arXiv subjects

P. Tozzi

Publications and source records attributed to P. Tozzi.

At least 163 records · Page 9Linked to original sources

arXiv2003

Tracing the large scale structure in the Chandra Deep Field South

We report the discovery of large scale structures of X-ray sources in the 1Msec observation of the Chandra Deep Field South. Two main structures appear as narrow (Delta_z < 0.02) spikes in the source redshift distribution at z=0.67 and z=0.73, respectively. Their angular distribution spans a region at least ~ 17 arcmin wide, corresponding to a physical size of 7.3 h_{70}^{-1} Mpc at a redshift of z ~ 0.7 (Omega_m=0.3, Omega_{Lambda}=0.7). These spikes are populated by 19 sources each, which are mainly identified as Active Galactic Nuclei (AGN). 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 AGN. The X-ray data have been complemented with the spectroscopic data from the K20 near infrared survey (Cimatti et al. 2002), which covers ~1/10 of the X-ray field. Also in this survey the source redshift distribution shows several spikes. Two narrow structures at z=0.67 and z=0.73 (again with Delta_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.[abridged]

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arXiv2003

Constraining the cosmological parameters with the gas mass fraction in local and z>0.7 Galaxy Clusters

We present a study of the baryonic fraction in galaxy clusters aimed at constraining the cosmological parameters Omega_m, Omega_Lambda and the ratio between the pressure and density of the ``dark'' energy, w. We use results on the gravitating mass profiles of a sample of nearby galaxy clusters observed with the BeppoSAX X-ray satellite (Ettori, De Grandi, Molendi 2002) to set constraints on the dynamical estimate of Omega_m. We then analyze Chandra observations of a sample of eight distant clusters with redshift in the range 0.72 and 1.27 and evaluate the geometrical limits on the cosmological parameters Omega_m, Omega_Lambda and w by requiring that the gas fraction remains constant with respect to the look-back time. By combining these two independent probability distributions and using a priori distributions on both Omega_b and H0 peaked around primordial nucleosynthesis and HST-Key Project results respectively, we obtain that, at 95.4 per cent level of confidence, (i) w < -0.49, (ii) Omega_m = 0.34^+0.11_-0.05, Omega_Lambda = 1.30^+0.44_-1.09 for w=-1 (corresponding to the case for a cosmological constant), and (iii) Omega_m = 1-Omega_Lambda = 0.33^+0.07_-0.05 for a flat Universe. These results are in excellent agreement with the cosmic concordance scenario which combines constraints from the power spectrum of the Cosmic Microwave Background, the galaxy and cluster distribution, the evolution of the X-ray properties of galaxy clusters and the magnitude-redshift relation for distant type Ia supernovae. By combining our results with the latter method we further constrain Omega_Lambda =0.94^+0.28_-0.32 and w < -0.89 at the 2 sigma level.

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arXiv2002

Moderate Temperature Clusters of Galaxies from the RDCS and the High Redshift Luminosity-Temperature Relation

We present our discovery observations and analysis of RDCS1317+2911, z = 0.805, and RDCS1350+6007, z= 0.804, two clusters of galaxies identified through X-ray emission in the ROSAT Deep Cluster Survey (RDCS). We find a temperature of 3.7 +1.5 -0.9 keV and a bolometric luminosity of 8.2e43 +1.7e43 -1.6e43 erg/s for RDCS1317+2911, and a temperature of 4.9 +1.3 -0.9 keV and a bolometric luminosity of 4.1e44 +0.5e44 -0.4e44 erg/s for RDCS1350+6007. Our weak lensing analysis of RDCS1350+6007 confirms the general shape of the inner density profile but predicts twice the mass of the model based on the X-ray profile. We combine the X-ray luminosities and temperatures for RDCS clusters of galaxies with such measurements of other clusters at high redshift (z>0.7) and fit the luminosity-temperature relation. We find no statistically significant evolution in the slope or zero-point of this relation at a median of z=0.83. This result is in agreement with models of intracluster medium evolution with significant pre-heating or high initial entropy values. We discuss how low temperature, high redshift clusters of galaxies will allow us to improve on this result and announce the discovery of two such objects, CXOU J0910.1+5419 and CXOU J1316.9+2914.

