arXiv subjects

S. A. Bonometto

Publications and source records attributed to S. A. Bonometto.

36 records · Page 2Linked to original sources

arXiv2003

Halo properties in models with dynamical Dark Energy

We study properties of dark matter halos in a variety of models which include Dark Energy (DE). We consider both DE due to a scalar field self--interacting through Ratra-Peebles or SUGRA potentials, and DE with constant negative w=pρ>-1. We find that at redshift zero the nonlinear power spectrum of the dark matter, and the mass function of halos, practically do not depend on DE state equation and are almost indistinguishable from predictions of the LCDM model. This is consistent with the nonlinear analysis presented in the accompanying paper. It is also a welcome feature because LCDM models fit a large variety of data. On the other hand, at high redshifts DE models show substantial differences from LCDM and substantial differences among themselves. Halo profiles differ even at z=0. DE halos are denser than LCDM in their central parts because the DE halos collapse earlier. Nevertheless, differences between the models are not so large. For example, the density at 10 kpc of a DE \~10^{13}Msun halo deviates from LCDM by not more than 50%. This, however, means that DE is not a way to ease the problem with cuspy dark matter profiles. Addressing another cosmological problem - abundance of subhalos -- we find that the number of satellites of halos in various DE models does not change relative to the LCDM, when normalized to the same circular velocity of the parent halo. To summarize, the best way to find which DE model fits the observed Universe is to look for evolution of halo properties. For example, the abundance of galaxy groups with mass larger than 10^{13}\Msun at z> 2 can be used to discriminate between the models, and, thus, to constrain the nature of DE.

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arXiv2003

Mixed Dark Matter from Axino Distribution

We study the possibility of mixed dark matter obtained through the phase space distribution of a single particle. An example is offered in the context of SUSY models with a Peccei-Quinn symmetry. Axinos in the 100 keV range can naturally have both thermal and non-thermal components. The latter one arises from the lightest neutralino decays and derelativizes at z ~ 10^4.

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arXiv2003

Mass of clusters in simulations

We show that DM halos, in n--body simulations, have a boundary layer (BL), separating bound from unbound mass. Let T(r) and W(r) be the kinetic and potential energies in shells of halos. We find that, in almost all halos: (i) The virial ratio R=-2T/W has at least one persistent (i.e. resolution independent) minimum r_c, such that, close to it, (ii) the function w=-d log W/d log r has a maximum and (iii) the relation R(r_c)=w(r_c) is almost exactly fulfilled. The radius r_c is the position of BL's, in halos found in simulations of TCDM and LCDM models, run using ART and GADGET codes at various resolutions. We find that 97% of the ~300 halos (per model) with M>4.2 10^14 M_s h^-1 owns a BL. Those with no BL are undergoing major mergings. The mass M_c enclosed in r_c almost coincides with the mass evaluated from velocities, according to virial theorem. Particles at r>r_c are not in virial equilibrium. Using r_c we have a density contrast Δ_c for each halo. For each mass scale, Δ_v=178 Omega_m^0.45 is within the range of Δ_c's, but the spread in Δ_c is wide and the average Δ_c is ~25% smaller than the corresponding Δ_v. The matching of properties derived under the assumption of spherical symmetry is a consequence of violent relaxation destroying features related to previous ellipsoidal non-linear growth. In turn, the spread of final Δ_c's is an imprint of the different 3-D geometries and of the variable environment during collapses, as suggested by a comparison with Sheth & Tormen analysis.

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arXiv2002

Non linear predictions from linear theories in models with Dark Energy

We study the cluster mass function and its evolution in different models with Dark Energy arising from a self--interacting scalar field, with Ratra-Peebles and SUGRA potentials. Computations are based on a Press & Schechter approximation. The mass functions we obtain are compared with results holding for open models or models with Dark Energy due to a cosmological constant. Evolution results, in the Dark Energy cases, closely approach open models.

