arXiv subjects

August E. Evrard

Publications and source records attributed to August E. Evrard.

79 records · Page 5Linked to original sources

arXiv1995

ROSAT Observations of Compact Groups of Galaxies

We have systematically analyzed a sample of 13 new and archival ROSAT PSPC observations of compact groups of galaxies: 12 Hickson Compact Groups plus the NCG 2300 group. We find that approximately two-thirds of the groups have extended X-ray emission and, in four of these, the emission is resolved into diffuse emission from gas at a temperature of $k$T $\sim 1$ keV in the group potential. All but one of the groups with extended emission have a spiral fraction of less than 50\%. The baryon fraction of groups with diffuse emission is 5--19\%, similar to the values in clusters of galaxies. However, with a single exception (HCG 62), the gas-to-stellar-mass ratio in our groups has a median value near 5\%, somewhat greater than the values for individual early-type galaxies and two orders of magnitude lower than in clusters of galaxies. The X-ray luminosities of individual group galaxies are comparable to those of similar field galaxies, although the L$_X$-L$_B$ relation for early-type galaxies may be flatter in compact groups than in the field.

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arXiv1994

Simulations of Clusters of Galaxies

The degree of complexity and, to a somewhat lesser degree, realism in simulations has advanced rapidly in the past few years. The simplest approach - modeling a cluster as collisionless dark matter and collisonal, non--radiative gas is now fairly well established. One of the most fruitful results of this approach is the {\sl morphology--cosmology connection} for X-ray clusters. Simulations have provided the means to make concrete predictions for the X-ray morphologies of clusters in cosmologies with different $Ω_o$, with the result that low $Ω_o$ cosmologies fair rather poorly when compared to observations. Another result concerns the accuracy of \xray binding mass estimates. The standard, hydrostatic, isothermal model estimator is found to be accurate to typically better than $50\%$ at radii where the density contrast is between $10^2$ and $10^3$. More complicated approaches, which attempt to explicitly follow galaxy formation within the proto--cluster environment are slowly being realized. The key issue of {\sl dynamical biasing} of the galaxy population within a cluster is being probed, but conclusive answers are lacking. The dynamics of multi--phase gas, including conversion of cold, dense gas into stars and the feedback therefrom, is the largest obstacle hindering progress. An example demonstrating the state--of--the--art in this area is presented.

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arXiv1994

The Cosmological Dependence of Cluster Density Profiles

We use N-body simulations to study the shape of mean cluster density and velocity profiles formed via gravitational instability. The dependence of the final structure on both cosmology and initial density field is examined, using a grid of cosmologies and scale-free initial power spectra P\propto k^n. For each model, we stack clusters to define an average density profile in the non-linear regime. The profiles are well fit by a power law over 99% of the cluster volume, with a clear trend toward steeper slopes with both increasing n and decreasing Omega_o. For models with a Omega_o = 0.2, the profile slopes are consistently higher than those for Omega-1.0. Cluster density profiles are thus potentially useful cosmological diagnostics. We find no evidence for a constant density core in any of the models, although the density profiles do tend to flatten at small radii. Much of the flattening is due to the force softening required by the simulations, and an attempt is made to recover the unsoftened profiles assuming angular momentum invariance. The recovered profiles in the Omega=1 cosmologies are consistent with a pure power law up to the highest density contrasts (10^6) accessible with our resolution. The low density models show significant deviations from a power law above density contrasts \sim 10^5. We interpret this curvature as reflecting the non scale-invariant nature of the background cosmology in these models.

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arXiv1994

A Comparison of Cosmological Hydrodynamic Codes

We present a detailed comparison of the simulation results of various cosmological hydrodynamic codes. Starting with identical initial conditions based on the Cold Dark Matter scenario for the growth of structure, we integrate from redshift $z=20$ to $z=0$ to determine the physical state within a representative volume of size $L^3$ where $L=64 h^{-1} {\rm Mpc}$. Five independent codes are compared: three of them Eulerian mesh based and two variants of the Smooth Particle Hydrodynamics "SPH" Lagrangian approach. The Eulerian codes were run at $N^3=(32^3,~64^3,~128^3,~{\rm and},~256^3)$ cells, the SPH codes at $N^3= 32^3$ and $64^3$ particles. Results were then rebinned to a $16^3$ grid with the expectation that the rebinned data should converge, by all techniques, to a common and correct result as $N \rightarrow \infty$. We find that global averages of various physical quantities do, as expected, tend to converge in the rebinned model, but that uncertainties in even primitive quantities such as $\langle T \rangle$, $\langle ρ^2\rangle^{1/2}$ persists at the 3\%-17\% level after completion of very large simulations. The two SPH codes and the two shock capturing Eulerian codes achieve comparable and satisfactory accuracy for comparable computer time in their treatment of the high density, high temperature regions as measured in the rebinned data; the variance among the five codes (at highest resolution) for the mean temperature (as weighted by $ρ^2$) is only 4.5\%. Overall the comparison allows us to better estimate errors, it points to ways of improving this current generation of hydrodynamic codes and of suiting their use to problems which exploit their individually best features.

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arXiv1993

Dynamics of Clusters of Galaxies

The abundance and internal characteristics of rich clusters of galaxies can provide useful constraints on models of large--scale structure formation. This article will review some recent three dimensional, multi--fluid simulations of cluster dynamics and discuss their impact on issues raised from optical and X-ray observations of clusters. In `bottom--up' formation scenarios (such as the ubiquitous cold dark matter model), galaxies form before rich clusters; hence, cluster formation is intimately linked to galaxy formation which, in turn, is tied to star formation. I will examine two issues which appear relatively insensitive to galaxy/star formation --- the baryon fraction in clusters and the connection between X--ray morphology and $Ω_o$ --- and end with a topic that is intimately linked to it --- dynamical biases in the cluster galaxy population.

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arXiv1993

A Simulation of the Intracluster Medium with Feedback from Cluster Galaxies

The formation and evolution of an X--ray cluster is studied using a 3--D N-body + hydrodynamical simulation which includes feedback of energy and iron from cluster galaxies. The differences in evolution and final state between the simulation and a similar run without feedback are highlighted. We address the energetics of cluster gas; the distribution of dark matter, gas, and galaxies; the thermodynamic state of cluster gas, the beta model, and the beta discrepancy; x-ray evolution; the validity of estimates of the binding mass and of the baryon fraction of the cluster; dynamics of the galaxy population and velocity bias; and of the evolution of the iron abundance distribution in the cluster. We find that preheating by galactic winds results in a lower core density for the ICM, which in turn makes the simulated cluster fainter at high redshift and undergo rapid luminosity evolution to the present. A strong iron abundance gradient is present, even at high redshift. Velocity bias is present in the galaxy population; this bias combined with the preheating from winds drives the value of $β_{spec}\,=\,σ^{2}/\left(kT/μm_{p}\right)$ to values less than one. The two fluid simulation, modelling infall only, predicts $β_{spec}\,\geq\,1$, less in line with observation.

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arXiv1992

Structure in a Loitering Universe

We study the formation of structure for a universe that undergoes a recent loitering phase. We compare the nonlinear mass distribution to that in a standard, matter dominated cosmology. The statistical aspects of the clustered matter are found to be robust to changes in the expansion law, an exception being that the peculiar velocities are lower by a factor of $\sim 3$ in the loitering model. Further, in the loitering scenario, nonlinear growth of perturbation occurs more recently ($z\sim 3-5$) than in the matter dominated case. Differences in the high redshift appearances of the two models will result but observable consequences depend critically on the chosen form, onset and duration of the loitering phase.

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