arXiv subjects

Hugo Martel

Publications and source records attributed to Hugo Martel.

72 records · Page 4Linked to original sources

arXiv2001

Explosions and Outflows during Galaxy Formation

We consider an explosion at the center of a halo which forms at the intersection of filaments inside a cosmological pancake, a convenient test-bed model for galaxy formation. ASPH/P3M simulations reveal that such explosions are anisotropic. The energy and metals are channeled into the low density regions, away from the pancake. The pancake remains essentially undisturbed, even if the explosion is strong enough to blow away all the gas located inside the halo and reheat the IGM surrounding the pancake. Infall quickly replenishes this ejected gas and gradually restores the gas fraction as the halo continues to grow. Estimates of the collapse epoch and SN energy-release for galaxies of different mass in the CDM model can relate these results to scale-dependent questions of blow-out and blow-away and their implication for early IGM heating and metal enrichment and the creation of gas-poor dwarf galaxies.

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arXiv2001

The Evolution of Dark-Matter Dominated Cosmological Halos

Adaptive SPH and N-body simulations were carried out to study the evolution of the equilibrium structure of dark matter halos that result from the gravitational instability and fragmentation of cosmological pancakes. Such halos resemble those formed by hierarchical clustering from realistic initial conditions in a CDM universe and, therefore, serve as a test-bed model for studying halo dynamics. The dark matter density profile is close to the universal halo profile identified previously from N-body simulations of structure formation in CDM, with a total mass and concentration parameter which grow linearly with scale factor a. When gas is included, this concentration parameter is slightly larger than the pure N-body result. We also find that the dark matter velocity distribution is less isotropic and more radial than found by N-body simulations of CDM.

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arXiv2001

Light Propagation in Inhomogeneous Universes II. Cosmological Parameter Survey

Using a multiple-lens plane algorithm, we study light propagation in inhomogeneous universes, for 43 different COBE-normalized Cold Dark Matter models, with various values of the density parameter Omega_0, cosmological constant lambda_0, Hubble constant H_0, and rms density fluctuation sigma_8. This is the largest cosmological parameter survey ever done in this field. We performed a total of 3,798 experiments, each experiment consisting of propagating a square beam of angular size 21.9'' x 21.9'' composed of 116,281 light rays from the observer up to redshift z=3. These experiments provide statistics of the magnification, shear, and multiple imaging of distant sources. The results of these experiments can be compared with observations, and eventually help constraining the possible values of the cosmological parameters. Additionally, they provide insight into the gravitational lensing process and its complex relationship with the various cosmological parameters.

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arXiv2000

COBE- and Cluster-Normalized CDM Simulations

We introduce a set of four new publicly available N-body simulations, the most recent additions to the Texas P^3M Database. Our models probe the less studied parameter space region of moderate volume (100/h Mpc box) combined with fine mass resolution (~10^12 M_Sol, roughly comparable to a L_* galaxy), making these simulations especially suitable for study of major large-scale structure features such as voids, and for comparison with the largest three-dimensional redshift surveys currently available. Our cosmological models (LCDM, TOCDM, OCDM, TCDM) are all COBE-normalized, and when possible (LCDM and TOCDM) also cluster-normalized, based on the X-ray cluster M--T relation. The COBE- and cluster-normalized LCDM model reiterates the attractiveness of this currently favored model which does not require the introduction of tilt in order to fit the constraints imposed by observations of other cosmological parameters.

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arXiv2000

Explosions During Galaxy Formation

As an idealized model of the effects of energy release by supernovae during galaxy formation, we consider an explosion at the center of a halo which forms at the intersection of filaments in the plane of a cosmological pancake by gravitational instability during pancake collapse. Such halos resemble the virialized objects found in N-body simulations in a CDM universe and, therefore, serve as a convenient, scale-free test-bed model for galaxy formation. ASPH/P3M simulations reveal that such explosions are anisotropic. The energy and metals are channeled into the low density regions, away from the pancake plane. The pancake remains essentially undisturbed, even if the explosion is strong enough to blow away all the gas located inside the halo at the onset of the explosion and reheat the IGM surrounding the pancake. Infall quickly replenishes this ejected gas and gradually restores the gas fraction as the halo mass continues to grow. Estimates of the collapse epoch and SN energy-release for galaxies of different mass in the CDM model can relate these results to scale-dependent questions of blow-out and blow-away and their implication for early IGM heating and metal enrichment and the creation of dark-matter-dominated dwarf galaxies.

