1,721,120 research outputs found
Hierarchical Clustering: Structure Formation in the Universe
Full text linkIn order to understand galaxy formation models it is necessary to have a reasonably clear idea of dark matter clustering. This because, in the standard cosmological scenario, galaxies are thought to reside in larger dark matter haloes, extending beyond the galaxy observable radius. Haloes form as consequence of gravitational instability of dark matter density perturbations, and collapse at a density about two hundred times that of the surrounding environment. Clustering
happens at allmasses at any time.
Until now no direct observations of the existence of these darkmatter haloes have been done; however, their presence may be indirectly tested by their gravitational influence. For example, galaxies in groups have a velocity dispersion much higher than that caused only by visiblematter. Astronomers thus assumed the existence of large amounts of dark matter, an hypothesis later found consistent with other independent observations like gravitational lensing, galaxy clustering on very large scales and anisotropies in the cosmic microwave background radiation.
In particle physics, supersymmetry predicts the existence of a particle named neutralino (Jungman et al., 1996; Bertone et al., 2005), today regarded as the most likely candidate for the darkmatter. This particle is heavy and slow-moving (mass ? 100 Gev), so that dark matter density fluctuations can collapse for any mass larger than 10?6M? (Hofmann et al., 2001; Green et al., 2004, 2005). This places amass cut-off on the smallest darkmatter haloes that can collapse. Neutralino
can also annihilatewith its anti-particle, generating ?-ray photons (Bergström, 2000; Bertone et al., 2005), with annihilation rates growing as the square of the density. Due to this process, it is expected that future ?-ray telescopes (like
GLAST, Morselli (1997)) should be able to detect some excess in the ?-ray background signal from the center of theMilky-Way halo and from its satellites. This would be the first time of an in-”direct” detection of dark matter.
In this PhD dissertation we study the evolution of dark matter haloes, using two complementary approaches: numerical simulations and analytical modeling (through the extended Press & Schechter formalism). The work is organized as follows. In the first three chapters we describe and review some properties of the early universe and the theory underlying models of dark matter clustering. We discuss how density perturbations evolve and formdarkmatter haloes inside
which baryons can shock and cool, eventually form stars and galaxies. We also show how the number density of haloes can be estimated at any redshift using the excursion set approach, both for the spherical and ellipsoidal collapsemodels. These model mass functions are compared with those from numerical simulations in Chapter 4. We show that the ellipsoidal collapse model (Sheth et al., 2001; Sheth and Tormen, 2002) perfectly reproduces the global mass function in N-Body simulations, while, on the other hand, the spherical collapse model (Press and Schechter, 1974; Lacey and Cole, 1993; Bond et al., 1991) overpredicts the aboundace of smallmasses and underpredicts that of large ones.
Dark matter clustering is hierarchical, i.e. small systems collapse first (at higher redshift), and subsequentlymerge together forming larger haloes. In this scenario, if we define a formation time as the earliest redshift when an halo assembles
half of its present-daymass, small haloes formfirst and large ones form later. The top of the hierarchical pyramid is occupied by galaxy clusters, which represent the largest virialized structures in the universe.
Another important quantity describing dark matter clustering is any conditional mass function. One example is the probability that an halo observed at redshift z1, will be part of a larger halo at z0 < z1. This distribution is also called progenitor mass function; theoretical predictions and N-Body simulations are
compared at the end of Chapter 4. There we show that, also in this case, the ellipsoidal collapse prediction well reproduces the distribution found in numerical simulations atmost redshifts.
In Chapter 5 we will discuss how it is possible to estimate the formation time distribution from the conditional mass function, and present a new formula, based on the ellipsoidal collapse, that better fits the formation redshift distributionmeasured
in N-Body simulations.