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arXiv2002

Cosmic opacity to CMB photons and polarization measurements

Anisotropy data analysis leaves a significant degeneracy between primeval spectral index (n_s) and cosmic opacity to CMB photons (τ). Low--l polarization measures, in principle, can remove it. We perform a likelihood analysis to see how cosmic variance possibly affects such a problem. We find that, for a sufficiently low noise level (σ_{pix}) and if τis not negligibly low, the degeneracy is greatly reduced, while the residual impact of cosmic variance on n_s and τdeterminations is under control. On the contrary, if σ_{pix} is too high, cosmic variance effects appear to be magnified. We apply general results to specific experiments and find that, if favorable conditions occur, it is possible that a 2--σdetection of a lower limit on τis provided by the SPOrt experiment. Furthermore, if the PLANCK experiment will measure polarization with the expected precision, the error on low--l harmonics is adequate to determine τ, without significant magnification of the cosmic variance. This however indicates that high sensitivity might be more important than high resolution in τdeterminations. We also outline that a determination of τis critical to perform detailed analyses on the nature of dark energy and/or on the presence of primeval gravitational waves.

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arXiv2002

Mass of Clusters in Simulations

We show that dark matter haloes, in n--body simulations, have a boundary layer (BL) with precise features. In particular, it encloses all dynamically stable mass while, outside it, dynamical stability is lost soon. Particles can pass through such BL, which however acts as a confinement barrier for dynamical properties. BL is set by evaluating kinetic and potential energies (T(r) and W(r)) and calculating R=-2T/W. Then, on BL, R has a minimum which closely approaches a maximum of w= -dlog W/dlog r. Such $Rw$ ``requirement'' is consistent with virial equilibrium, but implies further regularities. We test the presence of a BL around haloes in spatially flat CDM simulations, with or without cosmological constant. We find that the mass M_c, enclosed within the radius r_c, where the $Rw$ requirement is fulfilled, closely approaches the mass M_{dyn}, evaluated from the velocities of all particles within r_c, according to the virial theorem. Using r_c we can then determine an individual density contrast Delta_c for each virialized halo, which can be compared with the "virial" density contrast $Δ_v ~178 Ω_m^{0.45}$ (Omega_m: matter density parameter) obtained assuming a spherically symmetric and unperturbed fluctuation growth. The spread in Delta_c is wide, and cannot be neglected when global physical quantities related to the clusters are calculated, while the average Delta_c is ~25 % smaller than the corresponding Delta_v; moreover if $M_{dyn}$ is defined from the radius linked to Delta_v, we have a much worse fit with particle mass then starting from {\it Rw} requirement.

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arXiv2001

Constraining Lambda using cluster quadrupoles

We examine how the statistics of the quadrupoles of (projected) cluster masses can discriminate between flat cold dark matter (CDM) universes with or without a cosmological constant term. Even in the era of high precision cosmology that cosmic microwave background experiments should open soon, it is important to devise self consistency tests of cosmogonic theories tuned at the matter radiation decoupling epoch using data from the non--linear evolved universe. We build cluster catalogs from two large volume simulations of a ``tilted'' CDM model and a $Λ$CDM model with cosmic density parameter $Ω_m=0.35$ and cosmological constant contribution $Ω_Λ=0.65$. From the projected mass distribution of the clusters we work out the quadrupoles Q and examine their dependence on cluster mass and the cosmological model. We find that TCDM clusters have systematically larger quadrupoles than their \lcdm counterpart. The effect is mass dependent: massive clusters ($M\gsim 10^{15}\Msunh$) have quadrupoles differing by more than 30% in the two models, while for $M\lsim 4\times10^{14}\Msunh$ the difference rapidly drops to $\sim 1$%. Performing a K-S test of the Q distributions, we estimate that using just the 15 most massive clusters in the simulation volume ($360\Mpch$ a side) we can discriminate between TCDM and \lcdm at a confidence level better than 99.9%. In the volume probed by exhisting observations, there are potentially several hundred clusters with masses above the threshold for which the differences in the quadrupoles become relevant. Should weak lensing data become available for this whole set, a quadrupole analysis may be expected to discriminate among different values of $Λ$.

