P. Tozzi

Dark Matter Substructure within Galactic Halos

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 Ways halo should contain about 500 satellites with circular velocities larger than the Draco and Ursa Minor systems, i.e., bound masses 108 M☉ and tidally limited sizes 1 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 New Photometric Technique for the Joint Selection of Star-forming and Passive Galaxies at 1.4 z 2.5

A simple two-color selection based on B-, z-, and K-band photometry is proposed for culling galaxies at 1.4 z 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 = (z - K)AB - (B - z)AB > -0.2 allows us to select actively star-forming galaxies at z 1.4, independently of their dust reddening. On the other hand, objects with BzK 2.5 colors include passively evolving galaxies at z 1.4, 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 ~1011 M☉ and sky and volume densities of ~1 arcmin-2 and ~10-4 Mpc-3, respectively. Based on their UV (reddening-corrected), X-ray, and radio luminosities, the BzK-selected star-forming galaxies with K < 20 turn out to have average SFR ≈ 200 M☉ yr-1 and median reddening E(B - 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 = 2 star-forming galaxies may be among the precursors of z = 0 early-type galaxies. A V/Vmax 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 = 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 z 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.

The Chandra Deep Field-South: Optical Spectroscopy. I.

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 1044 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 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 [LX(0.5-10 keV) > 1042 ergs s-1]. Photometric redshifts will be necessary to derive the properties and evolution of the X-ray selected EROs.

The Evolution of X-Ray Clusters and the Entropy of the Intracluster Medium

The thermodynamics of the diffuse, X-ray-emitting 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 effect 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 flat 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 first 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 find 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 fitted for the following range of values of the adiabatic constant: K* ≡ kBT/μmpρ2/3 = (0.4 ± 0.1) × 1034 ergs cm2 g-5/3 for clusters with average temperatures kT > 2 keV and K* = (0.2 ± 0.1) × 1034 ergs cm2 g-5/3 for groups and clusters with average temperatures kBT < 2 keV. These values correspond to different excess energy per particle of kBT ≥ 0.1(K*/0.4 × 1034) keV. The dependence of K* on the mass scale can be well reproduced by an epoch-dependent external entropy: the relation K* = 0.8(1 + z)-1 × 1034 ergs cm2 g-5/3 fits 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 fit 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 typical epoch and the source of the heating processes. The X-ray satellites Chandra and XMM can add to our knowledge of the history of the cosmic baryons, already derived from the high-redshift, low-density gas observed in the QSO absorption-line clouds, by imaging the hot, higher density plasma observed in groups and clusters of galaxies.

X-ray properties of galaxy clusters and groups from a cosmological hydrodynamical simulation

We present results on the X-ray properties of clusters and groups of galaxies, extracted from a large cosmological hydrodynamical simulation. We used the TREE+SPH code GADGET to simulate a concordance A cold dark matter cosmological model within a box of 192 h -1 Mpc on a side, 480 3 dark matter particles and as many gas particles. The simulation includes radiative cooling assuming zero metallicity, star formation and supernova feedback. The very high dynamic range of the simulation allows us to cover a fairly large interval of cluster temperatures. We compute X-ray observables of the intracluster medium (ICM) for simulated groups and clusters and analyse their statistical properties. The simulated mass-temperature relation is consistent with observations once we mimic the procedure for mass estimates applied to real clusters. Also, with the adopted choices of Ω m = 0.3 and σ 8 = 0.8 for matter density and power spectrum normalization, respectively, the resulting X-ray temperature functton agrees with the most recent observational determinations. The luminosity-temperature relation also agrees with observations for clusters with T ≥ 2 keV. At the scale of groups, T ≥ 1 keV, we find no change of slope in this relation. The entropy in central cluster regions is higher than predicted by gravitational heating alone, the excess being almost the same for clusters and groups. We also find that the simulated clusters appear to have suffered some overcooling. We find f * ≃ 0.2 for the fraction of baryons in stars within clusters, thus approximately twice as large as the value observed. Interestingly, temperature profiles of simulated clusters are found to increase steadily toward cluster centres. They decrease in the outer regions, much like observational data do at r ≥ 0.2r vir , while not showing an isothermal regime followed by a smooth temperature decline in the innermost regions. Our results thus demonstrate the need for yet more efficient sources of energy feedback and/or the need to consider additional physical process which may be able to further suppress the gas density at the scale of poor clusters and groups, and, at the same time, to regulate the cooling of the ICM in central regions.

The Chandra Deep Field-South: The 1 Million Second Exposure*

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.

