arXiv subjects

S. Ghigna

Publications and source records attributed to S. Ghigna.

8 recordsLinked to original sources

arXiv2003

Intracluster stellar population properties from N-body cosmological simulations -- I. Constraints at $z=0$

We use a high resolution collisionless simulation of a Virgo--like cluster in a $Λ$CDM cosmology to determine the velocity and clustering properties of the diffuse stellar component in the intracluster region at the present epoch. The simulated cluster builds up hierarchically and tidal interactions between member galaxies and the cluster potential produce a diffuse stellar component free-flying in the intracluster medium. Here we adopt an empirical scheme to identify tracers of the stellar component in the simulation and hence study its properties. We find that at $z=0$ the intracluster stellar light is mostly unrelaxed in velocity space and clustered in structures whose typical clustering radii are about 50 kpc at R=400--500 kpc from the cluster center, and predict the radial velocity distribution expected in spectroscopic follow-up surveys. Finally, we compare the spatial clustering in the simulation with the properties of the Virgo intracluster stellar population, as traced by ongoing intracluster planetary nebulae surveys in Virgo. The preliminary results indicate a substantial agreement with the observed clustering properties of the diffuse stellar population in Virgo.

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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

The descendents of Lyman Break Galaxies in galaxy clusters: spatial distribution and orbital properties

We combine semi-analytical methods with a ultra-high resolution simulation of a galaxy cluster (of mass 2.3 10^14h-1Msolar, and 4 10^6 particles within its virial radius) formed in a standard CDM universe to study the spatial distribution and orbital properties of the present-day descendents of Lyman Break Galaxies (LBGs). At the present time only five (out of 12) of halos containing LBGs survive as separate entities inside the cluster virial radius. Their circular velocities are in the range 200 - 550 km/sec. Seven halos merged together to form the central object at the very center of the cluster. Using semi-analytical modeling of galaxy evolution we show that descendents of halos containing LBGs now host giant elliptical galaxies. Galaxy orbits are radial, with a pericenter to apocenter ratio of about 1:5. The orbital eccentricities of LBGs descendents are statistically indistinguishable from those of the average galaxy population inside the cluster, suggesting that the orbits of these galaxies are not significantly affected by dynamical friction decay after the formation of the cluster's main body. In this cluster, possibly due to its early formation time, the descendents of LBGs are contained within the central 60% of the cluster virial radius and have an orbital velocity dispersion lower than the global galaxy population, originating a mild luminosity segregation for the brightest cluster members. Mass estimates based only on LBGs descendents (especially including the central cD) reflect this bias in space and velocity and underestimate the total mass of this well virialized cluster by up to a factor of two compared to estimates using at least 20 cluster members.

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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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arXiv1994

Sizes of Voids as a test for Dark Matter Models

We use the void probability statistics to study the redshift-space galaxy distribution as described by a volume-limited subsample of the Perseus-Pisces survey. We compare the results with the same analysis realized on artificial samples, extracted from high-resolution N-body simulations by reproducing the observational biases of the real data set. Simulations are run for the Cold+HotDM model (CHDM) and for unbiased and biased (b=1.5) CDM models in a 50 Mpc/h box. We identify galaxies as residing in peaks of the evolved density field. We fragment overmerged structures into individual galaxies so as to reproduce both the correct luminosity function (after assuming M/ L values for the resulting galaxy groups) and the two-point correlation function. Our main result is that a void-probability function (VPF) from the standard CHDM model with fractions 60% cold, 30% hot, 10% barions, exceeds the observational VPF with a high confidence level. CDM models produce smaller VPF independent of the biasing parameter. We verify the robustness of this result against changing the observer position in the simulations and the galaxy identification in the evolved density field.

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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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