arXiv subjects

Frazer Pearce

Publications and source records attributed to Frazer Pearce.

28 records · Page 2Linked to original sources

arXiv2008

The Mass Assembly of Fossil Groups of Galaxies in the Millennium Simulation

The evolution of present-day fossil galaxy groups is studied in the Millennium Simulation. Using the corresponding Millennium gas simulation and semi-analytic galaxy catalogues, we select fossil groups at redshift zero according to the conventional observational criteria, and trace the haloes corresponding to these groups backwards in time, extracting the associated dark matter, gas and galaxy properties. The space density of the fossils from this study is remarkably close to the observed estimates and various possibilities for the remaining discrepancy are discussed. The fraction of X-ray bright systems which are fossils appears to be in reasonable agreement with observation, and the simulations predict that fossil systems will be found in significant numbers (3-4% of the population) even in quite rich clusters. We find that fossils assemble a higher fraction of their mass at high redshift, compared to non-fossil groups, with the ratio of the currently assembled halo mass to final mass, at any epoch, being about 10 to 20% higher for fossils. This supports the paradigm whereby fossils represent undisturbed, early-forming systems in which large galaxies have merged to form a single dominant elliptical.

astro-ph↗

arXiv2008

The Aspen--Amsterdam Void Finder Comparison Project

Despite a history that dates back at least a quarter of a century studies of voids in the large--scale structure of the Universe are bedevilled by a major problem: there exist a large number of quite different void--finding algorithms, a fact that has so far got in the way of groups comparing their results without worrying about whether such a comparison in fact makes sense. Because of the recent increased interest in voids, both in very large galaxy surveys and in detailed simulations of cosmic structure formation, this situation is very unfortunate. We here present the first systematic comparison study of thirteen different void finders constructed using particles, haloes, and semi--analytical model galaxies extracted from a subvolume of the Millennium simulation. The study includes many groups that have studied voids over the past decade. We show their results and discuss their differences and agreements. As it turns out, the basic results of the various methods agree very well with each other in that they all locate a major void near the centre of our volume. Voids have very underdense centres, reaching below 10 percent of the mean cosmic density. In addition, those void finders that allow for void galaxies show that those galaxies follow similar trends. For example, the overdensity of void galaxies brighter than $m_B = -20 $ is found to be smaller than about -0.8 by all our void finding algorithms.

astro-ph↗

arXiv2006

Fundamental differences between SPH and grid methods

We have carried out a hydrodynamical code comparison study of interacting multiphase fluids. The two commonly used techniques of grid and smoothed particle hydrodynamics (SPH) show striking differences in their ability to model processes that are fundamentally important across many areas of astrophysics. Whilst Eulerian grid based methods are able to resolve and treat important dynamical instabilities, such as Kelvin-Helmholtz or Rayleigh-Taylor, these processes are poorly or not at all resolved by existing SPH techniques. We show that the reason for this is that SPH, at least in its standard implementation, introduces spurious pressure forces on particles in regions where there are steep density gradients. This results in a boundary gap of the size of the SPH smoothing kernel over which information is not transferred.

astro-ph↗

arXiv2006

The Group Evolution Multiwavelength Study (GEMS): the Sample and Datasets

Galaxy groups have been under-studied relative to their richer counterparts -- clusters. The Group Evolution Multiwavelength Study (GEMS) aims to redress some the balance. Here we describe the GEMS sample selection and resulting sample of 60 nearby (distance < 130 Mpc) galaxy groups and our multiwavelength dataset of X-ray, optical and HI imaging. ROSAT X-ray images of each group are presented. GEMS also utilizes near-infrared imaging from the 2MASS survey and optical spectra from the 6dFGS. These observational data are complemented by mock group catalogues generated from the latest LCDM simulations with gas physics included. Existing GEMS publications are briefly highlighted as are future publication plans.

astro-ph↗

arXiv2005

Simulating the joint evolution of quasars, galaxies and their large-scale distribution

The cold dark matter model has become the leading theoretical paradigm for the formation of structure in the Universe. Together with the theory of cosmic inflation, this model makes a clear prediction for the initial conditions for structure formation and predicts that structures grow hierarchically through gravitational instability. Testing this model requires that the precise measurements delivered by galaxy surveys can be compared to robust and equally precise theoretical calculations. Here we present a novel framework for the quantitative physical interpretation of such surveys. This combines the largest simulation of the growth of dark matter structure ever carried out with new techniques for following the formation and evolution of the visible components. We show that baryon-induced features in the initial conditions of the Universe are reflected in distorted form in the low-redshift galaxy distribution, an effect that can be used to constrain the nature of dark energy with next generation surveys.