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arXiv2000

Cluster Correlation in Mixed Models

We evaluate the dependence of the cluster correlation length r_c on the mean intercluster separation D_c, for three models with critical matter density, vanishing vacuum energy (Lambda = 0) and COBE normalized: a tilted CDM (tCDM) model (n=0.8) and two blue mixed models with two light massive neutrinos yielding Omega_h = 0.26 and 0.14 (MDM1 and MDM2, respectively). All models approach the observational value of sigma_8 (and, henceforth, the observed cluster abundance) and are consistent with the observed abundance of Damped Lyman_alpha systems. Mixed models have a motivation in recent results of neutrino physics; they also agree with the observed value of the ratio sigma_8/sigma_25, yielding the spectral slope parameter Gamma, and nicely fit LCRS reconstructed spectra. We use parallel AP3M simulations, performed in a wide box (side 360/h Mpc) and with high mass and distance resolution, enabling us to build artificial samples of clusters, whose total number and mass range allow to cover the same D_c interval inspected through APM and Abell cluster clustering data. We find that the tCDM model performs substantially better than n=1 critical density CDM models. Our main finding, however, is that mixed models provide a surprisingly good fit of cluster clustering data.

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arXiv1999

Limber equation for luminosity dependent correlations

The passage from angular to spatial correlations, in the case of spatial clustering length depending on the average distance between nearby objects is studied. We show that, in a number of cases, the scaling law of angular correlation amplitudes, which holds for constant spatial clustering length, is still true also for a luminosity dependent spatial correlation. If the Limber equation is then naively used to obtain `the' spatial clustering length from the angular function amplitude, a quantity close to the average object separation is obtained. The case of cluster clustering is explicitly considered.

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arXiv1999

Cluster mass function in mixed models

We study the cluster mass function in mixed dark matter (MDM) models, using two COBE normalized simulations with Omega_h = 0.26 and n=1.2, and Omega_h = 0.14 and n = 1.05, both with 2 massive nu's (MDM1 and MDM2, respectively). For the sake of comparison, we also simulate a CDM model with spectral index n=0.8 (TCDM), also COBE normalized. We argue that, in our non--hydro simulations, where CDM particles describe both actual CDM and baryons, the galaxy distribution traces CDM particles. Therefore, we use them to define clusters and their velocities to work out cluster masses. As CDM particles are more clustered than HDM and therefore have, in average, greater velocities, this leads to significant differences from PS predictions. Such predictions agree with simulations if both HDM and CDM are used to define clusters. Clusters defined through CDM in MDM models, instead, are less numerous than PS estimates, by a factor ~0.3, at the low mass end; the factor becomes \~0.6-0.8, depending on the mix, on intermediate mass scales (~4-5, h^-1 10^14 Msun) and almost vanishes on the high mass end. Therefore: (i) MDM models expected to overproduce clusters over intermediate scales are viable; (ii) the greater reduction factor at small scales agrees with the observational data dependence on the cluster mass M (which, however, may be partially due to sample incompleteness); (iii) the higher spectral normalization is felt at large scales, where MDM models produce more objects (hence, large clusters) than CDM. MDM1 even exceeds Donahue et al. (1998) findings, while MDM2 is consistent with them. (abridged)

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arXiv1999

Global cluster morphology and its evolution: X-ray data vs CDM, LCDM and mixed models

The global structure of galaxy clusters and its evolution are tested within a large set of TREESPH simulations, so to allow a fair statistical comparison with available X-ray data. Structure tests are based on the "power ratios", introduced by Buote & Tsai. Cosmological models considered are CDM, LCDM (Omega_L=0.7) and CHDM (1 mass.neu., Omega_h = 0.2). All models are normalized to provide a fair number density of clusters. For each model we run a P3M simulation in a large box, where we select the most massive 40 clusters. Going back to the initial redshift we run a hydro-TREESPH simulation for each of them. In this way we perform a statistical comparison of the global morphology of clusters, for each cosmological model, with ROSAT data, using Student t-test, F-test and K-S test. The last test and its generalization to 2--D distributions are also used to compare the joint distributions of 2 or 3 power ratios. We find that, using DM distribution, instead of gas, as done by some authors, leads to biased results, as baryons are distributed in a less structured way than DM. We also find that the cosmological models considered have different behaviours in these tests: LCDM has the worst performance. CDM and our CHDM have similar scores. The general trend of power ratio distributions is already fit by these models, but a further improvement is expected either from a different DM mix or a non-flat CDM model.

