We use numerical simulations to examine the substructure within galactic and cluster mass halos that form within a hierarchical universe. Clusters are easily reproduced with a steep mass spectrum of thousands of substructure clumps that closely matches the observations. However, the survival of dark matter substructure also occurs on galactic scales, leading to the remarkable result that galaxy halos appear as scaled versions of galaxy clusters. The model predicts that the virialized extent of the Milky Way's halo should contain about 500 satellites with circular velocities larger than the Draco and Ursa Minor systems, i.e., bound masses տ10 8 M , and tidally limited sizes տ1 kpc. The substructure clumps are on orbits that take a large fraction of them through the stellar disk, leading to significant resonant and impulsive heating. Their abundance and singular density profiles have important implications for the existence of old thin disks, cold stellar streams, gravitational lensing, and indirect/ direct detection experiments.

A simple two-color selection based on B-, z-, and K-band photometry is proposed for culling galaxies at 1:4 P z P 2:5 in K-selected samples and classifying them as star-forming or passive systems. The method is calibrated on the highly complete spectroscopic redshift database of the K20 survey, verified with simulations and tested on other data sets. Requiring BzK ¼ (z À K ) AB À (B À z) AB > À0:2 allows us to select actively starforming galaxies at z k1:4, independently of their dust reddening. On the other hand, objects with BzK < À0:2 and (z À K ) AB > 2:5 colors include passively evolving galaxies at z k1:4, often with spheroidal morphologies. Simple recipes to estimate the reddening, star formation rates (SFRs), and masses of BzK-selected galaxies are derived and are calibrated on K < 20 galaxies. These K < 20 galaxies have typical stellar masses of $10 11 M and sky and volume densities of $1 arcmin À2 and $10 À4 Mpc À3 , respectively. Based on their UV (reddeningcorrected ), X-ray, and radio luminosities, the BzK-selected star-forming galaxies with K < 20 turn out to have average SFR % 200 M yr À1 and median reddening E(B À V ) $ 0:4. This SFR is a factor of 10 higher than that of z $1 dusty extremely red objects, and a factor of 3 higher than found for z $ 2 UV-selected galaxies, both at similar K limits. Besides missing the passively evolving galaxies, the UV selection appears to miss some relevant fraction of the z $ 2 star-forming galaxies with K < 20, and hence of the (obscured) SFR density at this redshift. The high SFRs and masses add to other existing evidence that these z ¼ 2 star-forming galaxies may be among the precursors of z ¼ 0 early-type galaxies. A V/V max test suggests that such a population may be increasing in number density with increasing redshift. Theoretical models cannot reproduce simultaneously the space density of both passively evolving and highly star-forming galaxies at z ¼ 2. In view of Spitzer Space Telescope observations, an analogous technique based on RJL photometry is proposed to complement the BzK selection and to identify massive galaxies at 2:5 P z P 4:0. By selecting passively evolving galaxies as well as actively star-forming galaxies (including strongly dust-reddened ones), these color criteria should help in completing the census of the stellar mass and of the SFR density at high redshift.

We present source catalogs for the 4 Ms Chandra Deep Field-South (CDF-S), which is the deepest Chandra survey to date and covers an area of 464.5 arcmin 2 . We provide a main Chandra source catalog, which contains 740 X-ray sources that are detected with WAVDETECT at a false-positive probability threshold of 10 −5 in at least one of three X-ray bands (0.5-8 keV, full band; 0.5-2 keV, soft band; and 2-8 keV, hard band) and also satisfy a binomial-probability source-selection criterion of P < 0.004 (i.e., the probability of sources not being real is less than 0.004); this approach is designed to maximize the number of reliable sources detected. A total of 300 main-catalog sources are new compared to the previous 2 Ms CDF-S main-catalog sources. We determine X-ray source positions using centroid and matched-filter techniques and obtain a median positional uncertainty of ≈ 0.42 ′′ . We also provide a supplementary catalog, which consists of 36 sources that are detected with WAVDETECT at a false-positive probability threshold of 10 −5 , satisfy the condition of 0.004 < P < 0.1, and have an optical counterpart with R < 24. Multiwavelength identifications, basic optical/infrared/radio photometry, and spectroscopic/photometric redshifts are provided for the X-ray sources in the main and supplementary catalogs. 716 (≈ 97%) of the 740 main-catalog sources have multiwavelength counterparts, with 673 (≈ 94% of 716) having either spectroscopic or photometric redshifts. The 740 main-catalog sources span broad ranges of full-band flux and 0.5-8 keV luminosity; the 300 new main-catalog sources span similar ranges although they tend to be systematically lower. Basic analyses of the X-ray and multiwavelength properties of the sources indicate that > 75% of the main-catalog sources are AGNs; of the 300 new main-catalog sources, about 35% are likely normal and starburst galaxies, reflecting the rise of normal and starburst galaxies at the very faint flux levels uniquely accessible to the 4 Ms CDF-S. Near the center of the 4 Ms CDF-S (i.e., within an off-axis angle of 3 ′ ), the observed AGN and galaxy source densities have reached 9800 +1300 −1100 deg −2 and 6900 +1100 −900 deg −2 , respectively. Simulations show that our main catalog is highly reliable and is reasonably complete. The mean backgrounds (corrected for vignetting and exposure-time variations) are 0.063 and 0.178 count Ms −1 pixel −1 (for a pixel size of 0.492 ′′ ) for the soft and hard bands, respectively; the majority of the pixels have zero background counts. The 4 Ms CDF-S reaches on-axis flux limits of ≈ 3.2 × 10 −17 , 9.1 × 10 −18 , and 5.5 × 10 −17 erg cm −2 s −1 for the full, soft, and hard bands, respectively. An increase in the CDF-S exposure time by a factor of ≈ 2-2.5 would provide further significant gains and probe key unexplored discovery space.

