Beyond the Spectroscopic Limit in the GOODS/CDFS Survey

Abstract

A quarter of the X-ray sources from recent Chandra and XMM-Newton deep surveys has optical counterparts too faint (R≥25) for spectroscopy, even with the 8-10 m class telescopes. In this work we focus on this subsample of X-ray sources with R≥25 and will refer to them as Optically Faint Sources (OFS). This criterium reflects an observational limit beyond which we need to rely on accurate photometric measurements over a large baseline and X-ray spectral information to estimate their redshift and to establish their nature. Since the OFS represent a significant fraction of the entire X-ray sample, they have an important impact on statistical studies, such as the X-ray luminosity function, the evolution of the TypeI/TypeII ratio and the overall N_H distribution. A first study on the nature of the OFS (I≥24) in the Chandra Deep Field North was done by [1].