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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 1999 October and 2000 December 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 hard (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 cataloged sources. Our primary optical data are 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 2.2 m telescope, as part of the ESO Imaging Survey (EIS), which cover 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 3 σ error box (≳1 5 depending on off-axis angle and X-ray 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 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). A total of 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 colors 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 active galactic nuclei (AGNs) but also type 1 AGN and X-ray normal galaxies: the one 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 sheetlike 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 color, 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 X-ray 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 ergs 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 color of most X-ray type 2 AGNs 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 AGNs. 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 half of them should be fairly bright X-ray sources [ L X (0.5-10 keV) > 10 42 ergs s -1 ]. Photometric redshifts will be necessary to derive the properties and evolution of the X-ray selected EROs.

As part of a project to assess the value of new energy technologies, an international group of researchers created a linear-programming model of national energy systems. This model, MARKAL, is driven by useful energy demands, optimizes over several time periods collectively, and allows multiobjective analyses to be performed quite easily. We describe here the technical structure of the model, defining the functions determined when satisfying the model's relations and the parameters that must be supplied to give the model content.

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 ~7 net counts). We reach a flux of 2 × 10 -16 erg s -1 cm -2 in the 0.5-2 keV soft band and 2 × 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 VRI with the FORS-1 imaging spectrometer on the Antu telescope (UT-1 at VLT) complete to R ≤ 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 ≃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 fit in the range 2-10 keV using the Galactic value of N H ≃ 8 × 10 19 cm -2 yields a photon index of Γ = 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 Γ = 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 γ ~ 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) × 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 × 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%-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 ≃10% of the sources in our sample are not immediately identifiable at R > 26. For these sources, S X / S opt > 15, 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.

Abstract Twenty-six years ago a small committee report, building on earlier studies, expounded a compelling and poetic vision for the future of astronomy, calling for an infrared-optimized space telescope with an aperture of at least 4 m. With the support of their governments in the US, Europe, and Canada, 20,000 people realized that vision as the 6.5 m James Webb Space Telescope. A generation of astronomers will celebrate their accomplishments for the life of the mission, potentially as long as 20 yr, and beyond. This report and the scientific discoveries that follow are extended thank-you notes to the 20,000 team members. The telescope is working perfectly, with much better image quality than expected. In this and accompanying papers, we give a brief history, describe the observatory, outline its objectives and current observing program, and discuss the inventions and people who made it possible. We cite detailed reports on the design and the measured performance on orbit.

We report new radio imaging of the large-scale radio structure of M87 with the VLA at 90 cm. These new images show the complex structure of the radio emission more clearly than previous attempts, some of which date back to the 1940s. The images suggest that the outward flow from the M87 nucleus extends well beyond the 2 kpc jet. Two "bubbles" of synchrotron emission appear to be inflated by this flow. A simple model of the emission, combined with our knowledge of the inner jet, suggest that the energy input into this region from the M87 nucleus exceeds the energy being radiated away as X-rays. This argues that the region within 40 kpc of the center of M87 is currently dominated by energy input from the M87 nucleus. The gas in the region is expanding, not flowing inward as is envisioned in the cooling flow model.

Occurrence of cerebrovascular accidents has been associated with cocaine abuse. We investigated the relative distribution of cerebral blood flow (CBF) in groups of chronic cocaine users, and of normal controls. Relative CBF was measured using positron emission tomography and 15 oxygen-labelled water. The cocaine users showed areas of deranged CBF as evidenced by patchy regions of defective isotope accumulation throughout their brain. The chronic cocaine users showed decreased relative CBF in the prefrontal cortex when compared with normal subjects. The repeated scans of some cocaine users, after 10 days of cocaine withdrawal, continued to show decreased relative CBF of the prefrontal cortex. We hypothesise that some of the widespread defects in CBF in the cocaine users could reflect the effects of vasospasm in cerebral arteries exposed chronically to the sympathomimetic actions of cocaine.