The progenitors accreted along themerging history tree of a halo can survive today in their host system, and constitute the so-called substructure population (Ghigna et al., 1998; Tormen et al., 2004; Gao et al., 2004; De Lucia et al., 2004; van den Bosch et al., 2005). In Chapter 6 we discuss how it is possible to analytically estimate this population using the conditionalmass function, assuming no tidal stripping andmerging among substructures. By extrapolating the power
spectrumof density perturbations down to the typical neutralino Jeansmass, we estimate the substructure population in aMilky-Way size halo, both for a spherical and ellipsoidal collapse model. Modeling the neutralino annihilation rate, we then estimate the ?-ray emission from this population and its detectability with a GLAST-like telescope.
In Chapter 7 we study the growth of the main progenitor halo, and the mass it accretes along itsmerging history tree using numerical simulations. Themass function of accreted haloes, called “unevolved subhalomass function”, turns out to be independent of the final host halo mass, both before and after its formation redshift. The accreted haloes, called satellites, are then followed snapshot by snapshot in order to compute their mass loss rate. This allow us to interpret the present-day subhalo population in term of the mass loss from the accreted
satellites. Since smaller hosts form earlier than larger ones, the former will accrete satellites at earlier times; these satellites will therefore spend a longer time inside the host halo and lose a larger fraction of their initialmass. This translates the (mass-independent) unevolved subhalo population in a present-day subhalo distribution that depends on the host halo mass: at fixed subhalo-to-host halo mass: msb/M0, more massive hosts contain more subhaloes than smaller hosts do.
Subhaloes defined in this way may contain other subhaloes within themselves (Diemand et al., 2007b; Li and Helmi, 2007), which were accreted when they were still isolated systems. In Chapter 8 we show how subhaloes within subhaloes can be identified following all branches of themerging history tree of
an host halo. We also compare our definition of substructures with that of other authors (Gao et al., 2004), finding very good agreement.
In the last chapter of this dissertation, we show how the merging history tree of a halo can be followed using Monte Carlo realizations. The partition code, on which the tree is based, is very fast, time step independent, and provides results
in excellent agreement with the spherical collapse conditional mass function down to any required mass resolution (Sheth and Lemson, 1999). The tree has been run following the main branch and resolving all satellites down to the typical neutralino Jeans mass, in order to study the Milky-Way subhalo population
Weak lensing light-cones in modified gravity simulations with and without massive neutrinos
Full text linkWe present a novel suite of cosmological N-body simulations called the DUSTGRAIN-pathfinder, implementing simultaneously the effects of an extension to general relativity in the form of f(R) gravity and of a non-negligible fraction of massive neutrinos. We describe the generation of simulated weak lensing and cluster counts observables within a past light-cone extracted from these simulations. The simulations have been performed by means of a combination of the MG-GADGET code and a particle-based implementation of massive neutrinos, while the light-cones have been generated using the MAPSIM pipeline allowing us to compute weak lensing maps through a ray-tracing algorithm for different values of the source plane redshift. The mock observables extracted from our simulations will be employed for a series of papers focused on understanding and possibly breaking the well-known observational degeneracy between f(R) gravity and massive neutrinos, i.e. the fact that some specific combinations of the characteristic parameters for these two phenomena (the f(R0) scalar amplitude and the total neutrino mass Sigma m(v) ) may result indistinguishable from the standard Lambda CDM cosmology through several standard observational probes. In particular, in this work we show how a tomographic approach to weak lensing statistics could allow - especially for the next generation of wide-field surveys - to disentangle some of the models that appear statistically indistinguishable through standard single-redshift weak lensing probe
Some like it triaxial: the universality of dark matter halo shapes and their evolution along the cosmic time
No full textWe present a detailed analysis of dark matter halo shapes, studying how the distributions of ellipticity, prolateness and axial ratios evolve as a function of time and mass. With this purpose in mind, we analysed the results of three cosmological simulations, running an ellipsoidal halo finder to measure triaxial halo shapes. The simulations have different scales, mass limits and cosmological parameters, which allows us to ensure a good resolution and statistics in a wide mass range, and to investigate the dependence of halo properties on the cosmological model. We confirm the tendency of haloes to be prolate at all times, even if they become more triaxial going to higher redshifts. Regarding the dependence on mass, more massive haloes are also less spherical at all redshifts, since they are the most recent forming systems and so still retain memory of their original shape at the moment of collapse. We then propose a rescaling of the shape-mass relations, using the variable ν = δc/σ to represent the mass, which absorbs the dependence on both cosmology and time, allowing us to find universal relations between halo masses and shape parameters (ellipticity, prolateness and the axial ratios) which hold at any redshift. This may be very useful to determine prior distributions of halo shapes for observational studies
Accretion of satellites on to central galaxies in clusters: merger mass ratios and orbital parameters
No full textWe study the statistical properties of mergers between central and satellite
galaxies in galaxy clusters in the redshift range 0<1, using a sample of
dark-matter only cosmological N-body simulations from Le SBARBINE dataset.
