arXiv subjects

Nicos Hiotelis

Publications and source records attributed to Nicos Hiotelis.

9 recordsLinked to original sources

arXiv2022

The excursion set model a step beyond: Environmental dependence

In terms of the excursion set model, we used Monte Carlo methods in order to study the non-Markovian stochastic evolution of the smoothed overdensity $δ$ at scale $S$. For a Gaussian density field, smoothed by the top-hat filter, in real space, we used random walks, with the correct correlation between scales, in order to calculate constrained probabilities and to connect the number of structures with the overdensity of their environment. A constant barrier is used, lower than the usual one but which improves the consistency of our results with those of N-body simulations.We present comparisons of multiplicity functions with those resulting from N-body simulations as well as number densities of descendant haloes and of their progenitors and we connect them to the density of their large environment.There exists a dependence of the number of haloes on their environment which is different for different masses of these haloes.This number increases for larger overdensity of the environment for large haloes and decreases for small ones. The number of progenitors of these haloes is insensitive to the overdensity of their large environment.

astro-ph.CO↗

arXiv2018

Mass functions from the excursion set model

Aims. We aim to study the stochastic evolution of the smoothed overdensity $δ$ at scale $S$ of the form $δ(S) = \int_{0}^S K(S,u)\mathrm{d}W(u)$, where $K$ is a kernel and $\mathrm{d}W$ is the usual Wiener process. Methods. For a Gaussian density field, smoothed by the top-hat filter, in real space, we used a simple kernel that gives the correct correlation between scales. A Monte Carlo procedure was used to construct random walks and to calculate first crossing distributions and consequently mass functions for a constant barrier. Results. We show that the evolution considered here improves the agreement with the results of N-body simulations relative to analytical approximations which have been proposed from the same problem by other authors. In fact, we show that an evolution which is fully consistent with the ideas of the excursion set model, describes accurately the mass function of dark matter haloes for values of $ν\leq 1$ and underestimates the number of larger haloes. Finally, we show that a constant threshold of collapse, lower than it is usually used, it is able to produce a mass function which approximates the results of N-body simulations for a variety of redshifts and for a wide range of masses. Conclusions. A mass function in good agreement with N-body simulations can be obtained analytically using a lower than usual constant collapse threshold.

astro-ph.CO↗

arXiv2017

On the first crossing distributions in fractional Brownian motion and the mass function of dark matter haloes

We construct an integral equation for the first crossing distributions for fractional Brownian motion in the case of a constant barrier and we present an exact analytical solution. Additionally we present first crossing distributions derived by simulating paths from fractional Brownian motion. We compare the results of the analytical solutions with both those of simulations and those of some approximated solutions which have been used in the literature. Finally, we present multiplicity functions for dark matter structures resulting from our analytical approach and we compare with those resulting from N-body simulations. We show that the results of analytical solutions are in good agreement with those of path simulations but differ significantly from those derived from approximated solutions. Additionally, multiplicity functions derived from fractional Brownian motion are poor fits of the those which result from N-body simulations. We also present comparisons with other models which exist in the literature and we discuss different ways of improving the agreement between analytical results and N-body simulations.

astro-ph.CO↗

arXiv2014

Cusps and Cores in the presence of galactic bulges

In this paper, we study how the presence of bulge formation in galaxies influence their inner density profile, by means of an extended version of the Del Popolo (2009) semi-analytical model. As in Del Popolo (2009), the model takes into account the effect of baryons adiabatic contraction, ordered and random angular momentum, dynamical friction, and adds to the previous the effect of gas cooling, star formation, supernova feedback, and reionization. Our model shows that dwarf galaxies are bulgeless, in agreement with observations showing that the large majority of them has no stellar bulges, and are characterized by a flat profile well described by a Burkert profile. {We then studied the effect of a bulge, added to the {cored} DM halo, on the density profile}. In the case of a galaxy having a mass $10^{11} M_{\odot}$ the inner density profile has a slope $α\simeq 0.65$, for a bulge of $4.5 \times 10^{9} M_{\odot}$, while if bulge formation is not considered, the slope would be $α\simeq 0.55$. If the bulge is larger, $6.5 \times 10^{9} M_{\odot}$ the slope is $α\simeq 0.7$. In the case of a larger galaxy with $10^{12} M_{\odot}$ the slope is $α\simeq 0.85$, while in absence of bulge it is $α\simeq 0.75$. We finally study how the inner slope $α$ changes with the bulge mass, and we find a correlation among the two quantities. The result shows that bulge formation has an important role in shaping the inner DM density profile in agreement with Inoue & Saitoh (2011) result. The result implies that going from Sc to SO Hubble type the slope is slightly steepening due to the bulge formation, and due to the fact that early type galaxies have larger bulges.