We present the main results from our 940 ks observation of the Chandra Deep Field-South using the source catalog described in an accompanying paper by Giacconi et al. We extend the measurement of source number counts to 5.5 × 10 -17 ergs cm -2 s -1 in the soft 0.5-2 keV band and 4.5 × 10 -16 ergs 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 ergs cm -2 s -1 , leaving at most 10%-15% of the X-ray background to be resolved, the main uncertainty lying in the measurement of the total flux of the X-ray background (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 ergs 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 nonnegligible population of faint hard sources to be discovered at energies not well probed by Chandra , which possibly contributes to the 30 keV bump in the spectrum of the XRB. We use optical redshifts and identifications, obtained with the Very Large Telescope, for one-quarter of the sample to characterize the combined optical and X-ray properties of the Chandra Deep Field-South sample. Different source types are well separated in a parameter space that includes X-ray luminosity, hardness ratio, and R - K color. Type II objects, while redder on average than the field population, have colors that are consistent with being hosted by a range of galaxy types. Type II active galactic nuclei are mostly found at z ≲ 1, in contrast with predictions based on active galactic nucleus population synthesis models, thus suggesting a revision of their evolutionary parameters.

Boron neutron capture therapy (BNCT) is based on the nuclear reaction that occurs when a stable isotope, boron-10 (10B), is irradiated with low-energy thermal neutrons (nth) to yield (4He) alpha-particles and 7Li nuclei (10B+nth-->[11B]-->4He+7Li+2.31 MeV). The success of BNCT as a tumoricidal modality is dependent on the delivery of a sufficient quantity of 10B and nth to individual cancer cells to sustain a lethal 10B(n, alpha) 7Li reaction. The current review covered the radiobiologic considerations on which BNCT is based, including a brief discussion of microdosimetry and normal tissue tolerance. The development of tumor-localizing boron compounds was discussed, including the sulfhydryl-containing polyhedral borane, sodium borocaptate (Na2B12H11SH), and boronophenylalanine (BPA), both of which are currently being used clinically in Japan as capture agents for malignant brain tumors and melanomas, respectively. Compounds currently under evaluation, such as boronated porphyrins, nucleosides, liposomes, and monoclonal antibodies (MoAbs), were also considered. Nuclear reactors have been used as the exclusive source of neutrons for BNCT. The use of low-energy (0.025 eV) thermal neutrons and higher-energy (1-10,000 eV) epithermal beams, beam optimization, and possible alternative neutron sources (accelerators) were also discussed. Clinical studies performed in the United States during the 1950s and 1960s for the treatment of malignant brain tumors were reviewed. Current studies in Japan and future studies in Europe and the United States concerning the treatment of glioblastomas and melanomas by BNCT were discussed, as were critical issues that must be addressed if BNCT is ever to be a useful therapeutic modality.

Radio imaging of the microquasar GRS 1915+105 with the Very Long Baseline Array (VLBA) over a range of wavelengths (13, 3.6, 2.0, and 0.7 cm), in different states of the black hole binary, always resolves the nucleus as a compact jet of length ~10λ cm AU. The nucleus is best imaged at the shorter wavelengths, on scales of 2.5-7 AU (0.2-0.6 mas resolution). The brightness temperature of the core is T B ≥ 10 9 K, and its properties are better fitted by a conically expanding synchrotron jet model rather than a thermal jet. The nuclear jet varies in ~30 minutes during minor X-ray/radio outbursts and reestablishes within ~18 hr of a major outburst, indicating the robustness of the X-ray/radio (or disk/jet) system to disruption. At lower resolution (80-240 AU), more extended ejecta are imaged at ~500 AU separation from the stationary core. Time-lapse images clearly detect the superluminal motion of the ejecta in a few hours. The measured velocity is 1.5 c ± 0.1 c ( D /12 kpc) for the approaching component and is consistent with ballistic motion of the ejecta from 500 AU outward, perhaps even since birth. The axis of the ejecta differs by ≤12° clockwise from the axis of the AU-scale jet, measured in the same observation. Both axes are stable in time (±5°), the AU scale for 2 yr and the large scale for over 4 yr. Astrometry over 2 yr relative to an extragalactic reference locates the black hole to ±1.5 mas, and its secular parallax due to Galactic rotation is 5.8 ± 1.5 mas yr -1 , consistent with a distance of 12 kpc. Finally, a limit of ≤100 km s -1 is placed on its proper motion with respect to its neighborhood. Some accreting black holes of stellar mass (e.g., Cyg X-1, 1E 1740-2942, GRS 1758-258, GX 339-4) and supermassive black holes at the center of galaxies (e.g., Sgr A*) lack evidence of large flares and discrete transient ejecta but have compact radio cores with steady, flat-spectrum "plateau" states, like GRS 1915+105. To the present day GRS 1915+105 is the only system where both AU-scale steady jets and large-scale superluminal ejections have been unambiguously observed. Our observations suggest that the unresolved flat-spectrum radio cores of accreting black holes are compact quasi-continuous synchrotron jets.

