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Issue A&A
Volume 493, Number 2, January II 2009
Page(s) 445 - 451
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:200809665
Published online 01 October 2008



A&A 493, 445-451 (2009)
DOI: 10.1051/0004-6361:200809665

An extreme EXO: a type 2 QSO at z = 1.87

A. Del Moro1, M. G. Watson1, S. Mateos1, M. Akiyama2, Y. Hashimoto3, N. Tamura4, K. Ohta5, F. J. Carrera6, and G. Stewart1

1  XROA-University of Leicester, University Road, Leicester LE1 7RH, UK
    e-mail: ad187@star.le.ac.uk
2  Astronomical Institute, Tohoku University, Sendai 980-8578, Japan
3  South African Astronomical Observatory, Observatory Road, Cape Town 7539, South Africa
4  Subaru Telescope, National Astronomical Observatory of Japan, Hilo, HI 96720, Japan
5  Department of Astronomy, Kyoto University, Kyoto 606-8502, Japan
6  Instituto de Física de Cantabria (CSIC-UC), Avenida de los Castros, 39005 Santander, Spain

Receievd 27 February 2008 / Accepted 23 September 2008

Abstract
Aims. We aim to understand the multi-wavelength properties of 2XMM J123204+215255, the source with the most extreme X-ray-to-optical flux ratio amongst a sample of bright X-ray selected EXOs drawn from a cross-correlation of the 2XMMp catalogue with the SDSS-DR5 catalogue.
Methods. We use 2XMMp  X-ray data, SDSS-DR5, NOT and UKIRT optical/NIR photometric data and Subaru MOIRCS IR spectroscopy to study the properties of 2XMM J123204+215255. We created a model SED including an obscured QSO and the host galaxy component to constrain the optical/IR extinction and the relative contribution of the AGN and the galaxy to the total emission.
Results. 2XMM J123204+215255 is a bright X-ray source with $f_{\rm X}\approx10^{-12}$ erg cm-2 s-1  (2–10 keV energy band) which has no detection down to a magnitude i'>25.2. NIR imaging reveals a faint K-band counterpart and NIR spectroscopy shows a single broad ( ${\it FWHM }\simeq5300$ km s-1) emission line, which is almost certainly H$\alpha$ at z=1.87. The X-ray spectrum shows evidence of significant absorption ( $N_{\rm H}>10^{23}~ \rm cm^{-2}$), typical of type 2 AGN, but the broad H$\alpha$ emission suggests a type 1 AGN classification. The very red optical/NIR colours (i'-K>5.3) strongly suggest significant reddening however. We find that simple modelling can successfully reproduce the NIR continuum and strongly constrain the intrinsic nuclear optical/IR extinction to $A_{V}\approx 4$, which turns out to be much smaller than the expected from the X-ray absorption (assuming Galactic gas-to-dust ratio).


Key words: galaxies: active -- galaxies: quasars: general -- X-rays: galaxies -- infrared: galaxies



© ESO 2009


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