuclear dusty toroidal structures, our results imply that the AGN emission at rest-frame 6 μm is not isotropic due to self-absorption in the dusty torus, as predicted by AGN torus models. Thus, AGN surveys at rest-frame ∼6 μm are subject to modest dust obscuration biases.