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arXiv2000

The Effect of Gasdynamics on the Structure of Dark Matter Halos

Adaptive SPH and N-body simulations were carried out to study the effect of gasdynamics on the structure of dark matter halos that result from the gravitational instability and fragmentation of cosmological pancakes. Such halos resemble those formed in a hierarchically clustering CDM universe and serve as a test-bed model for studying halo dynamics. With no gas, the density profile is close to the universal profile identified previously from N-body simulations of structure formation in CDM. When gas is included, the gas in the halo is approximately isothermal, and both the dark matter and the gas have singular central density profiles which are steeper than that of the dark matter with no gas. This worsens the disagreement between observations of constant density cores in cosmological halos and the singular ones found in simulations. We also find that the dark matter velocity distribution is less isotropic than found by N-body simulations of CDM, because of the strongly filamentary substructure.

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arXiv1999

Explosions During Galaxy Formation. Scale-Free Simulations

When density fluctuations collapse gravitationally out of the expanding cosmological background universe to form galaxies, the secondary energy release which results can affect their subsequent evolution profoundly. We focus here on the effects of one form of such energy release - explosions, such as might result from the supernovae which end the lives of the first generation of massive stars to form inside protogalaxies. We are particularly interested in the consequences of the nonspherical geometry and continuous infall which are characteristic of galaxy formation from realistic initial and boundary conditions. As an idealized model which serves to illustrate and quantify the importance of these effects, we study the effect of explosions on the quasi-spherical objects which form at the intersections of filaments in the plane of a cosmological pancake, as a result of gravitational instability and fragmentation of the pancake. We study the formation and evolution of these "galaxies," subject to the explosive injection of energy at their centers, by numerical gas dynamical simulation in 3D utilizing our new, anisotropic version of Smoothed Particle Hydrodynamics, Adaptive SPH ("ASPH"), with a P3M gravity solver.

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arXiv1999

The Asymptotic Collapsed Fraction in an Eternal Universe

We calculate the maximum fraction of matter which is able to condense out of the expanding background universe, for any universe which will expand forever. We use a simple spherical model for the growth of density fluctuations in the universe. This model includes open, matter-dominated universe and universes in which there is an uniform background component (e.g. the cosmological constant or "quintessence"). In these background universes, Omega eventually drops significantly below unity. When this happens, gravitational instability is suppressed and, with it, so is the growth of the collapsed fraction. We identify a limitation of the Press-Schechter (PS) approximation. In this approximation, the mass function determined from the predicted collapse of overdense regions is multiplied by a correction factor of 2, assuming that each bound fluctuation will accrete an equal share of mass from nearby underdense regions. Our model determines the actual value of the correction factor. We show that, while the factor of 2 adopted by the PS approximation is correct for an Einstein-de Sitter universe, it is not correct when the freeze-out of fluctuation growth inherent in the more general class of background universes described above occurs. When freeze-out occurs, the correction factor reduces to unity and the PS approximation must overestimate the collapsed fraction. We apply our model to open CDM and flat Lambda-CDM models. For H0=70 and Omega0=0.3, these models yield asymptotic collapsed fractions of 0.0361 and 0.0562, respectively, on the galaxy cluster mass-scale, only 55% of the values determined by the PS approximation. These results have implications for the use of the latter approximation to compare the observed space density of X-ray clusters today with that predicted by cosmological models.