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arXiv2002

The effect of non--gravitational gas heating in groups and clusters of galaxies

We present a set of gas-dynamical simulations of galaxy groups and clusters aimed at exploring the effect of non-gravitational heating. We use GASOLINE, a parallel Tree+SPH code, to simulate the formation of four cosmic halos with temperature 0.5<T<8 keV. Non-gravitational heating is implemented in two different ways: (1) by imposing a minimum entropy floor at a given redshift, 1<z<5; (2) by gradually heating gas, proportionally to the SN rate expected from semi-analytical modeling of galaxy formation. Our main results are the following. (a) An extra heating energy of about 1 keV per gas particle is required to reproduce the observed Lx-T relation, independent of whether it is provided so as to create an entropy floor of 50-100 keV cm^2, or is modulated in redshift; our SN feedback recipe provides only 1/3 keV/part. (b) The M-T relation is almost unaffected by non-gravitational heating and follows the M T^{3/2} scaling, with a normalization ~40% higher than observed, independent of the heating scheme. The inclusion of cooling in a run of a small group has the effects of increasing T_ew by ~30%, possibly reconciling simulated and observed M-T relations, and of decreasing Lx by ~40%. In spite of the inclusion of SN feedback energy, almost 40% of the gas becomes cold, in excess of current observational estimates. (abridged)

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arXiv2002

A Classic Type 2 QSO

In the Chandra Deep Field South 1Msec exposure we have found, at redshift 3.700 +- 0.005, the most distant Type 2 AGN 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 3sigma detection limit of ~3 10^{-19} ergs/s/cm^2/AA and shows narrow lines of Ly_alpha, CIV, NV, HeII, OVI, [OIII], and CIII]. Their FWHM line widths have a range of ~700-2300 km/s with an average of approximately ~1500 km/s. The emitting gas is metal rich (Z ~2.5-3 Z_solar). 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 K_alpha 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 2sigma level. If confirmed by our forthcoming XMM observations this would be the highest redshift detection of FeK_alpha. 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/s, which makes our source a classic example of the long sought Type 2 QSOs. 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.

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arXiv2001

Chandra Deep Field South: The 1Msec Catalog

In this Paper we present the source catalog obtained from a 942 ks exposure of the Chandra Deep Field South (CDFS), using ACIS-I on the Chandra X-ray Observatory. 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 5.5E-17 erg/s/cm^2 and 4.5E-16 erg/s/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 to a depth of R~26.5 (Vega). We found that the R-band/Chandra offsets are small, ~1 arcsec. Coordinate cross-correlation finds 85% of the Chandra sources covered in R to have counterparts within the 3-sigma error box (>~1.5 arcsec 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.

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arXiv2001

Agns and Clusters in Chandra Deep Fields

The Chandra X-ray Satellite already observed several deep fields, including the two 1 Megasec exposures of the Chandra Deep Field South (CDFS) and North. We review here the main findings from the CDFS. The LogN-LogS relations show the resolution of the X-ray background into point sources at the level of 83-99% in the 1-2 keV band and 65-98% in the 2-10 keV band, given the uncertainties in the unresolved value. The so called ``spectral paradox'' is solved by a hard, faint population of sources constituted mostly by nearby (z<1) absorbed (Type II) AGNs with hard-band luminosities L~10^{42}-10^{44} erg/s. When comparing these results to other deep fields in the X-ray band, we find that the AGNs detected in 0.1 deg^2 of the CDFS are representative of the AGNs population as a whole. However, we also noticed an excess in the hard counts in two Chandra deep fields. If we include this excess and average it among the observed fields, the total contribution of the XRB can grow of about 7%. Finally, we discuss briefly the properties of the Intra Cluster Medium imaged at high z, showing no evolution in clusters properties up to z~1.

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arXiv2001

The Chandra Deep Field South: the 1 Million Second

We 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 5.5e-17 erg/cm^2/s in the soft 0.5-2 keV band and 4.5e-16 erg/cm^2/s in the hard 2-10 keV band. The hard band LogN-LogS shows a significant flattening (slope~=0.6) below ~1e-14 erg/cm^2/s, 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 LogN-LogS (slope ~=1.3) down to 1e-15 erg/cm^2/s. 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.

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arXiv2001

HST Imaging in the Chandra Deep Field South: II. WFPC2 Observations of an X-Ray Flux-Limited Sample from the 1 Msec Chandra Catalog

We present HST/WFPC2 observations of a well-defined sample of 40 X-ray sources with X-ray fluxes above the detection threshold of the full 1 Msec Chandra Deep Field South (CDFS). The sensitivity and spatial resolution of our HST observations are sufficient to detect the optical counterparts of 37 of the X-ray sources, yielding information on their morphologies and environments. In this paper we extend the results obtained in our previous study on the 300 ks CDFS X-ray data (Schreier et al. 2001, Paper I). Specifically, we show that the optical counterparts to the X-ray sources are divided into two distinct populations: 1) an optically faint group with relatively blue colors, similar to the faint blue field galaxy population, and 2) an optically brighter group, including resolved galaxies with average colors significantly redder than the corresponding bright field galaxy population. The brighter objects comprise a wide range of types, including early and late type galaxies, starbursts, and AGN. By contrast, we show that the faint blue X-ray population are most consistent with being predominantly Type 2 AGN of low to moderate luminosity, located at higher redshifts (z ~ 1 - 2). This conclusion is supported by luminosity function models of the various classes of objects. Hence, the combination of deep X-ray data with the high spatial resolution of HST are for the first time allowing us to probe the faint end of the AGN luminosity function at cosmologically interesting redshifts.

