The puzzling case of XBONGs: Will 3D-spectroscopy explain their true nature?

Abstract

The discovery of luminous hard X-ray sources hosted by "normal" galaxies with optical spectra typical of early-type systems (XBONG; Comastri et al. 2002, (3)) represents one of the most surprising results of Chandra and XMM- Newton surveys. Why the relatively bright X-ray emission, typical of moderately luminous (1042-43 erg s-1) Active Galactic Nuclei (AGN), does not leave any optical signature of the presence of a nuclear source is still subject of debate. Although several possibilities have been discussed in the recent years, the main concern about the nature of these sources is that the AGN emission lines may be "hidden" by the host galaxy starlight. The already granted VIMOS-IFU observations will provide us with a unique opportunity to understand these enigmatic sources. High spatial resolution, integral field spectroscopy in the optical, isolating the nuclei of the XBONGs, would help to settle the AGN line absence vs. starlight dilution controversy. The 3D-spectroscopy will also provide a clear insight into the optical properties of these X-ray emitting galaxies and, therefore, into the nature of the obscured super-massive black holes (SMBH) they host. © 2007 Springer-Verlag Berlin Heidelberg.