2006A&A...445..457M

We present VLT near-IR spectroscopic observations of three X-ray sources characterized by extremely high X-ray-to-optical ratios (X/O>40), extremely red colors (6.3<R-K<7.4, i.e. EROs) and bright infrared magnitudes (17.6<K<18.3). These objects are very faint in the optical, making their spectroscopic identification extremely challenging. Instead, our near-IR spectroscopic observations have been successful in identifying the redshift of two of them (z=2.08 and z=1.35), and tentatively even of the third one (z=2.13). When combined with the X-ray properties, our results clearly indicate that all these objects host obscured QSOs (4x10⁴⁴<L_2–10keV<1.5x10⁴⁵erg/s, 2x10²²<N_H<4x10²³cm^–2) at high redshift. The only object with unresolved morphology in the K band shows broad Hα emission, but not broad Hβ, implying a type 1.9 AGN classification. The other two objects are resolved and dominated by the host galaxy light in the K band, and appear relatively quiescent: one of them has a LINER-like emission line spectrum and the other presents only a single, weak emission line which we tentatively identify with Hα. The galaxy luminosities for the latter two objects are an order of magnitude brighter than typical local L_K^* galaxies and the derived stellar masses are well in excess of 10¹¹M_☉. For these objects we estimate black hole masses higher than 10⁹M_☉ and we infer that they are radiating at Eddington ratios L/L_Edd≤0.1. We discuss the implications of these findings for the coevolution of galaxies and black hole growth. Our results provide further support that X-ray sources with high X/O ratios and very red colors tend to host obscured QSO in very massive galaxies at high redshift.