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arXiv2001

X-ray temperature and morphology of z>0.8 clusters of galaxies

We discuss our current progress in studying a sample of z>0.8 clusters of galaxies from the ROSAT Distant Cluster Survey. To date, we have Chandra observations for four of the ten clusters. We find that the morphology of two of these four are quite regular, with deviations from circular of less than 5%, while two are strikingly elliptical. When the temperatures and luminosities of our sample are grouped with six other high-redshift measurements, there is no measured evolution in the luminosity-temperature relation. We identify a number of X-ray emitting point sources that are potential cluster members. These could be sources of intracluster medium heating, adding the entropy necessary to explain the cluster luminosity-temperature relation.

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arXiv2001

Pre-heating the ICM in high resolution simulations: the effect on the gas entropy

We present results from high-resolution Tree+SPH simulations of galaxy clusters and groups, aimed at studying the effect of non-gravitational heating on the entropy of the ICM. We simulate three systems, having emission-weighted temperature T=0.6,1 and 3 keV, with spatial resolution better than 1% of the virial radius. We consider the effect of different prescriptions for non-gravitational ICM heating, such as SN energy feedback, as predicted by semi-analytical models of galaxy formation, and two different minimum entropy floors, S_fl=50 and 100 keV cm^2, imposed at z=3. Simulations with only gravitational heating nicely reproduce predictions from self-similar ICM models, while extra heating is shown to break the self-similarity, by a degree which depends on total injected energy and on cluster mass. We use observational results on the excess entropy in central regions of galaxy systems, to constrain the amount of extra-heating required. We find that setting the entropy floor S_fl=50 keV cm^2, which corresponds to an extra heating energy of about 1 keV per particle, is able to reproduce the observed excess of ICM entropy.

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arXiv2001

New Results from the X-ray and Optical Survey of the Chandra Deep Field South: The 300ks Exposure

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} erg s^{-1} cm^{-2} in the 0.5-2 keV soft band and 10^{-15} erg s^{-1} cm^{-2} in the 2-10 keV hard band, thus a factor 2 fainter than the previous 120 ks exposure. The total catalogue 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 have now the optical spectra for 86 optical counterparts. We compute the total contribution to the X-ray background in the 2-10 keV band, which now amounts to (1.45\pm 0.15)*10^{-11} erg cm^{-2} s^{-1} deg^{-2} (after the inclusion of the ASCA sources to account for the bright end) to a flux limit of 10^{-15} erg s^{-1} cm^{-2}. This corresponds to 60-90% of the unresolved hard X-ray background (XRB), given the uncertainties on its actual value.[ABRIDGED]

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arXiv2001

Measuring $Ω_m$ 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} cgs (RDCS-3) in the [0.5-2.0] keV energy band, with a high-z extension containing four clusters at 0.90 1 10^{-14} cgs (RDCS-1). Model predictions for the cluster mass function are converted into the X-ray luminosity function in two steps. First we convert mass into intra-cluster gas temperature by assuming hydrostatic equilibrium. Then temperature is converted into X-ray luminosity by using the most recent data on the Lx-T relation for nearby and distant clusters. These include the Chandra data for seven distant clusters at 0.57<z<1.27. From RDCS-3 we find Ω_m=0.35+/-0.12 and σ_8=0.66+/-0.06 for a spatially flat Universe with cosmological constant, with no significant constraint on Γ. Even accounting for theoretical and observational uncertainties in the mass/X-ray luminosity conversion, an Einstein-de-Sitter model is always excluded at far more than the 3sigma level.

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arXiv2001

RX J0848+4456: Disentangling a Moderate Redshift Cluster

We present a multi-wavelength study of RX J0848+4456, a cluster of galaxies discovered through X-ray emission in the ROSAT Deep Cluster Survey. Our observations consist of WFPC2 imaging, optical spectra, and X-ray data collected with the Chandra observatory. We find that RX J0848+4456 consists of an X-ray emitting cluster of galaxies at a redshift of z=0.570 and a group at slightly lower redshift, z=0.543, with little X-ray emission. This lower redshift system, however, is a gravitational lens, with the lensed galaxy an unusual AGN or star-forming system at z=3.356. The cluster has an X-ray temperature of kT = 3.6 +/- 0.4 keV, a bolometric luminosity of 1.0e44 +/- 0.3e44 erg/s and a velocity dispersion of 670 +/- 50 km/s. These values all agree with the low redshift correlations for clusters of galaxies, implying a relaxed system with the ICM in equilibrium with the dark matter potential. The lower redshift group of galaxies at z=0.543 has, at most 1/5, more likely 1/10, of the X-ray luminosity of RX J0848+4456. Despite being a gravitational lens, this is a low mass system, with an X-ray temperature of kT = 2.3 +0.5 -0.4 keV and a velocity dispersion of only 430 +/- 20 km/s. Our observations show the importance of detailed studies of clusters of galaxies when using them as probes of cosmological mass functions.

