Optical spectroscopy of active galactic nuclei in SA57

Context. The cosmological evolution of X-ray-selected and optically selected active galactic nuclei (AGNs) show different behaviours interpreted in terms of two different populations. The difference is evident mainly for low luminosity AGNs (LLAGNs), many of which are lost by optical photometric surveys. Aims. We are conducting a spectroscopical study of a composite sample of AGN candidates selected in SA57 following different searching techniques, to identify low luminosity AGNs and break down the sample into different classes of objects. Methods. AGN candidates were obtained through optical variability and/or X-ray emission. Of special interest are the extended variable objects, which are expected to be galaxies hosting LLAGNs. Results. Among the 26 classified objects a fair number (9) show typical AGN spectra. Ten objects show Narrow Emission Line Galaxy spectra, and in most of them (8/10) optical variability suggests the presence of LLAGNs.