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          Institute: MPI für Astronomie     Collection: Publikationen_mpia     Display Documents

ID: 693598.0, MPI für Astronomie / Publikationen_mpia
The obscured fraction of active galactic nuclei in the XMM-COSMOS survey: a spectral energy distribution perspective
Authors:Lusso, E.; Hennawi, J. F.; Comastri, A.; Zamorani, G.; Richards, G. T.; Vignali, C.; Treister, E.; Schawinski, K.; Salvato, M.; Gilli, R.
Date of Publication (YYYY-MM-DD):2013
Title of Journal:The Astrophysical Journal
Issue / Number:2
Start Page:id. 86 (28 pp)
Audience:Not Specified
Abstract / Description:The fraction of active galactic nucleus (AGN) luminosity obscured by dust and re-emitted in the mid-IR is critical for understanding AGN evolution, unification, and parsec-scale AGN physics. For unobscured (Type 1) AGNs, where we have a direct view of the accretion disk, the dust covering factor can be measured by computing the ratio of re-processed mid-IR emission to intrinsic nuclear bolometric luminosity. We use this technique to estimate the obscured AGN fraction as a function of luminosity and redshift for 513 Type 1 AGNs from the XMM-COSMOS survey. The re-processed and intrinsic luminosities are computed by fitting the 18 band COSMOS photometry with a custom spectral energy distribution fitting code, which jointly models emission from hot dust in the AGN torus, from the accretion disk, and from the host galaxy. We find a relatively shallow decrease of the luminosity ratio as a function of L bol, which we interpret as a corresponding decrease in the obscured fraction. In the context of the receding torus model, where dust sublimation reduces the covering factor of more luminous AGNs, our measurements require a torus height that increases with luminosity as h\propto L_bol^{\quad 0.3-0.4}. Our obscured-fraction-luminosity relation agrees with determinations from Sloan Digital Sky Survey censuses of Type 1 and Type 2 quasars and favors a torus optically thin to mid-IR radiation. We find a much weaker dependence of the obscured fraction on 2-10 keV luminosity than previous determinations from X-ray surveys and argue that X-ray surveys miss a significant population of highly obscured Compton-thick AGNs. Our analysis shows no clear evidence for evolution of the obscured fraction with redshift.
Free Keywords:galaxies: active; galaxies: evolution; quasars: general; methods: statistical
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Astronomie
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