The Calán-Yale Deep Extragalactic Research (CYDER) Survey: Optical Properties and Deep Spectroscopy of Serendipitous X-Ray Sources^*

We present the first results from the Calán-Yale Deep Extragalactic Research (CYDER) survey. The main goal of this survey is to study serendipitous X-ray sources detected by Chandra in an intermediate flux range (10^-15 to 10^-12 ergs s^-1) that comprises most of the X-ray background. A total of 267 X-ray sources spread over five archived fields were detected. The log N- log S distribution obtained for this sample is consistent with the results of other surveys. Deep V and I images were taken of these fields in order to calculate X-ray-to-optical flux ratios. Identifications and redshifts were obtained for 106 sources using optical spectroscopy from 8 m class telescopes to reach the optically faintest sources, to the same level as deeper X-ray fields like the Chandra Deep Fields, showing that the nature of sources detected depends mostly on the optical limit for spectroscopy. In general, sources optically classified as obscured active galactic nuclei (AGNs) have redder optical colors than unobscured AGNs. A rough correlation between f_X/f_opt and hard X-ray luminosity was found for obscured AGNs, confirming the prediction by existing models that in obscured AGNs the optical light is completely dominated by the host galaxy. The previously claimed decrease of the obscured-to-unobscured AGN ratio with increasing X-ray luminosity is observed. However, this correlation can be explained as a selection effect caused by the lower optical flux of obscured AGNs. Comparison between the observed N_H distribution and predictions by existing models shows that the sample appears complete up to N_H < 3 × 10²² cm^-2, while for more obscured sources incompleteness plays an important role in the observed obscured-to-unobscured AGN ratio.