arXiv subjects

Peter A. Thomas

Publications and source records attributed to Peter A. Thomas.

At least 73 records · Page 4Linked to original sources

arXiv2006

Evolution of X-ray cluster scaling relations in simulations with radiative cooling and non-gravitational heating

We investigate the redshift dependence of X-ray cluster scaling relations drawn from three hydrodynamic simulations of the LCDM cosmology: a Radiative model that incorporates radiative cooling of the gas, a Preheating model that additionally heats the gas uniformly at high redshift, and a Feedback model that self-consistently heats cold gas in proportion to its local star-formation rate. While all three models are capable of reproducing the observed local Lx-Tx relation, they predict substantially different results at high redshift (to z=1.5), with the Radiative, Preheating and Feedback models predicting strongly positive, mildly positive and mildly negative evolution, respectively. The physical explanation for these differences lies in the structure of the intracluster medium. All three models predict significant temperature fluctuations at any given radius due to the presence of cool subclumps and, in the case of the Feedback simulation, reheated gas. The mean gas temperature lies above the dynamical temperature of the halo for all models at z=0, but differs between models at higher redshift with the Radiative model having the lowest mean gas temperature at z=1.5. We have not attempted to model the scaling relations in a manner that mimics the observational selection effects, nor has a consistent observational picture yet emerged. Nevertheless, evolution of the scaling relations promises to be a powerful probe of the physics of entropy generation in clusters. First indications are that early, widespread heating is favored over an extended period of heating that is associated with galaxy formation.

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arXiv2006

On the population of primordial star clusters in the presence of UV background radiation

We use the algorithm of Cole et al. (2000) to generate merger trees for the first star clusters in a Lambda CDM cosmology under an isotropic UV background radiation field, parametrized by J_21. We have investigated the problem in two ways: a global radiation background and local radiative feedback surrounding the first star clusters. Cooling in the first halos at high redshift is dominated by molecular hydrogen, H_2 - we call these Generation 1 objects. At lower redshift and higher virial temperature, T_vir > 10^4K, electron cooling dominates - we call these generation 2. Radiation fields act to photo-dissociate H_2, but also generate free electrons that can help to catalyse its production. At modest radiation levels, J_{21}/(1+z)^3 ~ 10^{-12}-10^{-7}, the nett effect is to enhance the formation of Generation 1 star-clusters. At higher fluxes the heating from photo-ionisation dominates and halts their production. With a realistic build-up of flux over time, the period of enhanced H_2 cooling is so fleeting as to be barely discernable and the nett effect is to move primordial star cluster formation towards Generation 2 objects at lower redshift. A similar effect is seen with local feedback. Provided that enough photons are produced to maintain ionization of their host halo, they will suppress the cooling in Generation 1 halos and boost the numbers of primordial star clusters in Generation 2 halos. Significant suppression of Generation~1 halos occurs for specific photon fluxes in excess of about 10^{43} ph s^{-1} Msun^{-1}.

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arXiv2004

Cosmological simulations of the intracluster medium

We investigate the properties of the intracluster medium (ICM) that forms within N-body/hydrodynamical simulations of galaxy clusters in a \LambdaCDM cosmology. When radiative cooling and a simple model for galactic feedback are included, our clusters have X-ray luminosities and temperatures in good agreement with observed systems, demonstrating the required excess entropy in their cores. More generally, cooling and feedback increases the entropy of the ICM everywhere, albeit without significantly affecting the slope of the profile (S prop. to r) at large radii. The temperature of the ICM is only modestly increased by these processes, with projected temperature profiles being in reasonable agreement with the observations. Star/galaxy formation is still too efficient in our simulations, however, and so our gas mass fractions are around 60 per cent of the observed value at r_{2500}. Finally, we examine the reliability of using the hydrostatic equilibrium equation to estimate cluster masses and find that it underpredicts the true mass of our clusters by up to 20 per cent, due to incomplete thermalisation of the gas. Feedback reduces this discrepancy, however, with estimates being accurate to within 10 per cent out to r_{500}.

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arXiv2004

The merger history of clusters and its effect on the X-ray properties of the intracluster medium

We investigate the growth over time of 20 massive (> 3 keV) clusters in a hydrodynamical simulation of the LambdaCDM cosmology with radiative cooling. The clusters show a variety of formation histories: some accrete most of their mass in major mergers; others more gradually. During major mergers the long-term (temporally-smoothed) luminosity increases such that the cluster moves approximately along the Lx-Tx relation; between times it slowly decreases, tracking the drift of the Lx-Tx relation. We identify several different kinds of short-term luminosity and temperature fluctuations associated with major mergers including double-peaked mergers in which the global intracluster medium merges first (Lx and Tx increase together) and then the cluster cores merge (Lx increases and Tx decreases). At both luminosity peaks, clusters tend to appear spherical and relaxed, which may lead to biases in high-redshift, flux-limited samples. There is no simple relationship between scatter in the Lx-Tx relation and either recent or overall merger activity or cluster formation time. The scatter in the Lx-M and Tx-M relations is reduced if properties are measured within R_500 rather than R_vir.

