arXiv subjects

P. Tozzi

Publications and source records attributed to P. Tozzi.

171 records · Page 10Linked to original sources

arXiv1999

Hot Gas in Clusters of Galaxies: the Punctuated Equilibria Model

We develop a model to describe the evolution of the intra-cluster X-ray emitting baryons, as they are included in the dark matter potential wells of galaxy clusters evolving through subsequent merging events in the framework of hierarchical clustering. The gas is assumed to re-adjusts to a new hydrostatic equilibrium after each merging event. Before merging it is gravitationally heated at the local virial temperature when bound in subclusters; at early $z$ the gas is preheated by supernova activity following star formation. In detail, we compute analytically the following steps: the dynamic histories of dark matter halos with their merging events; the associated infall of gas into a halo, with compressions and shocks estabilishing the conditions at the cluster boundary; the updated disposition of the gas in the potential well matching such conditions; the statistical convolution of the key quantities over the merging histories. The model predicts the density and surface brightness profiles with no free parameters; the so-called $β$ parameter is itself an outcome of the model, and the polytropic index $γ$ is internally constrained to a narrow range. We obtain declining temperature profiles, and profiles for the density and for the surface brightness shallower in groups compared with clusters. Our model groups also contain a lower baryonic fraction on average, but with a scatter considerably larger than at cluster scales. Various statistics are obtained analytically upon averaging over the merging histories. In particular, we predict in different cosmologies the statistical correlation $L-T$ of luminosity with temperature; similarly we derivethe correlation $R_X-T$ for the size of the X-ray emitting region. The intrinsic scatter in both correlations is also predicted.

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arXiv1999

Cosmological Constraints from the ROSAT Deep Cluster Survey

The ROSAT Deep Cluster Survey (RDCS) has provided a new large deep sample of X-ray selected galaxy clusters. Observables such as the flux number counts n(S), the redshift distribution n(z) and the X-ray luminosity function (XLF) over a large redshift baseline (z\lesssim 0.8) are used here in order to constrain cosmological models. Our analysis is based on the Press-Schechter approach, whose reliability is tested against N-body simulations. Following a phenomenological approach, no assumption is made a priori on the relation between cluster masses and observed X-ray luminosities. As a first step, we use the local XLF from RDCS, along with the high-luminosity extension provided by the XLF from the BCS, in order to constrain the amplitude of the power spectrum, σ_8, and the shape of the local luminosity-temperature relation. We obtain σ_8=0.58 +/- 0.06 for Omega_0=1 for open models at 90% confidence level, almost independent of the L-T shape. The density parameter Ω_0 and the evolution of the L-T relation are constrained by the RDCS XLF at z>0 and the EMSS XLF at z=0.33, and by the RDCS n(S) and n(z) distributions. By modelling the evolution for the amplitude of the L-T relation as (1+z)^A, an Ω_0=1 model can be accommodated for the evolution of the XLF with 1<A<3 at 90% confidence level, while Ω_0=0.4^{+0.3}_{-0.2} and Ω_0<0.6 are implied by a non--evolving L-T for open and flat models, respectively.

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arXiv1998

Properties of Galaxy Clusters: Mass and Correlation Functions

We analyse parallel N-body simulations of three Cold Dark Matter (CDM) universes to study the abundance and clustering of galaxy clusters. The simulations cover a volume comparable to the forthcoming SDSS. We are able to make robust measurements of cluster properties to a redshift larger than unity. We extract halos using two independent, public domain group finders (FOF & HOP) and find consistent results. The correlation function of clusters is in very good agreement with a simple analytic prescription based upon a Lagrangian biasing scheme developed by Mo & White (1996) and the Press-Schechter (PS) formalism for the mass function. The R_0--D_c relation for the open CDM model is in good agreement with the results from the APM Cluster Survey. The SCDM universe shows a robust deviation in the shape and evolution of the mass function when compared with that predicted by the PS formalism. Critical models with a low sigma_8 normalization or small shape parameter Gamma show an excess of massive clusters compared with the PS prediction. When cluster normalized, the SCDM universe at z =1 contains 10 times more clusters with temperatures greater than 7keV, compared with the PS prediction. The agreement between the analytic and N-body mass functions of SCDM can be improved if the value of the delta_c (the extrapolated linear theory threshold for collapse) is revised to be $ delta_c(z) = 1.685[(0.7/sigma_8)(1+z)]^{-0.125}. Our best estimate for the amplitude of fluctuations inferred from the local cluster abundance for SCDM is sigma_{8} = 0.5 \pm 0.04. However, the discrepancy between the temperature function predicted in a critical density universe and that observed at z=0.33 (Henry et al. 1998) remains. (abridged)

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arXiv1998

Testing Omega0 with X-ray Clusters: a Physical Approach

The X-ray emission from clusters of galaxies is one of the most pursued observational probe to investigate the distribution of dark matter and the related density parameter Omega0. The crucial link to derive the statistics of observables from a dynamical theory is constituted by the physics for the diffuse baryons (or ICP) responsible of the X-ray emission. Here we present a physical model for the ICP which leads to a definite L-T relation. Then we perform a physically based cosmological test, pointing out three cold dark matter universes: a Tilted critical CDM, a flat CDM with Omega0=0.3, and an Open CDM with Omega0=0.5, which are discussed on the basis of the RDCS survey.