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...

We present a detailed X-ray spectral analysis of the sources in the 1Ms catalog of the Chandra Deep Field South (CDFS) taking advantage of optical spectroscopy and photometric redshifts for 321 extragalactic sources out of the total sample of 347 sources. As a default spectral model, we adopt a power law with slope Γ with an intrinsic redshifted absorption N H , a fixed Galactic absorption and an unresolved Fe emission line. For 82 X-ray bright sources, we are able to perform the X-ray spectral analysis leaving both Γ and N H free. The weighted mean value for the slope of the power law is Γ 1.75 ± 0.02, and the distribution of best fit values shows an intrinsic dispersion of σ int 0.30. We do not find hints of a correlation between the spectral index Γ and the intrinsic absorption column density N H . We then investigate the absorption distribution for the whole sample, deriving the N H values in faint sources by fixing Γ = 1.8. We also allow for the presence of a scattered component at soft energies with the same slope of the main power law, and for a pure reflection spectrum typical of Compton-thick AGN. We detect the presence of a scattered soft component in 8 sources; we also identify 14 sources showing a reflection-dominated spectrum. The latter are referred to as Compton-thick AGN candidates. By correcting for both incompleteness and sampling-volume effects, we recover the intrinsic N H distribution representative of the whole AGN population, f (N H )dN H , from the observed one. f (N H ) shows a lognormal shape, peaking around log(N H ) 23.1 and with σ 1.1. Interestingly, such a distribution shows continuity between the population of Compton-thin and that of Compton-thick AGN. We find that the fraction of absorbed sources (with N H > 10 22 cm −2 ) in the sample is constant (at the level of about 75%) or moderately increasing with redshift. Finally, we compare the optical classification to the X-ray spectral properties, confirming that the correspondence of unabsorbed (absorbed) X-ray sources to optical type I (type II) AGN is accurate for at least 80% of the sources with spectral identification (1/3 of the total X-ray sample).

The thermodynamics of the di †use, X-rayÈemitting gas in clusters of galaxies is determined by gravitational processes associated with infalling gas, shock heating and adiabatic compression, and nongravitational processes such as heating by supernovae, stellar winds, activity in central galactic nuclei, and radiative cooling. The e †ect of gravitational processes on the thermodynamics of the intracluster medium (ICM) can be expressed in terms of the ICM entropy. The entropy is a convenient variable as long as cooling is negligible, since it remains constant during the phase of adiabatic compression during accretion into the potential well, and it shows a single steplike increase during shock heating. Observations indicate that nongravitational processes also play a key role in determining the distribution of entropy in the ICM. In particular, an entropy excess with respect to that produced by purely gravitational processes has been recently detected in the centers of low-temperature systems. This type of entropy excess is believed to be responsible for many other properties of local X-ray clusters, including the L -T relation and the Ñat density cores in clusters and groups.In this paper we assume that the entropy excess is present in the intergalactic medium (IGM) baryons before the gas is accreted by the dark matter halos and reaches high densities. We use a generalized spherical model to compute the X-ray properties of groups and clusters for a range of initial entropy levels in the IGM and for a range of mass scales, cosmic epochs, and background cosmologies. In particular, we follow the formation of adiabatic cores during the Ðrst stages of the gravitational collapse and the subsequent evolution of the central entropy due to radiative energy loss. The model predicts the statistical properties of the cluster population at a given epoch and also allows study of the evolution of single X-ray halos as a function of their age.We Ðnd that the statistical properties of the X-ray clusters strongly depend on the value of the initial background entropy. Assuming a constant, uniform value for the background entropy, the present-day X-ray data are well Ðtted for the following range of values of the adiabatic constant : K * 4 ergs cm2 g~5@3 for clusters with average temperatures kT [ 2 keV and k B T /km p o2@3 \ (0.4^0.1) ] 1034 ergs cm2 g~5@3 for groups and clusters with average temperatures keV. K * \ (0.2^0.1) ] 1034 k B T \ 2 These values correspond to di †erent excess energy per particle of keV. The k B T º 0.1(K * /0.4 ] 1034) dependence of on the mass scale can be well reproduced by an epoch-dependent external entropy : K * the relation ergs cm2 g~5@3 Ðts the data over the whole temperature range.The model can be extended to include internal heating, but in this case the energy budget required to Ðt the X-ray properties would be much higher. Observations of both local and distant clusters can be used to trace the distribution and the evolution of the entropy in the cosmic baryons and to constrain the typi...

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...