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          Institute: MPI für Astronomie     Collection: Publikationen_mpia     Display Documents

ID: 710121.0, MPI für Astronomie / Publikationen_mpia
The incidence of obscuration in active galactic nuclei
Authors:Merloni, A.; Bongiorno, A.; Brusa, M.; Iwasawa, K.; Mainieri, V.; Magnelli, B.; Salvato, M.; Berta, S.; Cappelluti, N.; Comastri, A.; Fiore, F.; Gilli, R.; Koekemoer, A.; Le Floc'h, E.; Lusso, E.; Lutz, D.; Miyaji, T.; Pozzi, F.; Riguccini, L.; Rosario, D. J.; Silverman, J.; Symeonidis, M.; Treister, E.; Vignali, C.; Zamorani, G.
Date of Publication (YYYY-MM-DD):2014
Title of Journal:Monthly Notices of the Royal Astronomical Society
Issue / Number:4
Start Page:3550
End Page:3567
Audience:Not Specified
Abstract / Description:We study the incidence of nuclear obscuration on a complete sample of 1310 active galactic nuclei (AGN) selected on the basis of their rest-frame 2-10 keV X-ray flux from the XMM-COSMOS survey, in the redshift range 0.3 < z < 3.5. We classify the AGN as obscured or unobscured on the basis of either the optical spectral properties and the overall SED or the shape of the X-ray spectrum. The two classifications agree in about 70 per cent of the objects, and the remaining 30 per cent can be further subdivided into two distinct classes: at low luminosities X-ray unobscured AGN do not always show signs of broad lines or blue/UV continuum emission in their optical spectra, most likely due to galaxy dilution effects; at high-luminosities broad-line AGN may have absorbed X-ray spectra, which hints at an increased incidence of small-scale (sub-parsec) dust-free obscuration. We confirm that the fraction of obscured AGN is a decreasing function of the intrinsic X-ray luminosity, while the incidence of absorption shows significant evolution only for the most luminous AGN, which appear to be more commonly obscured at higher redshift. We find no significant difference between the mean stellar masses and star formation rates of obscured and unobscured AGN hosts. We conclude that the physical state of the medium responsible for obscuration in AGN is complex and mainly determined by the radiation environment (nuclear luminosity) in a small region enclosed within the gravitational sphere of influence of the central black hole, but is largely insensitive to the wider scale galactic conditions.
Free Keywords:catalogues; surveys; galaxies: active; galaxies: evolution; galaxies: fundamental parameters
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Astronomie
Identifiers:ISSN:0035-8711 %R 10.1093/mnras/stt2149
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