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...

Abstract. We present the results of the X-ray spectral analysis of the first deep X-ray survey with the XMM-Newton observatory during Performance Verification. The X-ray data of the Lockman Hole field and the derived cumulative source counts were reported by Hasinger et al. (2001). We restrict the analysis to the sample of 98 sources with more than 70 net counts (flux limit in the [0.5-7] keV band of 1.6 × 10 −15 erg cm −2 s −1 ) of which 61 have redshift identification. We find no correlation between the spectral index Γ and the intrinsic absorption column density N H and, for both the Type-1 and Type-2 AGN populations, we obtain Γ 2. The progressive hardening of the mean X-ray source spectrum with decreasing flux is essentially due to an increase in intrinsic absorption. The marked separation between the two AGN populations in several diagnostics diagrams, involving X-ray colour, X-ray flux, optical/near IR colour and optical brightness, is also a consequence of different absorption column densities and enables the classification of optically faint obscured AGN. The Type-2 and obscured AGN have weaker soft X-ray and optical fluxes and redder R − K colours. They follow the evolutionary tracks of their host galaxies in a colorredshift diagram. About 27% of the subsample with R − K colour are EROs (R − K ≥ 5) and most of these 18 X-ray selected EROs contain an obscured AGN as revealed by their high X-ray-to-optical/near IR flux ratios. There are six sources in our sample with L X [0.5−10] > 10 44 erg s −1 and log(N H ) > 10 22 cm −2 : which are likely Type-2 QSOs and we thus derive a density of ∼69 objects of this class per square degree.

Abstract. The ROSAT Deep Surveys in the direction of the Lockman Hole are the most sensitive X-ray surveys performed with the ROSAT satellite. About 70-80% of the X-ray background has been resolved into discrete sources at a flux limit of ∼10 −15 erg cm −2 s −1 in the 0.5-2.0 keV energy band. A nearly complete optical identification of the ROSAT Deep Survey (RDS) has shown that the great majority of sources are AGNs. We describe in this paper the ROSAT Ultra Deep Survey (UDS), an extension of the RDS in the Lockman Hole. The Ultra Deep Survey reaches a flux level of 1.2 10 −15 erg cm −2 s −1 in 0.5-2.0 keV energy band, a level ∼4.6 times fainter than the RDS. We present nearly complete spectroscopic identifications (90%) of the sample of 94 X-ray sources based on low-resolution Keck spectra. The majority of the sources (57) are broad emission line AGNs (type I), whereas a further 13 AGNs show only narrow emission lines or broad Balmer emission lines with a large Balmer decrement (type II AGNs) indicating significant optical absorption. The second most abundant class of objects (10) are groups and clusters of galaxies (∼11%). Further we found five galactic stars and one "normal" emission line galaxy. Eight X-ray sources remain spectroscopically unidentified. We see no evidence for any change in population from the RDS survey to the UDS survey. The photometric redshift determination indicates in three out of the eight sources the presence of an obscured AGN. Their photometric redshifts, assuming that the spectral energy distribution (SED) in the optical/near-infrared is due to stellar processes, are in the range of 1.2 ≤ z ≤ 2.7. These objects could belong to the long-sought population of type 2 QSOs, which are predicted by the AGN synthesis models of the X-ray background. Finally, we discuss the optical and soft X-ray properties of the type I AGN, type II AGN, and groups and clusters of galaxies, and the implication to the X-ray background.