In the Chandra Deep Field-South 1 Ms exposure, we have found, at redshift 3.700 ± 0.005, the most distant type 2 active galactic nucleus ever detected. It is the source with the hardest X-ray spectrum with redshift z > 3. The optical spectrum has no detected continuum emission to a 3 σ detection limit of ~3 × 10 -19 ergs s -1 cm -2 Å -1 and shows narrow lines of Lyα, C IV, N V, He II, O VI, [O III], and C III]. Their FWHM line widths have a range of ~700-2300 km s -1 with an average of approximately ~1500 km s -1 . The emitting gas is metal-rich ( Z ≃ 2.5-3 Z ☉ ). In the X-ray spectrum of 130 counts in the 0.5-7 keV band, there is evidence for intrinsic absorption with N H ≳ 10 24 cm -2 . An iron Kα line with rest-frame energy and equivalent width of ~6.4 keV and ~1 keV, respectively, in agreement with the obscuration scenario, is detected at a 2 σ level. If confirmed by our forthcoming XMM-Newton observations, this would be the highest redshift detection of Fe Kα. Depending on the assumed cosmology and the X-ray transfer model, the 2-10 keV rest frame luminosity corrected for absorption is ~10 45 ± 0.5 ergs cm -2 s -1 , which makes our source a classic example of the long-sought type 2 QSO. From standard population synthesis models, these sources are expected to account for a relevant fraction of the black hole-powered QSO distribution at high redshift.

We investigate morphological structure parameters and local environments of distant moderate-luminosity active galactic nucleus (AGN) host galaxies in the overlap between the HST /ACS observations of the Great Observatories Origins Deep Survey (GOODS) and the two Chandra Deep Fields. We compute near-neighbor counts and BViz asymmetry ( A ) and concentration ( C ) indices for ≈35,500 GOODS/ACS galaxies complete to z 850 ≈ 26.6, including the resolved hosts of 322 X-ray-selected AGNs. Distributions of (1) z 850 asymmetry for 130 z 850 < 23 AGN hosts and (2) near-neighbor counts for 173 z 850 < 24 AGN hosts are both consistent with non-AGN control samples. This implies no close connection between recent galaxy mergers and moderate-luminosity AGN activity out to appreciable look-back times ( z ≲ 1.3), approaching the epoch of peak AGN activity in the universe. The distribution of z 850 C for the AGN hosts is offset by Δ C ≈ +0.5 compared to the non-AGN, a 6.4 σ discrepancy much larger than can be explained by the possible influence of unresolved emission from the AGN or a circumnuclear starburst. The local universe association between AGN and bulge-dominated galaxies thus persists to substantial look-back time. We discuss implications in the context of the low-redshift supermassive central black hole mass correlation with host galaxy properties, including concentration.