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arXiv1998

Mixed Models with n>1 and Large Scale Structure constraints

Recent data on CBR anisotropies show a Doppler peak higher than expected in CDM cosmological models, if the spectral index $n=1$. However, CDM and LCDM models with n>1 can hardly be consistent with LSS data. Mixed models, instead, whose transfer function is naturally steeper because of free--streaming in the hot component, may become consistent with data if n>1, when Omega_h is large. This is confirmed by our detailed analysis, extended both to models with a hot component whose momentum space distribution had a thermal origin (like massive neutrinos), and to models with a non--cold component arising from heavier particle decay. In this work we systematically search models which fulfill all constraints which can be implemented at the linear level. We find that a stringent linear constraint arises from fitting the extra-power parameter Gamma. Other significant constraints arise comparing the expected abundances of galaxy clusters and high-z systems with observational data. Keeping to models with Gamma \geq 0.13, a suitable part of the space parameter still allows up to \sim 30% of hot component (it is worth outlining that our stringent criteria allow only models with 0.10 \mincir Omega_h \mincir 0.16, if n \leq 1). We also outline that models with such large non--cold component would ease the solution of the so--called baryon catastrophe in galaxy clusters.

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arXiv1998

CMB and Large Scale Structure as a test of Mixed Models with n>1

We compute CBR anisotropies in mixed models with different hot components, including neutrinos or volatile HDM arising from the decay of heavier particles. The CBR power spectra of these models exhibit a higher doppler peak than CDM, and the discrepancy is even stronger in volatile models when the decay gives rise also to a neutral scalar. CBR experiments, together with Large Scale Structure (LSS) data, are then used to constrain the space parameter of mixed models, when values of the primeval spectral index n > 1 are also considered. Even if n>1 is allowed, however, LSS alone prescribes that Ω_h \mincir 0.30. LSS can be fitted by taking simultaneously a low derelativization redshift z_der (down to \simeq 600) and a high n, while CBR data from baloon--borne experiment cause a severe selection on this part of the parameter space. In fact, while late derelativization and $n>1$ have opposite effects on the fluctuation spectrum P(k), they sum their action on the angular spectrum C_l. Henceforth n \magcir 1.3 seems excluded by baloon--borne experiment outputs, while a good fit of almost all CBR and LSS data is found for Omega_h values between 0.11 and 0.16, n \sim 1.1 and z_der \sim 2000--5000. A smaller n is allowed, but z_der should never be smaller than \simeq 1200.

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arXiv1996

Statistical Tests for CHDM and \LambdaCDM Cosmologies

We apply several statistical estimators to high-resolution N-body simulations of two currently viable cosmological models: a mixed dark matter model, having $Ω_ν=0.2$ contributed by two massive neutrinos (C+2\nuDM), and a Cold Dark Matter model with Cosmological Constant (\LambdaCDM) with $Ω_0=0.3$ and h=0.7. Our aim is to compare simulated galaxy samples with the Perseus-Pisces redshift survey (PPS). We consider the n-point correlation functions (n=2-4), the N-count probability functions P_N, including the void probability function P_0, and the underdensity probability function U_ε(where εfixes the underdensity threshold in percentage of the average). We find that P_0 (for which PPS and CfA2 data agree) and P_1 distinguish efficiently between the models, while U_εis only marginally discriminatory. On the contrary, the reduced skewness and kurtosis are, respectively, S_3\simeq 2.2 and S_4\simeq 6-7 in all cases, quite independent of the scale, in agreement with hierarchical scaling predictions and estimates based on redshift surveys. Among our results, we emphasize the remarkable agreement between PPS data and C+2\nuDM in all the tests performed. In contrast, the above \LambdaCDM model has serious difficulties in reproducing observational data if galaxies and matter overdensities are related in a simple way.

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arXiv1996

Void analysis as a test for dark matter composition?

