This Special Issue of the Astrophysical Journal Letters is dedicated to presenting initial results from the Great Observatories Origins Deep Survey (GOODS) that are primarily, but not exclusively, based on multi-band imaging data obtained with the Hubble Space Telescope (HST) and the Advanced Camera for Surveys (ACS). The survey covers roughly 320 square arcminutes in the ACS F435W, F606W, F814W, and F850LP bands, divided into two well-studied fields. Existing deep observations from the Chandra X-ray Observatory (CXO) and groundbased facilities are supplemented with new, deep imaging in the optical and

We present the results of our spectroscopic follow-up program of the X-ray sources detected in the 942 ks exposure of the Chandra Deep Field South (CDFS). 288 possible counterparts were observed at the VLT with the FORS1/FORS2 spectrographs for 251 of the 349 Chandra sources (including three additional faint X-ray sources). Spectra and R-band images are shown for all the observed sources and R−K colours are given for most of them. Spectroscopic redshifts were obtained for 168 X-ray sources, of which 137 have both reliable optical identification and redshift estimate (including 16 external identifications). The R< 24 observed sample comprises 161 X-ray objects (181 optical counterparts) and 126 of them have unambiguous spectroscopic identification. There are two spikes in the redshift distribution, predominantly populated by type-2 AGN but also type-1 AGN and X-ray normal galaxies: that at z = 0.734 is fairly narrow (in redshift space) and comprises two clusters/groups of galaxies centered on extended X-ray sources, the second one at z = 0.674 is broader and should trace a sheet-like structure. The type-1 and type-2 populations are clearly separated in X-ray/optical diagnostics involving parameters sensitive to absorption/reddening: X-ray hardness ratio (HR), optical/near-IR colour, soft X-ray flux and optical brightness. Nevertheless, these two populations cover similar ranges of hard X-ray luminosity and absolute K magnitude, thus trace similar levels of gravitational accretion. Consequently, we introduce a new classification based solely on X-ray properties, HR and X-ray luminosity, consistent with the unified AGN model. This Xray classification uncovers a large fraction of optically obscured, X-ray luminous AGNs missed by the classical optical classification. We find a similar number of X-ray type-1 and type-2 QSOs (L X (0.5-10 keV)> 10 44 erg s −1 ) at z > 2 (13 sources with unambiguous spectroscopic identification); most X-ray type-1 QSOs are bright, R 24, whereas most X-ray type-2 QSOs have R 24 which may explain the difference with the CDFN results as few spectroscopic redshifts were obtained for R> 24 CDFN X-ray counterparts. There are X-ray type-1 QSOs down to z ∼ 0.5, but a strong decrease at z < 2 in the fraction of luminous X-ray type-2 QSOs may indicate a cosmic evolution of the X-ray luminosity function of the type-2 population. An X-ray spectral analysis is required to confirm this possible evolution. The red colour of most X-ray type-2 AGN could be due to dust associated with the X-ray absorbing material and/or a substantial contribution of the host galaxy light. The latter can also be important for some redder X-ray type-1 AGN. There is a large population of EROs (R−K> 5) as X-ray counterparts and their fraction strongly increases with decreasing optical flux, up to 25% for the R≥ 24 sample. They cover the whole range of X-ray hardness ratios, comprise objects of various classes (in particular a high fraction of z 1 X-ray absorbed AGNs, but also elliptical and starburst galaxies) and more than ha...

In this Paper we present the source catalog obtained from a 942 ks exposure of the Chandra Deep Field South (CDFS), using the Advanced CCD Imaging Spectrometer (ACIS-I) on the Chandra X-ray Observatory. Eleven individual pointings made between October 1999 and December 2000 were combined to generate the final image used for object detection. Catalog generation proceeded simultaneously using two different methods; a method of our own design using a modified version of the SExtractor algorithm, and a wavelet transform technique developed specifically for Chandra observations. The detection threshold has been set in order to have less than 10 spurious sources, as assessed by extensive simulations. We subdivided the catalog into four sections. The primary list consists of objects common to the two detection methods. Two secondary lists contain sources which were detected by: 1) the SExtractor algorithm alone and 2) the wavelet technique alone. The fourth list consists of possible diffuse or extended sources. The flux limits at the aimpoint for the soft (0.5-2 keV) and -2hard (2-10 keV) bands are 5.5×10 −17 erg s −1 cm −2 and 4.5×10 −16 erg s −1 cm −2 respectively. The total number of sources is 346; out of them, 307 were detected in the 0.5-2 keV band, and 251 in the 2-10 keV band.We also present optical identifications for the catalogued sources. Our primary optical data is R band imaging from VLT/FORS1 to a depth of R ∼ 26.5 (Vega). In regions of the field not covered by the VLT/FORS1 deep imaging, we use R-band data obtained with the Wide Field Imager (WFI) on the ESO-MPI 2.2m, as part of the ESO Imaging Survey (EIS), which covers the entire X-ray survey. We found that the FORS1/Chandra offsets are small, ∼ 1 ′′ . Coordinate cross-correlation finds 85% of the Chandra sources covered by FORS1 R to have counterparts within the 3σ error box ( 1.5 ′′ depending on off-axis angle and signal-to-noise). The unidentified fraction of sources, approximately ∼ 10-15%, is close to the limit expected from the observed X-ray flux to R-band ratio distribution for the identified sample.

1 Based on observations made at the European Southern Observatory, Paranal, Chile. The EIS observations have been carried out using the ESO New Technology Telescope (NTT) at the La Silla observatory (ESO LP 164.O-O561).-2 - ABSTRACTWe present the main results from our 940 ksec observation of the Chandra Deep Field South (CDFS), using the source catalog described in an accompanying paper (Giacconi et al. 2001). We extend the measurement of source number counts to 5.5 × 10 −17 erg cm −2 s −1 in the soft 0.5-2 keV band and 4.5 × 10 −16 erg cm −2 s −1 in the hard 2-10 keV band. The hard band Log N-Log S shows a significant flattening (slope≃ 0.6) below ≈ 10 −14 erg cm −2 s −1 , leaving at most 10-15% of the X-ray background (XRB) to be resolved, the main uncertainty lying in the measurement of the total flux of the XRB. On the other hand, the analysis in the very hard 5-10 keV band reveals a relatively steep Log N-Log S (slope ≃ 1.3) down to 10 −15 erg cm −2 s −1 .Together with the evidence of a progressive flattening of the average X-ray spectrum near the flux limit, this indicates that there is still a non negligible population of faint hard sources to be discovered at energies not well probed by Chandra, which possibly contribute to the 30 keV bump in the spectrum of the XRB. We use optical redshifts and identifications, obtained with the VLT, for one quarter of the sample to characterize the combined optical and X-ray properties of the CDFS sample. Different source types are well separated in a parameter space which includes X-ray luminosity, hardness ratio and R − K color. Type II objects, while redder on average than the field population, have colors which are consistent with being hosted by a range of galaxy types. Type II AGN are mostly found at z ∼ < 1, in contrast with predictions based on AGN population synthesis models, thus suggesting a revision of their evolutionary parameters.