We present a table of redshifts for 2907 galaxies and stars in the 145 arcmin 2 HST ACS GOODS-North, making this the most spectroscopically complete redshift sample obtained to date in a field of this size. We also include the redshifts, where available, in a table containing just under 7000 galaxies from the ACS area with K s, AB < 24.5 measured from a deep K s image obtained with WIRCam on the CFHT, as well as in a table containing 1016 sources with NUV AB < 25 and 478 sources with FUV AB < 25.5 (there is considerable overlap) measured from the deep GALEX images in the ACS area. Finally, we include the redshifts, where available, in a table containing the 1199 24 μm sources to 80 μJy measured from the wider area Spitzer GOODS-North. The redshift identifications are greater than 90% complete to magnitudes of F 435W AB = 24.5, F 850LP AB = 23.3, and K s, AB = 21.5, and to 24 μm fluxes of 250 μJy. An extensive analysis of these data appear in a companion paper, but here we test the efficiency of color-selection techniques to identify populations of high-redshift galaxies and active galactic nuclei. We also examine the feasibility of doing tomography of the intergalactic medium with a 30 m telescope.

This manual was assembled for use as a guide for analyzing the nutrient content of seawater samples collected in the marine coastal zone of the Northeast United States and the Bering Sea. Some modifications (changes in dilution or sample pump tube sizes) may be necessary to achieve optimum measurements in very pronounced oligotrophic, eutrophic or brackish areas. Information is presented under the following section headings: theory and mechanics of automated analysis; continuous flow system description; operation of autoanalyzer system; cookbook of current nutrient methods; automated analyzer and data analysis software; computer interfacing and hardware modifications; and trouble shooting. The three appendixes are entitled: references and additional reading; manifold components and chemicals; and software listings. (JGB)

We present results from 300 ks of X-ray observations of the Chandra Deep Field-South. The field of the four combined exposures is now 0.1035 deg 2 , and we reach a flux limit of 10 -16 ergs s -1 cm -2 in the 0.5-2 keV soft band and 10 -15 ergs s -1 cm -2 in the 2-10 keV hard band, i.e., a factor of 2 fainter than the previous 120 ks exposure. The total catalog is composed of 197 sources including 22 sources detected only in the hard band, 51 only in the soft band, and 124 detected in both bands. We now have the optical spectra for 86 optical counterparts. The log N -log S relationship of the whole sample confirms the flattening with respect to the ASCA hard counts and the ROSAT soft counts. The average logarithmic slopes of the number counts are α = 0.66 ± 0.06 and α = 0.92 ± 0.12 in the soft and hard bands, respectively. Double power-law fits to the differential counts show evidence of further flattening at the very faint end to slopes of 0.5 ± 0.1 and 0.6 ± 0.2 in the soft and hard bands, respectively. We compute the total contribution to the X-ray background (XRB) in the 2-10 keV band, which now amounts to (1.45 ± 0.15) × 10 -11 ergs cm -2 s -1 deg -2 (after the inclusion of the ASCA sources to account for the bright end) to a lower flux limit of 10 -15 ergs s -1 cm -2 . This corresponds to 60%-90% of the unresolved hard XRB, given the uncertainties on its actual value. We confirm previous findings on the average spectrum of the sources, which is well described by a power law with Γ = 1.44 ± 0.03, and the progressive hardening of the sources at lower fluxes. In particular, we find that the average spectral slope of the sources is flatter than the average for fluxes lower than 9 × 10 -15 ergs s -1 cm -2 in the hard band. The hardening of the spectra is consistent with an increasing fraction of absorbed objects ( N H > 10 22 cm -2 ) at low fluxes. From 86 redshifts available at present, we find that hard sources have on average lower redshifts ( z ≤ 1) than soft sources. Their typical luminosities and optical spectra show that most of these sources are obscured active galactic nuclei (AGNs), as expected by AGN population synthesis models of the XRB. We are still in the process of finding hard sources that constitute the remaining fraction of the total XRB. Most of the sources detected only in the soft band appear to be optically normal galaxies with luminosities L X ≃ 10 40 -10 42 ergs s -1 . This population appears to be a mix of normal galaxies, possibly with enhanced star formation, and galaxies with low-level nuclear activity.

