The Calan‐Yale Deep Extragalactic Research (CYDER) Survey: Optical Properties and Deep Spectroscopy of Serendipitous X‐Ray Sources

We present the first results from the Cal\'an-Yale Deep ExtragalacticResearch (CYDER) survey. The main goal of this survey is to study serendipitousX-ray sources detected by Chandra in an intermediate flux range($10^{-15}-10^{-12}$ ergs s$^{-1}$) that comprises most of the X-raybackground. 267 X-ray sources spread over 5 archived fields were detected. The$\log N-\log S$ distribution obtained for this sample is consistent with theresults of other surveys. Deep $V$ and $I$ images were taken of these fields inorder to calculate X-ray-to-optical flux ratios. Identifications and redshiftswere obtained for 106 sources using optical spectroscopy from 8-m classtelescopes to reach the optically faintest sources, to the same level as deeperX-ray fields like the Chandra Deep Fields, showing that the nature of sourcesdetected depends mostly on the optical limit for spectroscopy. In general,sources optically classified as obscured Active Galactic Nuclei (AGNs) haveredder optical colors than unobscured AGN. A rough correlation between$f_X/f_{\rm opt}$ and hard X-ray luminosity was found for obscured AGNconfirming the prediction by existing models that in obscured AGN the opticallight is completely dominated by the host galaxy. The previously claimeddecrease of the obscured to unobscured AGN ratio with increasing X-rayluminosity is observed. However, this correlation can be explained as aselection effect caused by the lower optical flux of obscured AGN. Comparisonbetween the observed $N_H$ distribution and predictions by existing modelsshows that the sample appears complete up to $N_H<3\times 10^{22}$ cm$^{-2}$,while for more obscured sources incompleteness plays an important role in theobserved obscured to unobscured AGN ratio.Comment: 47 pages, 19 figures, Astrophysical Journal Accepte