The HELLAS2XMM survey

Abstract

We present VLT near-IR spectroscopic observations of three X-ray sources characterized by extremely high X-ray–to–optical ratios (), extremely red colors (, i.e. EROs) and bright infrared magnitudes (). 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 ( and ), and tentatively even of the third one (). When combined with the X-ray properties, our results clearly indicate that all these objects host obscured QSOs (, ) 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 galaxies and the derived stellar masses are well in excess of . For these objects we estimate black hole masses higher than and we infer that they are radiating at Eddington ratios . 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.