arXiv subjects

Tom Theuns

Publications and source records attributed to Tom Theuns.

230 records · Page 13Linked to original sources

arXiv2000

The thermal history of the intergalactic medium

At redshifts z >~ 2, most of the baryons reside in the smooth intergalactic medium which is responsible for the low column density Lyman-alpha forest. This photoheated gas follows a tight temperature-density relation which introduces a cut-off in the distribution of widths of the Lyman-alpha absorption lines (b-parameters) as a function of column density. We have measured this cut-off in a sample of nine high resolution, high signal-to-noise quasar spectra, and determined the thermal evolution of the intergalactic medium in the redshift range 2.0-4.5. At redshift z ~ 3, the temperature at the mean density shows a peak and the gas becomes nearly isothermal. We interpret this as evidence for the reionization of HeII.

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arXiv2000

A wavelet analysis of QSO spectra

The temperature of the intergalactic medium (IGM) is an important factor in determining the line-widths of the absorption lines in the Lyman-alpha forest. We present a method to characterise the line-widths distribution using a decomposition of a Lyman-alpha spectrum in terms of discrete wavelets. Such wavelets form an orthogonal basis so the decomposition is unique. We demonstrate using hydrodynamic simulations that the mean and dispersion of the wavelet amplitudes is strongly correlated with both the temperature of the absorbing gas and its dependence on the gas density. Since wavelets are also localised in space, we are able to analyse the temperature distribution as a function of position along the spectrum. We illustrate how this method could be used to identify fluctuations in the IGM temperature which might result from late reionization or local effects.

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arXiv2000

Lyman alpha Absorption Systems and the Intergalactic Medium

The last few years have seen a dramatic improvement in our understanding of the origin of Lyman alpha absorption systems. Hydrodynamic numerical simulations of cold dark matter dominated universes have shown that the many properties of the Lyman alpha absorption systems can be explained by a photoionized, space-filling, intergalactic medium. Lyman alpha lines offer promising probes of the photoionizing background, the amplitude of the mass fluctuations at high redshift and the evolution of the equation of state of the intergalactic medium.

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arXiv2000

Constraining reionization using the thermal history of the baryons

The thermal evolution of the intergalactic medium (IGM) depends on the reionization history of the universe. Numerical simulations indicate that the low density IGM, which is responsible for the low column density Ly-alpha forest, follows a well defined temperature-density relation. This results in a cut-off in the distribution of line widths as a function of column density. We use hydrodynamic simulations to calibrate the relation between the cut-off and the temperature-density relation and apply this relation to Keck spectra spanning a redshift range z=2-4.5. We find that the temperature peaks at z~3 and interpret this as evidence for reheating due to the reionization of helium.

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arXiv1999

Measuring the equation of state of the intergalactic medium

(Abridged) Numerical simulations indicate that the intergalactic medium (IGM) responsible for the low column density Lyman-alpha forest follows a well defined temperature-density relation. We demonstrate that such an equation of state results in a cutoff in the distribution of line widths (b-parameters) as a function of column density (N) for the low column density absorption lines. This explains the existence of the lower envelope which is clearly seen in scatter plots of the b(N)-distribution in observed QSO spectra. We show that the parameters of the cutoff in the b(N)-distribution are strongly correlated with the parameters of the underlying equation of state. We use simulations to determine these relations, which can then be applied to the observed cutoff in the b(N)-distribution to measure the equation of state of the IGM. Using Monte Carlo simulations of Keck spectra at z=3, we show that determining the slope of the equation of state will be difficult, but that the amplitude can be determined to within ten per cent, even from a single QSO spectrum. Measuring the evolution of the equation of state with redshift will allow us to put tight constraints on the reionization history of the universe.

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arXiv1999

Measuring the temperature of the intergalactic medium

Numerical simulations indicate that the smooth, photoionized intergalactic medium (IGM) responsible for the low column density Lyman-alpha forest follows a well defined temperature-density relation. We show that such an equation of state results in a cutoff in the distribution of line widths (b-parameters) as a function of column density (N) for the low column density absorption lines. This explains the existence of the lower envelope which is clearly seen in scatter plots of the b(N)-distribution in observed QSO spectra. The intercept and slope of this cutoff can be used to measure the equation of state of the IGM. Measuring the evolution of the equation of state with redshift will allow us to put tight constraints on the reionization history of the universe.