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arXiv2004

Hydrodynamical simulations of the Sunyaev-Zel'dovich effect: cluster scaling relations and X-ray properties

The Sunyaev-Zel'dovich effect is a powerful new tool for finding and studying clusters at high redshift, particularly in combination with their X-ray properties. In this paper we quantify the expected scaling relations between these properties using numerical simulations with various models for heating and cooling of the cluster gas. For a Non-radiative model, we find scaling relations in good agreement with self-similar predictions: $Y\propto T_X^{5/2}$ and $Y \propto L_X^{5/4}$. Our main results focus on predictions from Cooling and Preheating simulations, shown by Muanwong et al. (2002) to provide a good match to the X-ray scaling relations at z=0. For these runs we find slopes of approximately $Y \proptoT_X^3$ and $Y \propto L_X$, steeper and flatter than the self-similar scalings respectively. We also study the redshift evolution of the scaling relations and find the slopes show no evidence of evolution out to redshifts well beyond one, while the normalizations of relations between the SZ signal and X-ray properties do show evolution relative to that expected from self-similarity, particularly at z<1.

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arXiv2003

The impact of galaxy formation on X-ray groups

Using hydrodynamical simulations of the LCDM cosmology, that include both radiative cooling and a phenomenological model for star formation and supernovae feedback, we investigate the impact of galaxy formation on the X-ray properties of groups at redshift zero. Motivated by the observed ``break'' in the L_x-T_x relation at kT~1-2keV, our feedback model is based on the assumption that supernovae imprint a temperature scale on the hot gas, with the star formation rate and corresponding reheated gas mass then depending only on the available energy budget. We demonstrate that a strong feedback model with a heating temperature comparable to this break (kT_=2keV), and an energy budget twice that available from supernovae (epsilon=2), raises the core entropy of groups sufficiently to produce an adequate match to their observed X-ray properties. A lower value of epsilon increases the star formation rate without significantly affecting the X-ray properties of groups, and a model with epsilon~0.1 reproduces the observed fraction of baryons in stars. However, a heating temperature which is lower than the virial temperatures of the groups leads to an excess of cooling gas that boosts their X-ray luminosities, due to the failure of the reheated material to escape from the gravitational potential. A limited study of numerical resolution effects reveals that the temperature of poorly-resolved objects is underestimated, therefore (in our case) a fully-resolved group population would lead to a steeper L_x-T_x relation, bringing our results into even better agreement with the observations.

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arXiv2002

Simulated X-ray Cluster Temperature Maps

Temperature maps are presented of the 9 largest clusters in the mock catalogues of Muanwong et al. for both the Preheating and Radiative models. The maps show that clusters are not smooth, featureless systems, but contain a variety of substructure which should be observable. The surface brightness contours are generally elliptical and features that are seen include cold clumps, hot spiral features, and cold fronts. Profiles of emission-weighted temperature, surface brightness and emission-weighted pressure across the surface brightness discontinuities seen in one of the bimodal clusters are consistent with the cold front in Abell 2142 observed by Markevitch et al.

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arXiv2002

The mass-function of primordial star clusters

We use the block model to generate merger trees for the first star clusters in a LCDM cosmology. Using a simple collapse model and cooling criterion, we determine which halos are able to form stars before being disrupted by mergers. We contrast the mass functions of all the resulting star clusters and those of primordial composition, i.e. star clusters that have not been contaminated by subclusters inside them. In confirmation of previous work, two generations of primordial star clusters are identified: low-temperature clusters that cool via molecular hydrogen, and high-temperature clusters that cool via electronic transitions. The former dominate by number, but the two populations contain a similar mass with the precise balance depending upon the details of the model. We speculate on the current-day distribution of Population III stars.

