Survival of the obscuring torus in the most powerful active galactic nuclei

Autor: Mateos, S., Carrera, F. J., Barcons, X., Alonso-Herrero, A., Hernán-Caballero, A., Page, M., Almeida, C. Ramos, Caccianiga, A., Miyaji, T., Blain, A.
Rok vydání: 2017
Předmět:
Druh dokumentu: Working Paper
DOI: 10.3847/2041-8213/aa7268
Popis: Dedicated searches generally find a decreasing fraction of obscured Active Galactic Nuclei (AGN) with increasing AGN luminosity. This has often been interpreted as evidence for a decrease of the covering factor of the AGN torus with increasing luminosity, the so-called receding torus models. Using a complete flux-limited X-ray selected sample of 199 AGN, from the Bright Ultra-hard XMM-Newton Survey, we determine the intrinsic fraction of optical type-2 AGN at 0.05$\leq$z$\leq$1 as a function of rest-frame 2-10 keV X-ray luminosity from 10$^{42}$ to 10$^{45}$ erg/s. We use the distributions of covering factors of AGN tori derived from CLUMPY torus models. Since these distributions combined over the total AGN population need to match the intrinsic type-2 AGN fraction, we reveal a population of X-ray undetected objects with high-covering factor tori, which are increasingly numerous at higher AGN luminosities. When these "missing" objects are included, we find that Compton-thick AGN account at most for 37$_{-10}^{+9}$% of the total population. The intrinsic type-2 AGN fraction is 58$\pm$4% and has a weak, non-significant (less than 2$\sigma$) luminosity dependence. This contradicts the results generally reported by AGN surveys, and the expectations from receding torus models. Our findings imply that the majority of luminous rapidly-accreting supermassive black holes at z<1 reside in highly-obscured nuclear environments but most of them are so deeply embedded that they have so far escaped detection in X-rays in <10 keV wide-area surveys.
Comment: 6 pages, 2 figures, 1 table; accepted for publication in the Astrophysical Journal Letters, May 10, 2017
Databáze: arXiv
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