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arXiv1999

A Database of COBE-Normalized CDM Simulations (Abbreviated Version)

We have simulated the formation and evolution of large-scale structure in the universe, for 68 different COBE-normalized cosmological models. For each cosmological model, we have performed between 1 and 3 simulations, for a total of 160 simulations. This constitutes the largest database of cosmological simulations ever assembled, and the largest cosmological parameter space ever covered by such simulations. We are making this database available to the astronomical community. We provide instructions for accessing the database and for converting the data from computational units to physical units. The database includes Tilted Cold Dark Matter (TCDM) models, Tilted Open Cold Dark Matter (TOCDM) models, and Tilted Lambda Cold Dark Matter (TLCDM) models. (For several simulations, the primordial exponent n of the power spectrum is near unity, hence these simulations can be considered as "untilted.") The simulations cover a 4-dimensional cosmological parameter phase space, the parameters being the present density parameter Omega_0, cosmological constant lambda_0, and Hubble constant H_0, and the rms density fluctuation sigma_8 at scale 8/h Mpc. All simulations were performed using a P3M algorithm with 64^3 particles on a 128^3 mesh, in a cubic volume of comoving size 128 Mpc. Each simulation starts at a redshift of 24, and is carried up to the present. More simulations will be added to the database in the future.

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arXiv1999

Statistics of Weak Gravitational Lensing in Cold Dark Matter Models; Magnification Bias on Quasar Luminosity Functions

We compute statistical properties of weak gravitational lensing by large-scale structure in three Cold Dark Matter models. We use a P$^3$M $N$-body code to simulate the formation and evolution of large-scale structure in the universe. We perform $1.1\times10^7$ ray-tracing experiments for each model using the multiple lens-plane algorithm. From the results of these experiments, we calculate the probability distribution functions (PDF) of the convergences, shears, and magnifications, and their root-mean-square (rms) values. We find that the rms values of the convergence and shear agree with the predictions of a nonlinear analytical model. We also find that the PDFs of the magnifications $μ$ have a peak at values slightly smaller than $μ=1$, and are strongly skewed toward large magnifications. In particular, for the high-density model, a power-law tail appears in the magnification distribution at large magnifications for sources at redshifts $z_s>2$. The rms values of the magnifications essentially agree with the nonlinear analytical predictions for sources at low redshift, but exceed these predictions for high redshift sources, once the power-law tail appears. We study the effect of magnification bias on the luminosity functions of high-redshift quasars, using the calculated PDFs of the magnifications. We show that the magnification bias is moderate in the absence of the power-law tail in the magnification distribution, but depends strongly on the value of the density parameter. In presence of the power-law tail, the bias becomes considerable, especially at the bright end of the luminosity functions.

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arXiv1998

Light Propagation in inhomogeneous Universes

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arXiv1998

Light Propagation in inhomogeneous Universes

Using a multi-plane lensing method that we have developed, we follow the evolution of light beams as they propagate through inhomogeneous universes. We use a P3M code to simulate the formation and evolution of large-scale structure. The resolution of the simulations is increased to sub-Megaparsec scales by using a Monte Carlo method to locate galaxies inside the computational volume according to the underlying particle distribution. The galaxies are approximated by isothermal spheres, with each morphological type having its own distribution of masses and core radii. The morphological types are chosen in order to reproduce the observed morphology-density relation. This algorithm has an effective resolution of 9 orders of magnitudes in length, from the size of superclusters down to the core radii of the smallest galaxies. We consider cold dark matter models normalized to COBE, and perform a large parameter survey by varying the cosmological parameters Omega_0, lambda_0, H_0, and n (the tilt of the primordial power spectrum). The values of n are chosen by imposing particular values or sigma_8, the rms mass fluctuation at a scale of 8/h Mpc. We use the power spectrum given by Bunn & White. This is the largest parameter survey ever done is this field.