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arXiv2002

The Effect of Cooling and Preheating on the X-ray Properties of Clusters of Galaxies

We calculate X-ray properties of present-day galaxy clusters from hydrodynamical cosmological simulations of the LCDM cosmology and compare these with recent X-ray observations. Results from three simulations are presented, each of which uses the same initial conditions: a standard adiabatic, Non-radiative model, a Radiative model that includes radiative cooling of the gas, and a Preheating model that also includes cooling but in addition impulsively heats the gas prior to cluster formation. At the end of the simulations, the global cooled baryon fractions in the latter two runs are 15 per cent and 0.4 per cent respectively which bracket the recent result from the K-band luminosity function. We construct cluster catalogues which consist of over 500 clusters and are complete in mass down to 1.18*10^{13} Msun/h. While clusters in the Non-radiative model behave in accord with the self-similar picture, those of the other two models reproduce key aspects of the observed X-ray properties: the core entropy, temperature-mass and luminosity-temperature relations are all in good agreement with recent observations. This agreement stems primarily from an increase in entropy with respect to the Non-radiative clusters. Although the physics affecting the intra-cluster medium is very different in the two models, the resulting cluster entropy profiles are very similar.

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arXiv2001

Can simulations reproduce the observed temperature-mass relation for clusters of galaxies?

It has become increasingly apparent that traditional hydrodynamical simulations of galaxy clusters are unable to reproduce the observed properties of galaxy clusters, in particular overpredicting the mass corresponding to a given cluster temperature. Such overestimation may lead to systematic errors in results using galaxy clusters as cosmological probes, such as constraints on the density perturbation normalization sigma_8. In this paper we demonstrate that inclusion of additional gas physics, namely radiative cooling and a possible preheating of gas prior to cluster formation, is able to bring the temperature-mass relation in the innermost parts of clusters into good agreement with recent determinations by Allen, Schmidt & Fabian using Chandra data.

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arXiv2001

Hydrodynamic simulations of merging clusters of galaxies

We present the results of high-resolution AP3M+SPH simulations of merging clusters of galaxies. We find that the compression and shocking of the core gas during a merger can lead to large increases in bolometric X-ray luminosities and emission-weighted temperatures of clusters. Cooling flows are completely disrupted during equal-mass mergers, with the mass deposition rate dropping to zero as the cores of the clusters collide. The large increase in the cooling time of the core gas strongly suggests that cooling flows will not recover from such a merger within a Hubble time. Mergers with subclumps having 1/8th of the mass of the main cluster are also found to disrupt a cooling flow if the merger is head-on. However, in this case the entropy injected into the core gas is rapidly radiated away and the cooling flow restarts within a few Gyr of the merger. Mergers in which the subcluster has an impact parameter of 500kpc do not disrupt the cooling flow, although the mass deposition rate is reduced by \~30%. Finally, we find that equal mass, off-centre mergers can effectively mix gas in the cores of clusters, while head on mergers lead to very little mixing. Gas stripped from the outer layers of subclumps results in parts of the outer layers of the main cluster being well mixed, although they have little effect on the gas in the core of the cluster. None of the mergers examined here resulted in the ICM being well mixed globally.

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arXiv2000

Multiphase Smoothed-Particle Hydrodynamics

We adapt the Smoothed-Particle Hydrodynamics technique to allow a multiphase fluid in which SPH particles of widely differing density may be freely intermixed. Applications include modelling of galaxy formation and cooling flows.

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arXiv2000

Weak gravitational lensing in different cosmologies, using an algorithm for shear in three dimensions

We present the results of weak gravitational lensing statistics in four different cosmological $N$-body simulations. The data has been generated using an algorithm for the three-dimensional shear, which makes use of a variable softening facility for the $N$-body particle masses, and enables a physical interpretation for the large-scale structure to be made. Working in three-dimensions also allows the correct use of the appropriate angular diameter distances. Our results are presented on the basis of the filled beam approximation in view of the variable particle softening scheme in our algorithm. The importance of the smoothness of matter in the universe for the weak lensing results is discussed in some detail. The low density cosmology with a cosmological constant appears to give the broadest distributions for all the statistics computed for sources at high redshifts. In particular, the range in magnification values for this cosmology has implications for the determination of the cosmological parameters from high-redshift Type Ia Supernovæ. The possibility of determining the density parameter from the non-Gaussianity in the probability distribution for the convergence is discussed.

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arXiv1999

The local to global $H_0$ ratio and the SNe Ia results

The effects of differences between the local and global values of the Hubble parameter on the cosmologies consistent with studies of high-redshift Type Ia supernovæare discussed. It is found that with a local Hubble parameter around 10 per cent higher than the global value then open cosmological models (such as $Ω_{M} = 0.3, Ω_Λ=0$) are prefered and if the local value is around 20 per cent higher then standard cosmological models ($Ω_{M} = 1, Ω_Λ=0$) can be recovered. Even in the case where the Hubble parameter ratio is 1, low $Ω_M$ open cosmologies with $Ω_Λ = 0$ are not rejected at the 95 per cent confidence level.