astro-ph.GA↗

arXiv2012

Merger rates of dark matter haloes: a comparison between EPS and N-body results

We calculate merger rates of dark matter haloes using the Extended Press-Schechter approximation (EPS) for the Spherical Collapse (SC) and the Ellipsoidal Collapse (EC) models. Merger rates have been calculated for masses in the range $10^{10}M_{\odot}\mathrm{h}^{-1}$ to $10^{14}M_{\odot}\mathrm{h}^{-1}$ and for redshifts $z$ in the range 0 to 3 and they have been compared with merger rates that have been proposed by other authors as fits to the results of N-body simulations. The detailed comparison presented here shows that the agreement between the analytical models and N-body simulations depends crucially on the mass of the descendant halo. For some range of masses and redshifts either SC or EC models approximate satisfactory the results of N-body simulations but for other cases both models are less satisfactory or even bad approximations. We showed, by studying the parameters of the problem that a disagreement --if it appears-- does not depend on the values of the parameters but on the kind of the particular solution used for the distribution of progenitors or on the nature of EPS methods. Further studies could help to improve our understanding about the physical processes during the formation of dark matter haloes.

astro-ph.CO↗

arXiv2009

Merger rates of dark matter haloes from merger trees in the extended Press-Schechter theory

We construct merger trees based on the extended Press-Schechter theory (EPS) in order to study the merger rates of dark matter haloes over a range of present day mass ($10^{10}M_{\sun}\leq M_0 \leq10^{15}M_{\sun}$), progenitor mass $(5\times10^{-3}\leq ξ\leq1$) and redshift ($0\leq z\leq 3$). We used the first crossing distribution of a moving barrier of the form $B(S,z)=p(z)+q(z)S^γ$, proposed by Sheth & Tormen, to take into account the ellipsoidal nature of collapse. We find that the mean merger rate per halo $B_m/n$ depends on the halo mass $M$ as $M^{0.2}$ and on the redshift as $(\mathrm{d}δ_c(z)/\mathrm{d}z)^{1.1}$. Our results are in agreement with the predictions of N-body simulations and this shows the ability of merger-trees based on EPS theory to follow with a satisfactory agreement the results of N-body simulations and the evolution of structures in a hierarchical Universe.

astro-ph.CO↗

arXiv2003

Extended Press-Schechter theory and the density profiles of dark matter haloes

An inside-out model for the formation of haloes in a hierarchical clustering scenario is studied. The method combines the picture of the spherical infall model and a modification of the extended Press-Schechter theory. The mass accretion rate of a halo is defined to be the rate of its mass increase due to minor mergers. The accreted mass is deposited at the outer shells without changing the density profile of the halo inside its current virial radius. We applied the method to a flat $ΛCDM$ Universe. The resulting density profiles are compared to analytical models proposed in the literature, and a very good agreement is found. A trend is found of the inner density profile becoming steeper for larger halo mass, that also results from recent N-body simulations. Additionally, present-day concentrations as well as their time evolution are derived and it is shown that they reproduce the results of large cosmological N-body simulations.

astro-ph↗

arXiv2002

Density profiles in a spherical infall model with non-radial motions

A generalized version of the Spherical Infall Model (SIM) is used to study the effect of angular momentum on the final density profile of a spherical structure. The numerical method presented is able to handle a variety of initial density profiles (scale or not scale free) and no assumption of self-similar evolution is required. The realistic initial overdensity profiles used are derived by a CDM power spectrum. We show that the amount of angular momentum and the initial overdensity profile affect the slope of the final density profile at the inner regions. Thus, a larger amount of angular momentum or shallower initial overdensity profiles lead to shallower final density profiles at the inner regions. On the other hand, the slope at the outer regions is not affected by the amount of angular momentum and has an almost constant value equal to that predicted in the radial collapse case.

astro-ph↗

arXiv2001

The velocity field of collapsing spherical structures. Limitations of the spherical infall model in mass estimation

We assume that the amplitude of the caustics in redshift space is a sum of two components: the first one can be predicted by the spherical infall model with no random motion, and the second is due to the random motion distribution. Smooth model curves are used to estimate the maximum values of the first component for the Coma cluster. Then, an approximation of the radial component of the infall velocity --based on the above curves-- is derived and a mass profile of the cluster is calculated. This mass profile, that is an upper limit for the spherical infall model, combined with estimations given by other authors provides an approximation of a lower limit for the mass of the system.

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