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arXiv1998

Dependencies of QSO Lyman-alpha absorption line statistics on cosmological parameters

We have performed high-resolution hydrodynamic simulations of the Lyman-alpha forest in a variety of popular cold dark matter dominated cosmologies, including a low density and a vacuum-dominated model. The fluctuation amplitude of these models is chosen to match the observed abundance of galaxy clusters at low redshift. We assume that the intergalactic medium is photoionized and photoheated by a uniform UV-background with the required amplitude to give the observed mean hydrogen absorption. We produce simulated spectra, analyze them by fitting Voigt profiles and compare line statistics with those obtained from high resolution observations. All models give column density distributions in good agreement with observations. However, the distribution of line widths (the b-parameter distribution) reflects differences in the temperature of the intergalactic medium between the models, with colder models producing more narrow lines. All the models with a low baryon density, Omega_b h^2=0.0125, are too cold to produce a b-parameter distribution in agreement with observations. Models with a higher baryon density, Omega_b h^2=0.025, are hotter and provide better fits. Peculiar velocities contribute significantly to the line widths in models with low matter density, and this improves the agreement with observations further. We briefly discuss alternative mechanisms for reconciling the simulations with the observed b-parameter distributions.

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arXiv1998

P3M-SPH simulations of the Lyman-alpha Forest

(Abridged) We investigate the importance of several numerical artifacts such as lack of resolution on spectral properties of the Lyman alpha forest as computed from cosmological hydrodynamic simulations in a standard cold dark matter universe. We assume an ionising background produced by quasars as computed by Haardt & Madau. We use a new simulation code based on P3M and SPH, which we compare in detail with a modified version of HYDRA (Couchman et al.) and published results of TREESPH (Hernquist et al.). The agreement is very good between all three codes. We then use our new code to investigate several numerical effects such as resolution on spectral statistics deduced from Voigt profile fitting. Our highest resolution simulation has a mass resolution of 2.1x10^5 solar masses. The column density distribution is converged but the b-parameter distribution is only marginally converged. The simulation reproduces both the HI column density and b-parameter distribution when we assume a high baryon density, Omega_B h^2 > 0.028. In addition we need to impose a higher IGM temperature than predicted within our basic set of assumptions. The simulated HI optical depth is in good agreement with observations but the HeII optical depth is lower than observed. Fitting the latter requires a larger jump between the photon flux at the H and He edge than is present in the Haardt & Madau spectrum.

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arXiv1997

Non-linear evolution of the tidal angular momentum of protostructures II: non-Gaussian initial conditions

The formalism that describes the non-linear growth of the angular momentum L of protostructures from tidal torques in a Friedmann Universe, as developed in a previous paper, is extended to include non-Gaussian initial conditions. We restrict our analysis here to a particular class of non-Gaussian primordial distributions, namely multiplicative models. In such models, strongly correlated phases are produced by obtaining the gravitational potential via a nonlinear local transformation of an underlying Gaussian random field. The dynamical evolution of the system is followed by describing the trajectories of fluid particles using second-order Lagrangian perturbation theory. In the Einstein-de Sitter universe, the lowest-order perturbative correction to the variance of the linear angular momentum of collapsing structures grows as t^8/3 for generic non-Gaussian statistics, which contrasts with the t^10/3 growth rate characteristic of Gaussian statistics. This is a consequence of the fact that the lowest-order perturbative spin contribution in the non-Gaussian case arises from the third moment of the gravitational potential, which is identically zero for a Gaussian field. Evaluating these corrections at the maximum expansion time of the collapsing structure, we find that these non-Gaussian and non-linear terms can be as high as the linear estimate, without the degree of non-Gaussianity as quantified by skewness and kurtosis of the density field being unacceptably large. The results suggest that higher-order terms in the perturbative expansion may contribute significantly to galactic spin which contrasts with the straightforward Gaussian case.

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arXiv1996

Intergalactic stars in the Fornax Cluster

We have identified ten candidate intergalactic planetary nebulae in the Fornax galaxy cluster. These objects were found during observations in 1992 and 1993 in three fields chosen well away from any Fornax galaxy at 15 arcmin, 30 arcmin, and 45 arcmin from the centre of Fornax. We used the usual method of blinking images taken in a narrow OIII filter, with images taken in an adjacent broad filter. The measured fluxes in the narrow, broad, and I bands are consistent with these unresolved objects being planetary nebulae immersed in an intergalactic population of stars. Such a population is expected to arise as a consequence of tidal encounters between galaxies, and our findings strengthen the case for the existence of such tidal debris. The confirmation of some or all of these ten candidates as planetary nebulae would imply that intergalactic stars constitute a substantial fraction of all the stars in Fornax, up to an estimated 40 per cent. Intergalactic planetary nebulae could prove useful in probing the underlying cluster potential, since they would be far more abundant than galaxies. We discuss possible contamination of the sample by emission-line galaxies, but conclude that planetary nebulae is the most likely identification for the detected objects. keywords: intergalactic medium-- galaxies: interactions -- planetary nebulae: general -- clusters: individual: Fornax

