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

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_X≈10^-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 ({FWHM }≃5300 km s^-1) emission line, which is almost certainly Hα at z=1.87. The X-ray spectrum shows evidence of significant absorption (N_H>10²³ cm^-2), typical of type 2 AGN, but the broad Hα 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≈ 4, which turns out to be much smaller than the expected from the X-ray absorption (assuming Galactic gas-to-dust ratio).