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arXiv1999

In-shock Cooling in Numerical Simulations

We model a one-dimensional shock-tube using smoothed particle hydrodynamics and investigate the consequences of having finite shock-width in numerical simulations. We investigate the cooling of gas during passage through the shock for different cooling regimes. For a shock temperature of 10^5K, the maximum temperature of the gas is much reduced and the cooling time was reduced by a factor of 2. At lower temperatures, we are especially interested in the production of molecular Hydrogen and so we follow the ionization level and H_2 abundance across the shock. This regime is particularly relevent to simulations of primordial galaxy formation for halos in which the virial temperature of the galaxy is sufficiently high to partially re-ionize the gas. The effect of in-shock cooling is substantial: the maximum temperature the gas reaches compared to the theoretical temperature was found to vary between 0.15 and 0.81 for the simulations performed. The downstream ionization level is reduced from the theoretical level by a factor of between 2.4 and 12.5, and the resulting H_2 abundance was found to be reduced to a fraction of 0.45 to 0.74 of its theoretical value. At temperatures above 10^5K, radiative shocks are unstable and will oscillate. We reproduce these oscillations and find good agreement with the previous work of Chevalier and Imamura (1982), and Imamura, Wolff and Durisen (1984). The effect of in-shock cooling in such shocks is difficult to quantify, but is undoubtedly present.

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arXiv1999

Weak gravitational lensing in the standard Cold Dark Matter model, using an algorithm for three-dimensional shear

We investigate the effects of weak gravitational lensing in the standard Cold Dark Matter cosmology, using an algorithm which evaluates the shear in three dimensions. The algorithm has the advantage of variable softening for the particles, and our method allows the appropriate angular diameter distances to be applied to every evaluation location within each three-dimensional simulation box. We investigate the importance of shear in the distance-redshift relation, and find it to be very small. We also establish clearly defined values for the smoothness parameter in the relation, finding its value to be at least 0.88 at all redshifts in our simulations. From our results, obtained by linking the simulation boxes back to source redshifts of 4, we are able to observe the formation of structure in terms of the computed shear, and also note that the major contributions to the shear come from a very broad range of redshifts. We show the probability distributions for the magnification, source ellipticity and convergence, and also describe the relationships amongst these quantities for a range of source redshifts. We find a broad range of magnifications and ellipticities; for sources at a redshift of 4, 97{1/2}% of all lines of sight show magnifications up to 1.3 and ellipticities up to 0.195. There is clear evidence that the magnification is not linear in the convergence, as might be expected for weak lensing, but contains contributions from higher order terms in both the convergence and the shear.

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arXiv1998

Measuring the three-dimensional shear from simulation data, with applications to weak gravitational lensing

We have developed a new three-dimensional algorithm, based on the standard P$^3$M method, for computing deflections due to weak gravitational lensing. We compare the results of this method with those of the two-dimensional planar approach, and rigorously outline the conditions under which the two approaches are equivalent. Our new algorithm uses a Fast Fourier Transform convolution method for speed, and has a variable softening feature to provide a realistic interpretation of the large-scale structure in a simulation. The output values of the code are compared with those from the Ewald summation method, which we describe and develop in detail. With an optimal choice of the high frequency filtering in the Fourier convolution, the maximum errors, when using only a single particle, are about 7 per cent, with an rms error less than 2 per cent. For ensembles of particles, used in typical $N$-body simulations, the rms errors are typically 0.3 per cent. We describe how the output from the algorithm can be used to generate distributions of magnification, source ellipticity, shear and convergence for large-scale structure.

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arXiv1998

Galaxy Clusters in the Hubble Volume Simulations

We report on analyses of cluster samples obtained from the Hubble Volume Simulations. These simulations, an $Ω=1$ model named $τ$CDM and a flat low $Ω$ model with a cosmological constant ($Λ$CDM), comprise the largest computational efforts to date in numerical cosmology. We investigate the presence of massive galaxy clusters at $z\approx 0.8$. The $τ$CDM model fails to form clusters at such a redshift. However, due to the small number of observed clusters around $z\approx 0.8$ and the uncertainties in the determinations of their masses, this conclusion still is somewhat preliminary. We produce cluster catalogs at $z=0$ for both cosmologies and investigate their two--point correlation function $ξ$. We show that the relationship between the mean density of subsamples of clusters, expressed via their mean separation $d_{\rm c}$, and the correlation length $r_0$, defined through $ξ(r_0) = 1$, is not linear but turns over gently for large $d_{\rm c}$. An analytic prediction by Mo & White (1996) overpredicts $r_0$. The results from the analysis of the APM cluster data by Croft et al. (1997) are nicely matched by the $Λ$CDM model.

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