Sebastiano Ghigna

Dark Matter Substructure within Galactic Halos

We use numerical simulations to examine the substructure within galactic and cluster mass halos that form within a hierarchical universe. Clusters are easily reproduced with a steep mass spectrum of thousands of substructure clumps that closely matches the observations. However, the survival of dark matter substructure also occurs on galactic scales, leading to the remarkable result that galaxy halos appear as scaled versions of galaxy clusters. The model predicts that the virialized extent of the Milky Ways halo should contain about 500 satellites with circular velocities larger than the Draco and Ursa Minor systems, i.e., bound masses 108 M☉ and tidally limited sizes 1 kpc. The substructure clumps are on orbits that take a large fraction of them through the stellar disk, leading to significant resonant and impulsive heating. Their abundance and singular density profiles have important implications for the existence of old thin disks, cold stellar streams, gravitational lensing, and indirect/direct detection experiments.

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.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.

The Descendants of Lyman Break Galaxies in Galaxy Clusters: Spatial Distribution and Orbital Properties

We combine semianalytical methods with an ultra-high-resolution simulation of a cluster (of mass 2.3 × 1014 h-1 M☉ and 4 × 106 particles within its virial radius) formed in a standard cold dark matter universe to study the spatial distribution and orbital properties of the present-day descendents of Lyman break galaxies (LBGs). At redshift 3 we find on average a total of 12 halos containing at least one LBG in the region that will later collapse to form the cluster itself. At the present time only five of these halos survive as separate entities inside the cluster virial radius, having been stripped of most of their dark matter. Their circular velocities are in the range 200-550 km s-1. Seven halos merged together to form the central object at the very center of the cluster. Using semianalytical modeling of galaxy evolution we show that descendents of halos containing the most massive LBGs now host giant elliptical galaxies such as those typically found in rich galaxy clusters. Inside the simulated cluster, galaxy orbits are very radial, with a pericenter-to-apocenter ratio of about 1 : 5. The orbital eccentricities of LBG 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 clusters main body. In this cluster, possibly because of its early formation time, the descendents of massive 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 massive LBG 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 2 compared to estimates using at least 20 cluster members.We combine semi–analytical methods with a ultra-high resolution simulation of a cluster (of mass 2.3 × 10 14 h −1 M ⊙ , 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 (LBG). At redshift 3 we find a total of 12 halos containing at least one Lyman Break galaxy in the region that will later collapse to form the cluster itself. At the present time only five of these halos survive as separate entities inside the virial radius, having been stripped of most of their dark matter. 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 Lyman Break galaxies now host giant elliptical galaxies such as those typically found in rich galaxy clusters. All galaxy orbits are very radial, with a pericenter to apocenter ratio of about 1:5. The orbital eccentricities of LBG 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 clusters main body. In this cluster, possibly due to its early formation time, the descendents of Lyman break galaxies 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 LBG 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.

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 observers 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.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 observers 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.

Statistical Tests for CHDM and ΛCDM Cosmologies

We apply several statistical estimators to high-resolution N-body simulations of two currently viable cosmological models: a mixed dark matter model, having ?? = 0.2 contributed by two massive neutrinos (C + 2?DM), and a cold dark matter model with cosmological constant (?CDM) with ?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 PN, including the void probability function P0; and the underdensity probability function U (where fixes the underdensity threshold in percentage of the average). We find that P0 (for which PPS and CfA2 data agree) and P1 distinguish efficiently between the models, while U is only marginally discriminatory. On the contrary, the reduced skewness and kurtosis are, respectively, S3 2.2 and S4 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?DM in all the tests performed. In contrast, the above ?CDM model has serious difficulties in reproducing observational data if galaxies and matter overdensities are related in a simple way.

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~

Density Profiles and Substructure of Dark Matter Halos: Converging Results at Ultra-High Numerical Resolution

h^{-1}

Dark matter haloes within clusters

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.