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.

We present our Ðrst results from 120 ks of X-ray observations obtained with the Advanced CCD Imaging Spectrometer on the Chandra X-Ray Observatory. The Ðeld of the two combined exposures is 0.096 deg2 and the detection limit is to a S/N of 2 (corresponding to D7 net counts). We reach a Ñux of 2 ] 10~16 erg s~1 cm~2 in the 0.5È2 keV soft band and 2 ] 10~15 erg s~1 cm~2 in the 2È10 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 identiÐcation, 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 ¹ 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 1A in the central 6@. Optical identiÐcations are found for^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 Ðt in the range 2È10 keV using the Galactic value of cm~2 yields a N H^8] 1019 photon index of ! \ 1.70^0.12 and 1.35^0.20 (errors at 90% conÐdence level) for the bright and faint samples, respectively, showing a Ñattening of the spectrum at lower Ñuxes. Hardness ratio is given as a function of X-ray Ñux and conÐrms this result. The spectrum of our sources is approaching the spectrum of the X-ray background (XRB) in the hard band, which has an e †ective ! \ 1.4. Correlation function analysis for the angular distribution of the sources indicates that they are signiÐcantly clustered on scales as large as 100A. The scale dependence of the correlation function is a power law with index c D 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 NÈ log S relationship and the discrete source contribution to the integrated X-ray sky Ñux. In the soft band, the sources detected in the Ðeld at Ñuxes below 10~15 erg s~1 cm~2 contribute (4.0^0.3) ] 10~12 erg cm~2 s~1 deg~2 to the total XRB. The Ñux resolved in the hard band down to the Ñux limit of 2 ] 10~15 erg s~1 cm~2 contributes (1.05^0.2) ] 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 ] 10~11 erg cm~2 s~1 deg~2, which is 60%È80% of the total measured background. This result conÐrms 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 Ñuxes below 10~15 erg s~1 cm~2. We discuss the X-ray Ñux versus R magnitude relation for the identiÐed sources. We Ðnd that^10% o...

Abstract. We present a detailed analysis of the stellar mass content of galaxies up to z = 2.5 as obtained from the K20 spectrophotometric galaxy sample. We have applied and compared two different methods to estimate the stellar mass M * from broad-band photometry: a Maximal Age approach, where we maximize the age of the stellar population to obtain the maximal mass compatible with the observed R − K color, and a Best Fit model, where the best-fitting spectrum to the complete UBVRIzJK s multicolor distribution is used. We find that the M * /L ratio decreases with redshift: in particular, the average M * /L ratio of early type galaxies decreases with z, with a scatter that is indicative of a range of star-formation time-scales and redshift of formation. More important, the typical M * /L ratio of massive early type galaxies is larger than that of less massive ones, suggesting that their stellar population formed at higher z. We show that the final K20 galaxy sample spans a range of stellar masses from M * = 10 9 M to M * = 10 12 M : massive galaxies (M * ≥ 10 11 M ) are common at 0.5 < z < 1, and are detected also up to z 2. We compute the Galaxy Stellar Mass Function at various z, of which we observe only a mild evolution (i.e. by 20-30%) up to z 1. At z > 1, the evolution in the normalization of the GSMF appears to be much faster: at z 2, about 35% of the present day stellar mass in objects with M * 10 11 M appear to have assembled. We also detect a change in the physical nature of the most massive galaxies: at z < ∼ 0.7, all galaxies with M > 10 11 M are early type, while at higher z a population of massive star-forming galaxies progressively appears. We finally analyze our results in the framework of Λ-CDM hierarchical models. First, we show that the large number of massive galaxies detected at high z does not violate any fundamental Λ-CDM constraint based on the number of massive DM halos. Then, we compare our results with the predictions of several renditions of both semianalytic as well as hydro-dynamical models. The predictions from these models range from severe underestimates to slight overestimates of the observed mass density at ≤2. We discuss how the differences among these models are due to the different implementation of the main physical processes.

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.