First results from the X-ray and optical survey of the Chandra Deep Field South

We present our first results from 120 ks of X-ray observations obtained with the Advanced CCD Imaging Spectrometer on the Chandra X-Ray Observatory. The field of the two combined exposures is 0.096 deg(2) and the detection limit is to a S/N of 2 (corresponding to similar to7 net counts). We reach a flux of 2 x 10(-16) erg s(-1) cm(-2) in the 0.5-2 keV soft band and 2 x 10(-15) erg s(-1) cm(-2) in the 2-10 keV hard band. Our combined sample has 144 soft sources and 91 hard sources, for a total of 159 sources. Fifteen sources are detected only in the hard band, and 68 only in the soft band. For the optical identification, we carried out a survey in V RI with the FORS-1 imaging spectrometer on the Antu telescope (UT-1 at VLT) complete to R less than or equal to 26. This data set was complemented with data from the ESO Imaging Survey (EIS) in the UBJK bands and the ESO Wide Field Imager Survey (WFI) in the B band. The positional accuracy of the X-ray detections is of the order of 1" in the central 6'. Optical identifications are found for similar or equal to 90% of the sources. Optical spectra have been obtained for 12 objects. We obtain the cumulative spectra of the faint and bright X-ray sources in the sample and also the hardness ratios of individual sources. A power-law Dt in the range 2-10 keV using the Galactic value of N-H similar or equal to 8 x 10(19) cm(-2) yields a photon index of Gamma = 1.70 +/- 0.12 and 1.35 +/- 0.20 (errors at 90% confidence level) for the bright and faint samples, respectively, showing a flattening of the spectrum at lower fluxes. Hardness ratio is given as a function of X-ray flux and confirms this result. The spectrum of our sources is approaching the spectrum of the X-ray background (XRB) in the hard band, which has an effective Gamma = 1.4. Correlation function analysis for the angular distribution of the sources indicates that they are significantly clustered on scales as large as 100". The scale dependence of the correlation function is a power law with index gamma similar to 2, consistent with that of the galaxy distribution in the local universe. Consequently, the discrete sources detected by deep Chandra-pointed observations can be used as powerful tracers of the large-scale structure at high redshift. We discuss the log N- log S relationship and the discrete source contribution to the integrated X-ray sky flux. In the soft band, the sources detected in the field at fluxes below 10(-15) erg s(-1) cm(-2) contribute (4.0 +/- 0.3) x 10(-12) erg cm(-2) s(-1) deg(-2) to the total XRB. The flux resolved in the hard band down to the flux limit of 2 x 10(-15) erg s(-1) cm(-2) contributes (1.05 +/- 0.2) x 10(-11) erg cm(-2) s(-1) deg(-2). Once the contribution from the bright counts resolved by ASCA is included, the total resolved XRB amounts to 1.3 x 10(-11) erg cm(-2) s(-1) deg(-2), which is 60%-80% of the total measured background. This result confirms that the XRB is due to the integrated contribution of discrete sources, but shows that there is still a relevant fraction (at least 20%) of the hard XRB to be resolved at fluxes below 10(-15) erg s(-1) cm(-2). We discuss the X-ray flux versus R magnitude relation for the identified sources. We find that similar or equal to 10% of the sources in our sample are not immediately identifiable at R > 26. For these sources, S-x/S-opt whereas most of the ROSAT and Chandra sources have S-x/S-opt <10. We have also found a population of objects with unusually low S-x/S-opt that are identified as galaxies. The R-K versus R color diagram shows that the Chandra sources continue the trend seen by ROSAT. For our 12 spectroscopically studied objects with redshifts, we observe four QSOs, five Seyfert 2 galaxies, one elliptical, and two interacting galaxies. We compare L-x versus z obtained with these measurements and show that Chandra is achieving the predicted sensitivity.