astro-ph↗

arXiv2005

The extragalactic sub-mm population: predictions for the SCUBA Half-Degree Extragalactic Survey (SHADES)

We present predictions for the angular correlation function and redshift distribution for SHADES, the SCUBA HAlf-Degree Extragalactic Survey, which will yield a sample of around 300 sub-mm sources in the 850 micron waveband in two separate fields. Complete and unbiased photometric redshift information on these sub-mm sources will be derived by combining the SCUBA data with i) deep radio imaging already obtained with the VLA, ii) guaranteed-time Spitzer data at mid-infrared wavelengths, and iii) far-infrared maps to be produced by BLAST, the Balloon-borne Large-Aperture Sub-millimeter Telescope. Predictions for the redshift distribution and clustering properties of the final anticipated SHADES sample have been computed for a wide variety of models, each constrained to fit the observed number counts. Since we are dealing with around 150 sources per field, we use the sky-averaged angular correlation function to produce a more robust fit of a power-law shape w(theta)=(theta/A)^{-delta} to the model data. Comparing the predicted distributions of redshift and of the clustering amplitude A and slope delta, we find that models can be constrained from the combined SHADES data with the expected photometric redshift information.

astro-ph↗

arXiv2004

Citation measures and impact within astronomy

By utilising the inbuilt citation counts from NASA's astrophysics data system (ADS) I derive how many citations refereed articles receive as a function of time since publication. After five years, one paper in a hundred has accumulated 91 or more citations, a figure which rises to 145 citations after ten years. By adding up the number of citations active researchers have received over the past five years I have estimated their relative impact upon the field both for raw citations and citations weighted by the number of authors per paper.

astro-ph↗

arXiv2001

Revisiting the Cosmic Cooling Crisis

Recent measurements of the K-band luminosity function now provide us with strong, reliable constraints on the fraction of baryons which have cooled. Globally, this fraction is only about 5%, and there is no strong evidence that it is significantly higher in clusters. Without an effective sub-grid feedback prescription, the cooled gas fraction in any numerical simulation exceeds these observational constraints, and increases with increasing resolution. This compromises any discussion of galaxy and cluster properties based on results of simulations which include cooling but do not implement an effective feedback mechanism.

astro-ph↗

arXiv2000

Simulating Supernovae Remnants in Gas Clouds

The Hydra $N$-body hydrodynamics code has been modified to model, from the end of the Sedov phase, the effects of supernovae on the surrounding medium. The motivation is to investigate the feedback of energy into the interstellar/intergalactic medium. We compare our results for supernova remnants (SNRs) in a uniform medium to previous detailed work on the late evolution of SNRs. The code is found to reproduce the bulk characteristics of SNRs well. Results on the effects of a single central SNR on Plummer clouds are presented. The feedback of kinetic energy and the percentage mass loss can be parameterised in terms of the cloud mass and characteristic radius in a simple way. The kinetic energy fraction returned to the ISM from a SNR is $<3$ per cent. The removal of gas from the cold, dense phase and the addition of energy due to the lowering of the potential energy of a cloud is at least as significant, if not much more so, than the kinetic energy leaving a cloud.

astro-ph↗

arXiv1997

A Parallel P^3M Code for Very Large Scale Cosmological Simulations

We have developed a parallel Particle-Particle, Particle-Mesh (P^3M) simulation code for the T3E well suited to studying the time evolution of systems of particles interacting via gravity and gas forces in cosmological contexts. The parallel code is based upon the public-domain serial Adaptive P^3M code of Couchman et al(1). The algorithm resolves gravitational forces into a long range component computed by discretizing the mass distribution and solving Poisson's equation on a grid using an FFT convolution method, and a short range component computed by direct force summation for sufficiently close particle pairs. The code consists primarily of a particle-particle computation parallelized by domain decomposition over blocks of neighbor-cells, a more regular mesh calculation distributed in planes along one dimension, and several transformations between the two distributions. Great care was taken throughout to make optimal use of the available memory, so that the current implementation is capable of simulating systems approaching 10^9 particles using a 1024^3 mesh for the long range force computation. These are thus among the largest N-body simulations ever carried out. We discuss these memory optimizations as well as those motivated by computational performance. Results from production runs have been very encouraging, and even prior to the implimentation of the full adaptive scheme the code has been used effectively for simulations in which the particle distribution becomes highly clustered as well as for other non-uniform systems of astrophysical interest.

astro-ph↗