We present results of optical identiÐcation of the X-ray sources detected in the ASCA Large Sky Survey. Optical spectroscopic observations were done for 34 X-ray sources that were detected with the SIS in the 2È7 keV band above 3.5 p. The Ñux limit corresponds to D1 ] 10~13 ergs cm~2 s~1 in the 2È10 keV band. The sources are identiÐed with 30 active galactic nuclei (AGNs), two clusters of galaxies, and one Galactic star. Only one source is still unidentiÐed.All of the X-ray sources that have a hard X-ray spectrum with an apparent photon index of smaller than 1 in the 0.7È10 keV band are identiÐed with narrow-line or weak-broad-line AGNs at redshifts smaller than 0.5. This fact supports the idea that absorbed X-ray spectra of narrow-line and weak-broadline AGNs make the cosmic X-ray background (CXB) spectrum harder in the hard X-ray band than that of a broad-line AGN, which is the main contributor in the soft X-ray band. Assuming their intrinsic spectra are same as a broad-line AGN (a power-law model with a photon index of 1.7), their X-ray spectra are Ðtted with hydrogen column densities of at the objectÏs redshift. On log N H (cm~2) \ 22È23 the other hand, X-ray spectra of the other AGNs are consistent with that of a nearby type 1 Seyfert galaxy. In the sample, four high-redshift luminous broad-line AGNs show a hard X-ray spectrum with an apparent photon index of 1.3^0.3. The hardness may be explained by the reÑection component of a type 1 Seyfert galaxy. The hard X-ray spectra may also be explained by absorption with at the objectÏs redshift, if we assume an intrinsic photon index of 1.7. The origin log N H (cm~2) \ 22È23 of the hardness is not clear yet.Based on the log NÈlog S relations of each population, contributions to the CXB in the 2È10 keV band are estimated to be 9% for less-absorbed AGNs including the four high-(log N H (cm~2) \ 22) redshift broad-line AGNs with a hard X-ray spectrum, 4% for absorbed AGNs (22 \ log N H (cm~2) \ 23, without the four hard broad-line AGNs), and 1% for clusters of galaxies in the Ñux range from 3 ] 10~11 ergs cm~2 s~1 to 2 ] 10~13 ergs cm~2 s~1. If the four hard broad-line AGNs are included in the absorbed AGNs, the contribution of the absorbed AGNs to the CXB is estimated to be 6%.In optical spectra, there is no high-redshift luminous cousin of a narrow-line AGN in our sample. The redshift distribution of the absorbed AGNs is limited below z \ 0.5 excluding the four hard broad-line AGNs, in contrast to the existence of 15 less-absorbed AGNs above z \ 0.5. The redshift distribution of the absorbed AGNs suggests a deÐciency of AGNs with column densities of in log N H (cm~2) \ 22È23 the redshift range 0.5È2, or in the X-ray luminosity range larger than 1044 ergs s~1, or both. If the large column densities of the four hard broad-line AGNs are real, they could complement the deÐciency of X-ray absorbed luminous high-redshift AGNs.

Abstract.We have obtained a deep radio image with the Very Large Array at 6 cm in the Lockman Hole. The noise level in the central part of the field is ∼11 µJy. From these data we have extracted a catalogue of 63 radio sources with a maximum distance of 10 arcmin from the field center and with peak flux density greater than 4.5 times the local rms noise. The di fferential source counts are in good agreement with those obtained by other surveys. The analysis of the radio spectral index suggests a flattening of the average radio spectra and an increase of the population of flat spectrum radio sources in the faintest flux bin. Cross correlation with the ROSAT/XMM X-ray sources list yields 13 reliable radio/X-ray associations, corresponding to ∼21% of the radio sample. Most of these associations (8 out of 13) are classified as type II AGN. Using optical CCD (V and I) and K band data with approximate limits of V ∼ 25.5 mag, I ∼ 24.5 mag and K ∼ 20.2 mag, we found an optical identification for 58 of the 63 radio sources. This corresponds to an identification rate of ∼92%, one of the highest percentages so far available. From the analysis of the colour-colour diagram and of the radio flux -optical magnitude diagram we have been able to select a subsample of radio sources whose optical counterparts are likely to be high redshift (z > 0.5) early-type galaxies, hosting an Active Galactic Nucleus responsible of the radio activity. This class of objects, rather than a population of star-forming galaxies, appears to be the dominant population ( 50%) in a 5 GHz selected sample with a flux limit as low as 50 µJy. We also find evidence that at these faint radio limits a large fraction (∼60%) of the faintest optical counterparts (i.e. sources in the magnitude range 22.5 < I < 24.5 mag) of the radio sources are Extremely Red Objects (EROs) with I − K > 4 and combining our radio data with existing ISO data we conclude that these EROs sources are probably associated with high redshift, passively evolving elliptical galaxies. The six radio selected EROs represent only ∼2% of the optically selected EROs present in the field. If their luminosity is indeed a sign of AGN activity, the small number of radio detections suggests that a small fraction of the EROS population contains an active nucleus.