The HELLAS2XMM survey - IX. Spectroscopic identification of super-EROs hosting AGNs

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 ( 4 x 10(44) < L2-10 (keV) < 1.5 x 10(45) erg s(-1), 2 x 10(22) < N-H < 4 x 10(23) cm(-2)) at high redshift. The only object with unresolved morphology in the K band shows broad Ha emission, but not broad H beta, 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 Ha. 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 10(11) M-circle dot. For these objects we estimate black hole masses higher than 10(9) M-circle dot 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.