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arXiv1996

Evolution of the angular momentum of protogalaxies from tidal torques: Zel'dovich approximation

The growth of the angular momentum L of protogalaxies induced by tidal torques is reconsidered within the Zel'dovich approximation. We obtain a general expression for the ensemble expectation value of the square of L in terms of the first and second invariant of the inertia tensor of the Lagrangian volume enclosing the protoobject's collapsing mass. We then specialize the formalism to the particular case in which this volume is centered on a peak of the smoothed Gaussian density field and approximated by an isodensity ellipsoid. The result is the appropriate analytical estimate for the rms angular momentum of peaks to be compared against simulations that make use of the Hoffman-Ribak algorithm to set up a constrained density field that contains a peak with given shape. Extending the work of Heavens & Peacock, we calculate the joint probability distribution function for several spin parameters and peak mass M using the distribution of peak shapes, for different initial power spectra. The values of observed specific angular momentum versus mass are well fitted by our theoretical isoprobability contours. In contrast, the observed lower values for the specific angular momentum for ellipticals of the same mass cannot be accounted for within our linear regime investigation, highlighting the importance of strongly non-linear phenomena to explain the spin of such objects.

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arXiv1996

Non-linear evolution of the angular momentum of protostructures from tidal torques

We discuss the non-linear evolution of the angular momentum L acquired by protostructures, like protogalaxies and protoclusters, due to tidal interactions with the surrounding matter inhomogeneities. The primordial density distribution is assumed to be Gaussian and the non-linear dynamics of the collisionless mass fluid is followed using Lagrangian perturbation theory. For a Cold Dark Matter spectrum, the inclusion of the leading-order Lagrangian correction terms results in a value of the rms ensemble average ^{1/2} which is only a factor of 1.3 higher than the corresponding linear estimate, irrespective of the scale. Consequently, the predictions of linear theory are rather accurate in quantifying the evolution of the angular momentum of protostructures before collapse sets in. In the Einstein-de Sitter universe, the initial torque is a good estimate for the tidal torque over the whole period during which the object is spun up.

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arXiv1996

Wind accretion in binary stars II. Accretion rates

Smoothed particle hydrodynamics (SPH) is used to estimate accretion rates of mass, linear and angular momentum in a binary system where one component undergoes mass loss through a wind. Physical parameters are chosen such as to model the alleged binary precursors of barium stars, whose chemical peculiarities are believed to result from the accretion of the wind from a companion formerly on the asymptotic giant branch (AGB). The binary system modelled consists of a 3 solar masses AGB star on the main sequence, in a 3AU circular orbit. Three-dimensional simulations are performed for gases with polytropic indices gamma=1, 1.1 and 1.5, to bracket more realistic situations that would include radiative cooling. Mass accretion rates are found to depend on resolution and we estimate typical values of 1-2% for the gamma=1.5 case and 8% for the other models. The highest resolution obtained (with 400k particles) corresponds to an accretor of linear size 16 solar radii. Despite being (in the gamma = 1.5 case) about ten times smaller than theoretical estimates based on the Bondi-Hoyle prescription, the SPH accretion rates remain large enough to explain the pollution of barium stars. Uncertainties in the current SPH rates remain however, due to the simplified treatment of the wind acceleration mechanism, as well as to the absence of any cooling prescription and to the limited numerical resolution. Angular momentum transfer leads to significant spin up of the accretor and can account for the rapid rotation of HD165141, a barium star with a young white dwarf companion and a rotation rate unusually large among K giants.

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arXiv1995

Numerical study of energy diffusion in King models

The energy diffusion coefficients D_n(E) (n=1,2) for a system of equal mass particles moving self-consistently in an N-body realisation of a King model are computed from the probability per unit time, P(E, Delta E), that a star with initial energy E will undergo an energy change Delta E. In turn, P is computed from the number of times during the simulation that a particle in a state of given energy undergoes a transition to another state. These particle states are defined directly from the time evolution of E by identifying them with the event occuring between two local maxima in the E(t) curve. If one assumes next that energy changes are uncorrelated between different states, one can use diffusion theory to compute D_n(E). The simulations employ N=512, 2048,... , 32768 particles and are performed using an implementation of Aarseth's direct integrator N-body1 on a massively parallel computer. The more than seven million transitions measured in the largest N simulation provide excellent statistics. The numerically determined D(E)'s are compared against their theoretical counterparts which are computed from phase-space averaged rates of energy change due to independent binary encounters. The overall agreement between them is impressive over most of the energy range, notwithstanding the very different type of approximations involved, giving considerable support to the valid usage of these theoretical expressions to simulate dynamical evolution in Fokker-Planck type calculations.

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