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arXiv1997

Adaptive Smoothed Particle Hydrodynamics: Methodology II

This paper presents an alternative formulation of the ASPH algorithm for evolving anisotropic smoothing kernels, in which the geometric approach of Shapiro et al. (1996; Paper I) is replaced by an approach involving a local transformation of coordinates to those in which the underlying anisotropic volume changes appear to be isotropic. The ASPH method is presented in 2D and 3D, including a number of details not previously included in Paper I, some of which represent either advances or different choices with respect to Paper I. Among the advances included here are an asynchronous time-integration scheme with different time steps for different particles and the generalization of the ASPH method to 3D. The shock-tracking algorithm described in Paper I for locally adapting the artificial viscosity to restrict viscous heating just to particles encountering shocks, is not included here. Instead, we adopt a different interpolation kernel for use with the artificial viscosity, which has the effect of spatially localizing effects of the artificial viscosity. This version of the ASPH method in 2D and 3D is then applied to a series of 1D, 2D, and 3D test problems, and the results are compared to those of standard SPH applied to the same problems. These include the problem of cosmological pancake collapse, the Riemann shock tube, cylindrical and spherical Sedov blast waves, the collision of two strong shocks, and problems involving shearing disks intended to test the angular momentum conservation properties of the method. These results further support the idea that ASPH has significantly better resolving power than standard SPH for a wide range of problems, including that of cosmological structure formation. (Abridged)

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arXiv1997

A Convenient Set of Comoving Cosmological Variables and Their Application

We present a set of cosmological variables, called "supercomoving variables," which are particularly useful for describing the gas dynamics of cosmic structure formation. For ideal gas with gamma=5/3, the supercomoving position, velocity, density, temperature, and pressure are constant in time in a uniform, isotropic, adiabatically expanding universe. Expressed in terms of these supercomoving variables, the cosmological fluid conservation equations and the Poisson equation closely resemble their noncosmological counterparts. This makes it possible to generalize noncosmological results and techniques to cosmological problems, for a wide range of cosmological models. These variables were initially introduced by Shandarin for matter-dominated models only. We generalize supercomoving variables to models with a uniform component corresponding to a nonzero cosmological constant, domain walls, cosmic strings, a nonclumping form of nonrelativistic matter (e.g. massive nettrinos), or radiation. Each model is characterized by the value of the density parameter Omega0 of the nonrelativistic matter component in which density fluctuation is possible, and the density parameter OmegaX of the additional, nonclumping component. For each type of nonclumping background, we identify FAMILIES within which different values of Omega0 and OmegaX lead to fluid equations and solutions in supercomoving variables which are independent of Omega0 and OmegaX. We also include the effects of heating, radiative cooling, thermal conduction, viscosity, and magnetic fields. As an illustration, we describe 3 familiar cosmological problems in supercomoving variables: the growth of linear density fluctuations, the nonlinear collapse of a 1D plane-wave density fluctuation leading to pancake formation, and the Zel'dovich approximation.

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arXiv1997

Morphological Evolution of Galaxies

We simulate the growth of large-scale structure in the universe using a N-body code. By combining these simulations with a Monte-Carlo method, we generate galaxy distributions at present that reproduces the observed morphology-density relation, with most ellipticals concentrated in the densest regions. By "tying" each galaxy to its nearest particle, we trace the trajectory of that galaxy back in time. This enables us to reconstruct the distribution of galaxies at high redshift, and the trajectory of each galaxy from its formation epoch to the present. Our goal is to determine whether the morphological type of galaxies is primarily determined by the initial conditions in which these galaxies form, or by evolutionary processes occurring later. We compare the environment in which galaxies are at the epoch of galaxy formation (z=3) and at the present. Making the null hypothesis that morphological types do not evolve, we compare the galaxies that form in low density environments but end up later in high density environments to the ones that form also in low density environment but remain in low density environment. The first group contains a larger proportion of E and S0 galaxies than the second group. We assume that the galaxy formation process cannot distinguish a low density environment that will always remain low density from one that will eventually become high density. Hence, these results force us to discard the null hypothesis of no morphological evolution. Our study suggests that 75% of the E and S0 galaxies observed at present formed as such, while the remaining 25% formed as spirals, and underwent morphological evolution. We conclude that most galaxies did not undergo morphological evolution, but a non-negligible fraction did.