Using a spherical overdensity algorithm to identify dark-matter haloes, we
construct halo merger trees for different values of the over-density
. While the virial overdensity definition allows us to probe the
accretion of satellites at the cluster virial radius , higher
overdensities probe satellite mergers in the central region of the cluster,
down to , which can be considered a proxy for the
accretion of satellite galaxies onto central galaxies. We find that the
characteristic merger mass ratio increases for increasing values of :
more than of the mass accreted by central galaxies since
comes from major mergers. The orbits of satellites accreting onto central
galaxies tend to be more tangential and more bound than orbits of haloes
accreting at the virial radius. The obtained distributions of merger mass
ratios and orbital parameters are useful to model the evolution of the
high-mass end of the galaxy scaling relations without resorting to hydrodynamic
cosmological simulations
Strong lensing cosmography in the frontier fields
No full textThe wealth of strong lensing features observed in the Frontier Fields clusters offers insights on the nature of dark energy. The large number of multiple-images systems with redshifts allows to simultaneously estimate the lens model parameters and the cosmological parameters involved in the distances calculations. In particular for the ΛCDM model, it is possible to estimate the matter density Ω m and the dark energy equations parameters w X. In this talk, I will present recent analyses of systematic errors based on Frontier Fields observed and simulated data
Formation times, mass growth histories and concentrations of dark matter haloes
No full textWe develop a simple model for estimating the mass growth histories of dark matter haloes. The model is based on a fit to the formation time distribution, where formation is defined as the earliest time that the main branch of the merger tree contains a fraction f of the final mass M. Our analysis exploits the fact that the median formation time as a function of f is the same as the median of the main progenitor mass distribution as a function of time. When coupled with previous work showing that the concentration c of the final halo is related to the formation time tf associated with f ∼ 0.04, our approach provides a simple algorithm for estimating how the distribution of halo concentrations may be expected to depend on mass, redshift and the expansion history of the background cosmology. We also show that one can predict log10c with a precision of about 0.13 and 0.09 dex if only its mass or both mass and tf are known. Moreover, conversely, one can predict log10tf from mass or c with a precision of 0.12 and 0.09 dex, approximately independent of f . Adding the mass to the c-based estimate does not result in further improvement. These latter results may be useful for studies which seek to compare the age of the stars in the central galaxy in a halo with the time the core was first assembled
GLAMER Part II: Multiple Plane Gravitational Lensing
No full textWe present an extension to multiple planes of the gravitational lensing code GLAMER. The method entails projecting the mass in the observed light-cone onto a discrete number of lens planes and inverse ray-shooting from the image to the source plane. The mass on each plane can be represented as halos, simulation particles, a projected mass map extracted form a numerical simulation or any combination of these. The image finding is done in a source oriented fashion, where only regions of interest are iteratively refined on an initially coarse image plane grid. The calculations are performed in parallel on shared memory machines. The code is able to handle different types of analytic halos (NFW, NSIE, power-law, etc.), haloes extracted from numerical simulations and clusters constructed from semi-analytic models (MOKA). Likewise, there are several different options for modeling the source(s) which can be distributed throughout the light-cone. The distribution of matter in the light-cone can be either taken from a pre-existing N-body numerical simulations, from halo catalogs, or are generated from an analytic mass function. We present several tests of the code and demonstrate some of its applications such as generating mock images of galaxy and galaxy cluster lenses. Key words:
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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