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arXiv2001

HST Imaging in the Chandra Deep Field South: I. Multiple AGN Populations

We present preliminary results from imaging three HST/WFPC2 fields in V and I within the Chandra Deep Field South (CDFS). HST's sensitivity and resolution are sufficient to reveal optical counterparts for 24 of the 26 CDFS X-ray sources detected in the 300 ksec X-ray catalog and to determine the morphologies of most of these. We find that the X-ray sources comprise two apparently distinct populations of optical candidates: one optically faint (I > 24) with V - I colors consistent with the I > 24 field population; the other significantly brighter (I < 22) with colors redder than the I < 22 field population. More than 2/3 of the X-ray source counterparts are resolved galaxies. The brighter sources are mostly AGN, based on their high X-ray luminosity. The optically resolved sources in the brighter population have a very narrow range of V - I color and appear to be a mix of both late and early type morphologies at low to moderate redshift. We show that the second population, with fainter optical counterparts, can be explained as higher redshift Type 2 AGN.

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arXiv2001

First Results from the X-ray and Optical Survey of the Chandra Deep Field South

We present our first results from 130 ks of X-ray observations obtained with the Advanced CCD Imaging Spectrometer on the Chandra X-ray Observatory. We reach a flux of 2 X 10^(-16) erg s^(-1) cm^(-2) in the 0.5-2 keV soft band and 2 X 10^(-15) erg s^(-1) cm^(-2) in the 2-10 keV hard 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 dataset 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 order of 1'' in the central 6'. Optical identifications are found for ~90% of the sources. We obtain the cumulative spectra of the faint and bright X-ray sources in the sample. A power law fit in the range 2-10 keV using the galactic value of N_H ~ 8 x 10^(19) cm^(-2), yields a photon index of Gamma = 1.70+-0.06 and 1.35+-0.10 for the bright and faint sample respectively, showing a flattening of the spectrum at lower fluxes. We discuss the LogN-LogS relationship and the discrete source contribution to the integrated X-ray sky flux. In the soft band, the sources now detected at fluxes below 10^(-15) erg s^(-1) cm^(-2) contribute (4.0 +- 0.3) X 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 X 10^(-15) erg s^(-1) cm^(-2) now contributes a total of 1.6 X 10^(-11) erg cm^(-2) s^(-1) deg^(-2) which amounts to a fraction of 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). (ABRIDGED)

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arXiv2000

The Intracluster Medium in z > 1 Galaxy Clusters

The Chandra X-ray Observatory was used to obtain a 190 ks image of three high redshift galaxy clusters in one observation. The results of our analysis of these data are reported for the two z > 1 clusters in this Lynx field, including the most distant known X-ray selected cluster. Spatially-extended X-ray emission was detected from both these clusters, indicating the presence of hot gas in their intracluster media. A fit to the X-ray spectrum of RX J0849+4452, at z=1.26, yields a temperature of kT = 5.8^{+2.8}_{-1.7} keV. Using this temperature and the assumption of an isothermal sphere, the total mass of RX J0849+4452 is found to be 4.0^{+2.4}_{-1.9} X 10^{14} h_{65}^{-1} M_{\sun} within r = 1 h_{65}^{-1} Mpc. The T_x for RX J0849+4452 approximately agrees with the expectation based on its L_{bol} = 3.3^{+0.9}_{-0.5} X 10^{44}$ erg s$^{-1} according to the low redshift L_x - T_x relation. The very different distributions of X-ray emitting gas and of the red member galaxies in the two z > 1 clusters, in contrast to the similarity of the optical/IR colors of those galaxies, suggests that the early-type galaxies mostly formed before their host clusters.

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arXiv1999

The ROSAT Deep Cluster Survey: Constraints on Cosmology

We use the ROSAT Deep Cluster Survey (RDCS) to trace the evolution of the cluster abundance out to $z\simeq 0.8$ and constrain cosmological models. We resort to a phenomenological prescription to convert masses into $X$-ray fluxes and apply a maximum-likelihood approach to the RDCS redshift- and luminosity-distribution. We find that, even changing the shape and the evolution on the $L_{bol}$-$T_X$ relation within the observational uncertainties, a critical density Universe is always excluded at more than $3σ$ level. By assuming a non-evolving $X$-ray luminosity-temperature relation with shape $L_{bol}\propto T_X^3$, it is $Ω_m=0.35^{+0.35}_{-0.25}$ and $σ_8=0.76^{+0.38}_{-0.14}$ for flat models, with uncertainties corresponding to $3σ$ confidence levels.

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