Measuring Ωm with the ROSAT Deep Cluster Survey

We analyze the ROSAT Deep Cluster Survey (RDCS) to derive cosmological constraints from the evolution of the cluster X-ray luminosity distribution. The sample contains 103 galaxy clusters out to z 0.85 and flux limit Flim = 3 × 10-14 ergs s-1 cm-2 (RDCS-3) in the [0.5-2.0] keV energy band, with a high-redshift extension containing four clusters at 0.90 ≤ z ≤ 1.26 and brighter than Flim = 1 × 10-14 ergs s-1 cm-2 (RDCS-1). We assume cosmological models to be specified by the matter density parameter Ωm, the rms fluctuation amplitude at the 8 h-1 Mpc scale σ8, and the shape parameter for the cold dark matter-like power spectrum Γ. Model predictions for the cluster mass function are converted into the X-ray luminosity function in two steps. First, we convert mass into intracluster gas temperature by assuming hydrostatic equilibrium. Then, temperature is converted into X-ray luminosity by using the most recent data on the LX-TX relation for nearby and distant clusters. These include the Chandra data for six distant clusters at 0.57 ≤ z ≤ 1.27. From RDCS-3 we find Ωm = 0.35 and σ8 = 0.66 for a spatially flat universe with a cosmological constant, with no significant constraint on Γ (errors correspond to 1 σ confidence levels for three fitting parameters). Even accounting for both theoretical and observational uncertainties in the mass-X-ray luminosity conversion, an Einstein-de Sitter model is always excluded at far more than the 3 σ level. We also show that the number of X-ray-bright clusters in RDCS-1 at z > 0.9 is expected from the evolution inferred at z < 0.9 data.

The Assembly History of Field Spheroidals: Evolution of Mass-to-Light Ratios and Signatures of Recent Star Formation

We present a comprehensive catalog of high signal-to-noise ratio spectra obtained with DEIMOS on the Keck II telescope for a sample of F850LP 2 for high-mass spheroidals and zf ~ 1.2 for lower mass systems, a more realistic picture is that most of the stellar mass formed in all systems at z > 2 with subsequent activity continuing to lower redshifts (z < 1.2). The fraction of stellar mass formed at recent times depends strongly on galactic mass, ranging from <1% for masses above 1011.5 M☉ to 20%-40% below 1011 M☉. Independent support for recent activity is provided by spectroscopic ([O II] emission, Hδ) and photometric (blue cores and broadband colors) diagnostics. Via the analysis of a large sample with many independent diagnostics, we are able to reconcile previously disparate interpretations of the assembly history of field spheroidals. We discuss the implications of this measurement for the determination of the evolution of the number density of E+S0 galaxies, suggesting that number density evolution of the morphologically selected population has occurred since z ~ 1.2.

A Classic Type 2 QSO

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 A-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 NH 1024 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 ~1045 ± 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.

The Extended Chandra Deep Field-South Survey: Chandra Point-Source Catalogs

We present Chandra point-source catalogs for the Extended Chandra Deep Field-South (E-CDF-S) survey. The E-CDF-S consists of four contiguous 250 ks Chandra observations covering an approximately square region of total solid angle ≈0.3 deg2, which flank the existing ≈1 Ms Chandra Deep Field-South (CDF-S). The survey reaches sensitivity limits of ≈1.1 × 10-16 and ≈6.7 × 10-16 ergs cm-2 s-1 for the 0.5-2.0 and 2-8 keV bands, respectively. We detect 762 distinct X-ray point sources within the E-CDF-S exposure; 589 of these sources are new (i.e., not previously detected in the ≈1 Ms CDF-S). This brings the total number of X-ray point sources detected in the E-CDF-S region to 915 (via the E-CDF-S and ≈1 Ms CDF-S observations). Source positions are determined using matched-filter and centroiding techniques; the median positional uncertainty is ≈035. The basic X-ray and optical properties of these sources indicate a variety of source types, although absorbed active galactic nuclei (AGNs) seem to dominate. In addition to our main Chandra catalog, we constructed a supplementary source catalog containing 33 lower significance X-ray point sources that have bright optical counterparts (R < 23). These sources generally have X-ray-to-optical flux ratios expected for normal and starburst galaxies, which lack a strong AGN component. We present basic number-count results for our main Chandra catalog and find good agreement with the ≈1 Ms CDF-S for sources with 0.5-2.0 and 2-8 keV fluxes greater than 3 × 10-16 and 1 × 10-15 ergs cm-2 s-1, respectively. Furthermore, three extended sources are detected in the 0.5-2.0 keV band, which are found to be likely associated with galaxy groups or poor clusters at z ≈ 0.1-0.7; these have typical rest-frame 0.5-2.0 keV luminosities of (1-5) × 1042 ergs s-1.