arXiv subjects

S. Borgani

Publications and source records attributed to S. Borgani.

At least 163 records · Page 9Linked to original sources

arXiv2004

Thermal Conduction in Simulated Galaxy Clusters

We study the formation of clusters of galaxies using high-resolution hydrodynamic cosmological simulations that include the effect of thermal conduction with an effective isotropic conductivity of 1/3 the classical Spitzer value. We find that, both for a hot ($T_{\rm ew}\simeq 12$ keV) and several cold ($T_{\rm ew}\simeq 2$ keV) galaxy clusters, the baryonic fraction converted into stars does not change significantly when thermal conduction is included. However, the temperature profiles are modified, particularly in our simulated hot system, where an extended isothermal core is readily formed. As a consequence of heat flowing from the inner regions of the cluster both to its outer parts and into its innermost resolved regions, the entropy profile is altered as well. This effect is almost negligible for the cold cluster, as expected based on the strong temperature dependence of the conductivity. Our results demonstrate that while thermal conduction can have a significant influence on the properties of the intra--cluster medium of rich galaxy clusters, it appears unlikely to provide by itself a solution for the overcooling problem in clusters, or to explain the current discrepancies between the observed and simulated properties of the intra--cluster medium.

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arXiv2003

Scaling laws in X-ray Galaxy Clusters at redshift between 0.4 and 1.3

(Abriged) We present a study of the integrated physical properties of a sample of 28 X-ray galaxy clusters observed with Chandra at redshift between 0.4 and 1.3. In particular, we have twelve objects in the redshift range 0.4-0.6, five between 0.6 and 0.8, seven between 0.8 and 1 and four at z>1, compounding the largest sample available for such a study. We focus particularly on the properties and evolution of the X-ray scaling laws. We fit both a single and a double beta-model with the former which provides a good representation of the observed surface brightness profiles, indicating that these clusters do not show any significant excess in their central brightness. By using the best-fit parameters of the beta-model together with the measured emission-weighted temperature (in the range 3-11 keV), we recover gas luminosity, gas mass and total gravitating mass out to R500. We observe scaling relations steeper than expected from self-similar model by a significant (>3 sigma) amount in the L-T and Mgas-T relations. We detect hints of negative evolution in the L-T, Mgas-T and L-Mtot relations, thus suggesting that systems at higher redshift have lower X-ray luminosity and gas mass for fixed temperature. In particular, when the 16 clusters at z>0.6 are considered, the evolution becomes more evident and its power-law modelization is a statistically good description of the data.

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arXiv2003

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 hydrodynamical simulation. We used the GADGET code to simulate a LambdaCDM model within a box of 192 Mpc/h on a side, with 480^3 dark matter particles and as many gas particles. The simulation includes radiative cooling, star formation and supernova feedback. The simulated M-T relation is consistent with observations once we mimic the procedure for mass estimates applied to real clusters. Also, with the adopted choices of Omega_m=0.3 and sigma_8=0.8 the resulting XTF agrees with observational determinations. The L-T relation also agrees with observations for clusters with T>2 keV, with no change of slope at the scale of groups. The entropy in central cluster regions is higher than predicted by gravitational heating alone, the excess being almost the same for clusters and groups. The simulated clusters appear to have suffered some overcooling, with f*~0.2, thus about twice as large as the value observed. Interestingly, temperature profiles are found to steadily increase toward cluster centers. 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.

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arXiv2003

Chandra and XMM-Newton Observations of RDCS1252.9-2927, A Massive Cluster at z=1.24

We present deep Chandra and XMM obervations of the galaxy cluster RDCS1252.9-2927, which was selected from the ROSAT Deep Cluster Survey (RDCS) and confirmed by extensive spectroscopy with the VLT at redshift z=1.237. With the Chandra data, the X-ray emission from the intra-cluster medium is well resolved and traced out to 500 kpc, thus allowing a measurement of the physical properties of the gas with unprecedented accuracy at this redshift. We detect a clear 6.7 keV Iron K line in the Chandra spectrum providing a redshift within 1% of the spectroscopic one. By augmenting our spectroscopic analysis with the XMM data (MOS detectors only), we significantly narrow down the 1 sigma error bar to 10% for the temperature and 30% for the metallicity, with best fit values kT = 6.0(+0.7,-0.5) keV, Z = 0.36(+0.12,-0.10) Z_sun. In the likely hypothesis of hydrostatic equilibrium, we measure a total mass of M_{500} = (1.9+-0.3)10^14 h_70^{-1}M_sun within R_{Delta=500} = 536 kpc. Overall, these observations imply that RDCS1252.9-2927 is the most X-ray luminous and likely the most massive bona-fide cluster discovered to date at z>1. When combined with current samples of distant clusters, these data lend further support to a mild evolution of the cluster scaling relations, as well the metallicity of the intra-cluster gas. Inspection of the cluster mass function in the current cosmological concordance model (h,Omega_m,Omega_Lambda)=(0.7,0.3,0.7) and sigma_8=0.7-0.8 shows that RDCS1252.9-2927 is an M* cluster at z=1.24, in keeping with number density expectations in the RDCS survey volume.

