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

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

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  erg cm^-2 s^-1  (2–10 keV energy band) which has no detection down to a magnitude . NIR imaging reveals a faint K-band counterpart and NIR spectroscopy shows a single broad ( km s^-1) emission line, which is almost certainly Hα at . The X-ray spectrum shows evidence of significant absorption (), typical of type 2 AGN, but the broad Hα emission suggests a type 1 AGN classification. The very red optical/NIR colours () 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 , which turns out to be much smaller than the expected from the X-ray absorption (assuming Galactic gas-to-dust ratio).