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arXiv1997

Light Propagation in Inhomogeneous Universes I: Methodology and Preliminary Results

We describe a numerical algorithm which simulates the propagation of light in inhomogeneous universes, using the multiple lens-plane method. The deformation and deflection of light beams as they interact with each lens plane are computed using the filled-beam approximation. We use a N-body code to simulate large-scale structure formation in the universe. We extend the length resolution of the simulations to sub-Megaparsec scales by using a Monte-Carlo method for locating galaxies according to the underlying distribution of background matter. The observed galaxy 2-point correlation function and morphology-density relation are reproduced. This algorithm constitutes a major improvement over previous methods. We test this algorithm for three different CDM models: an Einstein-de Sitter model, an open model, and a flat model with a nonzero cosmological constant. In each case, we compute the shear and magnification caused by the presence of inhomogeneities. Our results are the following: (1) The magnification is totally dominated by the convergence; the shear contributes less than one part in 10^4. (2) Most of the cumulative shear and magnification is contributed by matter located at intermediate redshifts z=1-2. (3) The redshift where the largest contribution to shear and magnification occurs is model-dependent. (4) The bulk of the shear and magnification does not originate from direct hits with galaxies, but rather from the tidal influence of nearby and more distant galaxies, and background matter. (5) The average contributions of background matter and nearby galaxies to the shear is comparable for models with small Omega0. For the Einstein-de Sitter model, the contribution of the background matter exceeds the one of nearby galaxies by nearly one order of magnitude.

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arXiv1997

Likely Values of the Cosmological Constant

In theories in which the cosmological constant Lambda takes a variety of values in different ``subuniverses,'' the probability distribution of its observed values is conditioned by the requirement that there must be someone to measure it. This probability is proportional to the fraction of matter which is destined to condense out of the background into mass concentrations large enough to form observers. We calculate this ``collapsed fraction'' by a simple, pressure-free, spherically symmetric, nonlinear model for the growth of density fluctuations in a flat universe with arbitrary value of the cosmological constant, applied in a statistical way to the observed spectrum of density fluctuations at recombination. From this, the probability distribution for the vacuum energy density rho_V=Lambda/8pi G for Gaussian random density fluctuations is derived analytically. It is shown that the results depend on only one quantity, sigma^3 RHO, where sigma^2 and RHO are the variance and mean value of the fluctuating matter density field at recombination, respectively. To calculate sigma, we adopt the flat CDM model with nonzero cosmological constant and fix the amplitude and shape of the primordial power spectrum in accordance with data on cosmic microwave background anisotropy from the COBE satellite DMR experiment. A comparison of the results of this calculation of the likely values of rho_V with present observational bounds on the cosmological constant indicates that the small, positive value of rho_V (up to 3 times greater than the present cosmic mass density) suggested recently by several lines of evidence is a reasonably likely value to observe, even if all values of rho_V are equally likely a priori.

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arXiv1996

Gravitational Instability in Collisionless Cosmological Pancakes

The gravitational instability of cosmological pancakes composed of collisionless dark matter in an Einstein-de Sitter universe is investigated numerically to demonstrate that pancakes are unstable with respect to fragmentation and the formation of filaments. A ``pancake'' is defined here as the nonlinear outcome of the growth of a 1D, sinusoidal, plane-wave, adiabatic density perturbation. We have used high resolution, 2D, N-body simulations by the Particle-Mesh (PM) method to study the response of pancakes to perturbation by either symmetric (density) or antisymmetric (bending or rippling) modes, with corresponding wavevectors k_s and k_a transverse to the wavevector k_p of the unperturbed pancake plane-wave. We consider dark matter which is initially ``cold'' (i.e. with no random thermal velocity in the initial conditions). We also investigate the effect of a finite, random, isotropic, initial velocity dispersion (i.e. initial thermal velocity) on the fate of pancake collapse and instability. Pancakes are shown to be gravitationally unstable with respect to all perturbations of wavelength l<l_p (where l_p= 2pi/k_p). These results are in contradiction with the expectations of an approximate, thin-sheet energy argument.

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