Metacognition fosters independent learning by providing personal insight into one's own thinking. Such awareness can lead to flexible and confident problem solving as well as feelings of self-efficacy and pride. This is especially important for students who encounter difficulty in school because they do not understand how to appraise and manage their own resources for learning. Too often, students develop debilitating expectations and behavior that undermine learning in school and inhibit transfer of effective learning strategies. We describe four general kinds of instruction that help students learn to think: metacognitive explanation, scaffolded instruction, cognitive coaching, and cooperative learning. Teachers can adapt and combine these methods to teach students how to think as they read, write, and compute in classrooms.

We present a new procedure to fit nonaxisymmetric flow patterns to two-dimensional velocity maps of spiral galaxies. We concentrate on flows caused by barlike or oval distortions to the total potential, which may arise either from a non-axially symmetric halo or a bar in the luminous disk. We apply our method to high-quality CO and Hα data for the nearby, low-mass spiral NGC 2976, previously obtained by Simon et al., and find that a barlike model fits the data at least as well as their model with large radial flows. We find supporting evidence for the existence of a bar in the baryonic disk. Our model suggests that the azimuthally averaged central attraction in the inner part of this galaxy is larger than estimated by these authors. It is likely that the disk is also more massive, which will limit the increase to the allowed dark halo density. Allowance for barlike distortions in other galaxies may either increase or decrease the estimated central attraction.

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 . 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 report the discovery of large-scale structures of X-ray sources in the 1 Ms observation of the Chandra Deep Field South. Two main structures appear as narrow (Δ z ≲ 0.02) spikes in the source redshift distribution at z = 0.67 and 0.73, respectively. Their angular distribution spans a region at least ~17' wide, corresponding to a physical size of 7.3 h Mpc at a redshift of z ~ 0.7 (Ω m = 0.3, Ω Λ = 0.7). These spikes are populated by 19 sources each, which are mainly identified as active galactic nuclei (AGNs). Two sources in each spike are extended in X-rays, corresponding to galaxy groups/clusters embedded in larger structures. The X-ray source redshift distribution shows other spikes, the most remarkable at z = 1.04, 1.62, and 2.57. This is one of the first evidences for large-scale structure traced by X-ray sources and for spatial clustering of X-ray-selected AGNs. The X-ray data have been complemented with the spectroscopic data from the K20 near-infrared survey (Cimatti et al.), which covers ~1/10 of the X-ray field. In this survey, too, the source redshift distribution shows several spikes. Two narrow structures at z = 0.67 and 0.73 (again with Δ z ~ 0.02) are the most significant features, containing 24 and 47 galaxies, respectively. While the K20 structure at z = 0.73 is dominated by a standard galaxy cluster with a significant concentration around a central cD galaxy and morphological segregation, the galaxies at z = 0.67 constitute a loose structure rather uniformly distributed along the K20 field. Moreover, we find a very good correlation (almost one-to-one) between less prominent peaks detected in the redshift distributions of X-ray and K20 sources. In particular, at z < 1.3 we find that five out of the six more significant K20 peaks have a corresponding peak in the X-ray-selected sources and, similarly, all five X-ray peaks below that redshift have a corresponding K20 peak. Since the K20 survey sensitivity drops beyond z ~ 1.3, structures at higher redshift are traced only by the X-ray sources. This correlation suggests that AGNs (from the X-ray data) and (early-type) galaxies (from the K20 survey) are tracing the same underlying structures. We also compared the X-ray and K -band catalogs to search for enhanced X-ray activity in the sources in the two main redshift spikes. While in the structure at z = 0.73 the fraction of X-ray sources is the same as in the field, in the structure at z = 0.67 it is higher by a factor of ~2, suggesting that X-ray activity may be triggered preferentially in the structure at z = 0.67. Given the limited statistics, this result is significant only at the ~2 σ level.

We report the discovery of a disk around the young high-mass star IRAS 20126+4104. In the NH 3 (1, 1) and (2, 2) lines, we detected a flattened disklike structure (6'' × 3'' or 10,000 × 5000 AU) roughly perpendicular to a bipolar molecular outflow. A velocity gradient exists along the major axis of the disk. The disk rotates faster toward the center. Our findings provide direct verification that disks exist during the formation of high-mass stars.