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arXiv2003

Iron abundance in the ICM at high redshift

We present the analysis of the X-ray spectra of 18 distant clusters of galaxies with redshift 0.3 < z < 1.3. Most of them were observed with the Chandra satellite in long exposures ranging from 36 ks to 180 ks. For two of the z>1 clusters we also use deep XMM-Newton observations. Overall, these clusters probe the temperature range 3 =0.8 is Z =0.25 +0.04 -0.06 Zo, consistent with the local canonical metallicity value, Z ~0.3 Zo, within 1sigma confidence level. Medium and low temperature clusters (kT <5 keV) tend to have larger iron abundances than hot clusters. At redshift ~1.2 (4 clusters at z>1) we obtain a statistically significant detection of the Fe-K line only in one cluster (Z>0.10 Zo at the 90% c.l.). Combining all the current data set from Chandra and XMM at z>1, the average metallicity is measured to be = 0.21 +0.10 -0.05 Zo (1sigma error), thus suggesting no evolution of the mean iron abundance out to z~1.2.

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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

Cooling and heating the ICM in hydrodynamical simulations

We discuss Tree+SPH simulations of galaxy clusters and groups, aimed at studying the effect of cooling and non-gravitational heating on observable properties of the ICM. We simulate at high resolution four halos,with masses in the range (0.2-4)10^{14}M_sol. We discuss the effects of using different SPH implementations and show that high resolution is mandatory to correctly follow the cooling pattern of the ICM. All of our heating schemes which correctly reproduce the X-ray scaling properties of clusters and groups do not succeed in reducing the fraction of collapsed gas below a level of 20 (30) per cent at the cluster (group) scale. Finally, gas compression in cooling cluster regions causes an increase of the temperature and a steepening of the temperature profiles, independent of the presence of non-gravitational heating processes. This is inconsistent with recent observational evidence for a decrease of gas temperature towards the center of relaxed clusters. Provided these discrepancies persist even for a more refined modeling of energy feedback, they may indicate that some basic physical process is still missing in hydrodynamical simulations.

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arXiv2003

On determining the cluster abundance normalization

Different determinations currently suggest scattered values for the power spectrum normalization on the scale of galaxy clusters, sigma_8. Here we concentrate on the constraints coming from the X-ray temperature and luminosity functions (XTF and XLF), and investigate several possible sources of discrepancies in the results. We conclude that the main source of error in both methods is the scaling relation involved, in particular the way its intrinsic scatter and systematic normalization are treated. For temperature derived constraints, we use a sample adapted from HIFLUGCS, and test for several sources of systematic error. We parameterize the mass-temperature relation with an overall factor T_ast, with 1.5 \le T_ast \le 1.9. After marginalising over the range of T_ast, we obtain a 68 per cent confidence range of sigma_8=0.77^{+0.05}_{-0.04} for a standard LambdaCDM model. For luminosity derived constraints we use the XLF from the REFLEX survey and explore how sensitive the final results are on the details of the mass-luminosity, M-L, conversion. Assuming a uniform systematic uncertainty of +/-20 per cent in the amplitude of the mass-luminosity relation by Reiprich and Bohringer, we derive sigma_8=0.79^{+0.06}_{-0.07} for the same standard LambdaCDM model. Although the XTF and XLF derived constraints agree very well with each other, we emphasize that such results can change by about 10--15 per cent, depending on how uncertainties in the L-T-M conversions are interpreted and included in the analysis. In order to achieve precision cosmology, with this probe, it is important to separate the uncertainty in the scaling relation into its intrinsic and overall normalization parts.

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arXiv2002

Reproducing the entropy structure in galaxy groups

We carry out a comparison between observations and hydrodynamic simulations of entropy profiles of groups and clusters of galaxies. We use the Tree+SPH GADGET code to simulate four halos of sizes in the M_500 = 1.0 - 16.e13 h^-1 Msun range, corresponding to poor groups up to Virgo-like clusters. We concentrate on the effect of introducing radiative cooling, star formation, and a variety of non-gravitational heating schemes on the entropy structure and the stellar fraction. We show that all the simulations result in a correct entropy profile for the Virgo-like cluster. With the heating energy budget of ~0.7 keV/particle injected at z_h=3, we are also able to reproduce the entropy profiles of groups. We obtain the flat entropy cores as a combined effect of preheating and cooling, while we achieve the high entropy at outskirts by preheating. The resulting baryon fraction locked into stars is in the 25-30% range, compared to 35-40% in the case of no preheating. Heating at higher redshift, z_h=9, strongly delays the star-formation, but fails to produce a sufficiently high specific entropy.

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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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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

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

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

Chemical Enrichment and Energetics of the ICM with Redshift

In this paper we show preliminary results concerning the chemical and energetic enrichment of the ICM by means of supernova-driven wind models in elliptical galaxies. These models are obtained by taking into account new prescriptions about supernova remnant evolution in the interstellar medium. We find that models, which can reproduce the Fe abundance and the [$α$/Fe] ratios observed in the ICM, predict that the SN energy input can provide about 0.3 keV per ICM particle. We have obtained this result by assuming that each SN explosion inject on the average into the ISM no more than 20% of its initial blast wave energy. The predicted energy per particle is not enough to break the cluster self-similarity but is more than predicted in previous models.

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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

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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