We use the void probability function to compare the redshift--space galaxy distribution in the Perseus--Pisces survey with artificial samples from N-body simulations of standard cold dark matter (CDM) and broken scale invariance (BSI) models. Galaxies are identified as residing in peaks of the evolved density field in such a way as to reproduce both the observed luminosity and two--point correlation functions. Using a similar approach, it was recently shown that the VPF can discriminate between CDM and a cold+hot dark matter (CHDM) model with cold/hot/baryon fractions = 0.6/0.3/0.1. Our main result is that both CDM (as expected from a previous analysis) and BSI fit observational data. The robustness of the result is checked against changing the observer's position in the simulations and the galaxy identification in the evolved density field. Therefore, while the void statistics is sensitive to the passage from CDM to CHDM (different spectrum and different nature of dark matter), it is not to the passage from CDM to BSI (different spectrum but same dark matter). On such a basis, we conjecture that the distribution of voids could be directly sensitive to the nature of dark matter, but scarcely sensitive to the shape of the transfer function.

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arXiv1995

Phase--space Distribution of Volatile Dark Matter

We discuss the phase--space distribution of $μ$ neutrinos if $τ$ neutrinos are unstable and decay into $ν_μ+ scalar$. If this scalar is a familon or a Majoron, in the generic case the $ν_μ$ background is NOT the straightforward overlap of neutrinos of thermal and decay origins. A delay in $ν_τ$ decay, due to the Pauli exclusion principle, can modify it in a significant way. We provide the equations to calculate the $ν_μ$ distribution and show that, in some cases, there exists a good approximate solution to them. However, even when such solution is not admitted, the equations can be numerically solved following a precise pattern. We give such a solution for a number of typical cases. If $ν_μ$ has a mass $\sim 2$ eV and the see--saw argument holds, $ν_τ$ must be unstable and the decay into $ν_μ+ scalar$ is a reasonable possibility. The picture leads to a delayed equivalence redshift, which could allow to reconcile COBE data with a bias parameter $b\ge 1$.

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arXiv1994

Deviations from Hierarchical Clustering in Real and Redshift Space

We discuss the effects of redshift--space distortions on the estimate of high order galaxy correlation functions. We use both the Perseus--Pisces redshift survey and the results of high--resolution N--body simulations to explore the consequences of working in redshift space on the detection of deviations from hierarchical clustering. Both for real and simulated data, significant deviations from hierarchical clustering seem to be present in real space. Their behaviour is coherent with that expected from an initially biased galaxy field, once the displacement from the sites where galaxies formed, due to the nonlinear gravitational evolution, is taken into account. The passage to redshift space has the net effect of filtering out the higher powers required to fit the distribution in real space. This magnifies the distortions operated by nonlinear evolution on the initial distribution, erasing the residual narrow scale range (2--5~$h^{-1}$Mpc) where deviations from hierarchical clustering are still detectable in real space. We conclude that such deviations can hardly be estimated using data in redshift space.

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arXiv1993

Non-Linear Clustering in the Cold+Hot Dark Matter Model

We use high resolution PM N-body simulations to follow the development of non-linear clustering in a flat Universe, dominated by Cold + Hot Dark Matter (CHDM) with 60% of CDM, 30% of HDM and 10% of baryons; a simulation box of 100 Mpc a side ($h=0.5$) is used. We analyze two CHDM simulations with $b =1.5$ (COBE normalization). We also compare them with CDM simulations with $b=1.5$ and $b=1$. We evaluate high-order correlation functions and the void-probability-function (VPF). Correlation functions are obtained both from counts in cells and counts of neighbors. The analysis is made for DM particles and for galaxies, identified as massive halos in the evolved density field. We also check the effects of dynamical evolution and redshift space distortions. We find that clustering of DM particles exhibits deviations from the hierarchical scaling, which decrease somewhat in redshift space. Galaxies follow hierarchical scaling far more closely, with coefficients S_3=2.5 and S_4=7.5, in general agreement with observational results, just marginally affected by redshift distortions and on the choices for the initial spectra. The hierarchical scaling of galaxy clustering is confirmed by the VPF analysis and in all the cases considered a good agreement with observational results is obtained.

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