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arXiv1998

A Physical Model for Baryons in Clusters of Galaxies

The X-ray emission from clusters of galaxies is one of the best observational probe to investigate the distribution of dark matter at intermediate and high redshifts. Since the disposition of the intracluster plasma (ICP) responsible of the emission is crucial to link X-ray properties to the global properties of the dark matter halos, we propose a semi--analytical approach for the diffuse baryons. This comprises the following blocks: Monte Carlo merging histories to describe the dynamics of dark matter halos; the central hydrostatic disposition for the ICP; conditions of shock, or of closely adiabatic compression at the boundary with the external gas, preheated by stellar energy feedbacks. From our model we predict the $L-T$ correlation, consistent with the data as for shape and scatter.

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arXiv1998

Diffuse Baryons in Groups and Clusters of Galaxies

To predict the X-ray observables associated to the diffuse baryons in clusters of galaxies, we develop a new physical approach to model such a hot intra-cluster plasma. Such approach is based on punctuated equilibria. and comprises the following blocks: Monte Carlo ``merging histories'' of dark matter potential wells; the central hydrostatic disposition for the ICP, reset to a new equilibrium after each merging episode; conditions of shock, or of closely adiabatic compression at the boundary with the external gas, preheated by stellar energy feedbacks. We predict the L-T relation, consistent with the data as for shape and scatter. This we combine with the mass distribution provided by the hierarchical clustering for different COBE-normalized CDM models, to predict the z-resolved luminosity functions, the source counts, the redshift distributions and contribution of the unresolved groups and clusters to the soft X-ray background. When compared with the recent ROSAT surveys, our results confirm that the critical cosmology with Standard CDM is ruled out by its overproduction of local clusters. On account of underproduction, instead, we rule out open cosmologies except for a narrow range around Ω_o=0.5; even there, we find the consistency with the full data base to be hardly marginal. For Ω_o=0.3 in flat geometry, we obtain acceptable fits. For the tilted CDM perturbation spectrum with high baryonic content in the critical universe, we obtain marginal consistency. Finally, we discuss the effective limitations of X-ray clusters and groups as cosmological signposts, and their brighter prospects toward the astrophysics of the ICP and the cosmogony of large, high-contrast structures.

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arXiv1997

The Cluster Temperature Function at High Redshift

We take advantage of the biggest cosmological simulation to date for a critical CDM universe in order to test robustness of the cluster mass function on a range of masses much wider than tested before. On the high mass end our results show an excess of hot clusters and a milder evolution compared to the analytical predictions based on the Press & Schechter formula. These features must be properly taken into account in deriving the cluster X-ray temperature function at moderate and high redshifts. On a general basis, the reduced negative evolution in the number of hot clusters could alleviate the discrepancies between the predictions for critical universes and incoming data, which instead seem to favour low Omega_0

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arXiv1997

The Luminosity-Temperature Relation for Groups and Clusters of Galaxies

We model the effects of shocks on the diffuse, X-ray emitting baryons in clusters of galaxies. Shocks separate the infalling from the inner gas nearly at equilibrium, and dominate the compression and the density gradients of the latter in the dark-matter potential of the cluster. We find that, independently of the detailed shape of the potential, the density gradient is steeper and the compression factor larger for the richer clusters. We show, considering the different merging histories, that in the hierarchical cosmogony the above effects lead, in X-rays, to a luminosity-temperature relation L \propto T^5 at the scale of groups which flattens down to L \propto T^{3} for rich clusters in accord with the observations, and then saturates toward L \propto T^2 for higher temperatures. From the merging histories we also compute statistical fluctuations of the L-T correlation.

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arXiv1996

Small groups of Galaxies as Cosmological Clocks

We study the formation and evolution of small groups of galaxies using new Monte Carlo simulations. These are directly based on the random walk approach to the statistics of condensations collapsing by gravitational instability, and the results are consistent with the Press and Schechter formula. We stress how observational features of groups, such as galaxy membership, depend on the global dynamics of the Universe. This is because the amount of aggregations between infalling galaxies is governed by the ratio of the group age to the time scale for dynamical friction. This ratio strongly depends on cosmology. Thus in low-density models, even if flat, collapse of group sized perturbations followed by fast mergers, would often originate a single massive remnant. This is contrast to observed associations as compact groups of galaxies.

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