Patrick M. Motl

The integrated sunyaev-zeldovich effect as a superior method for measuring the mass of clusters of galaxies

We investigate empirical scaling relations between the thermal Sunyaev-Zeldovich effect (SZE) and cluster mass in simulated clusters of galaxies. The simulated clusters have been compiled from four different samples that differ only in their assumed baryonic physics. We show that the strength of the thermal SZE integrated over a significant fraction of the virialized region of the clusters is relatively insensitive to the detailed heating and cooling processes in the cores of clusters, by demonstrating that the derived scaling relations are nearly identical among the four cluster samples considered. For our synthetic images, the central Comptonization parameter shows significant boosting during transient merging events, but the integrated SZE appears to be relatively insensitive to these events. Most importantly, the integrated SZE closely tracks the underlying cluster mass. Observations through the thermal SZE allow a strikingly accurate mass estimation from relatively simple measurements that do not require either parametric modeling or geometric deprojection and thus avoid assumptions regarding the physics of the intracluster medium or the symmetry of the cluster. This result offers significant promise for precision cosmology using clusters of galaxies.

Why Do Only Some Galaxy Clusters Have Cool Cores

Flux-limited X-ray samples indicate that about half of rich galaxy clusters have cool cores. Why do only some clusters have cool cores while others do not? In this paper, cosmological N-body + Eulerian hydrodynamic simulations, including radiative cooling and heating, are used to address this question as we examine the formation and evolution of cool core (CC) and noncool core (NCC) clusters. These adaptive mesh refinement simulations produce both CC and NCC clusters in the same volume. They have a peak resolution of 15.6 h−1 kpc within a (256 h−1 Mpc)3 box. Our simulations suggest that there are important evolutionary differences between CC clusters and their NCC counterparts. Many of the numerical CC clusters accreted mass more slowly over time and grew enhanced CCs via hierarchical mergers; when late major mergers occurred, the CCs survived the collisions. By contrast, NCC clusters experienced major mergers early in their evolution that destroyed embryonic CCs and produced conditions that prevented CC reformation. As a result, our simulations predict observationally testable distinctions in the properties of CC and NCC beyond the core regions in clusters. In particular, we find differences between CC versus NCC clusters in the shapes of X-ray surface brightness profiles, between the temperatures and hardness ratios beyond the cores, between the distribution of masses, and between their supercluster environs. It also appears that CC clusters are no closer to hydrostatic equilibrium than NCC clusters, an issue important for precision cosmology measurements.

Cluster Structure in Cosmological Simulations. I. Correlation to Observables, Mass Estimates, and Evolution

We use Enzo, a hybrid Eulerian adaptive mesh refinement/N-body code including nongravitational heating and cooling,toexplorethemorphologyof theX-raygasinclustersof galaxiesanditsevolutionincurrent-generationcosmological simulations. We employ and compare two observationally motivated structure measures: power ratios and centroidshift.Overall,thestructureof oursimulated clusterscompares remarkablywelltolow-redshift observations, although some differences remain that may point to incomplete gas physics. We find no dependence on cluster structure in the mass-observable scaling relations, TX-M and YX-M, when using the true cluster masses. However, estimates of the total mass based on the assumption of hydrostatic equilibrium, as assumed in observational studies, are systematicallylow.Weshowthatthehydrostaticmassbiasstronglycorrelateswithclusterstructureand,moreweakly, with cluster mass. When thehydrostatic massesareused,themass-observablescalingrelationsandgasmass fractions depend significantly on cluster morphology, and the true relations are not recovered even if the most relaxed clusters are used. We show that cluster structure, via the power ratios, can be used to effectively correct the hydrostatic mass estimatesandmassscalingrelations,suggestingthatwecancalibrateforthissystematiceffectincosmologicalstudies. Similar to observational studies, we find that cluster structure, particularly centroid shift, evolves with redshift. This evolutionismildbutwill leadtoadditionalerrorsathighredshift.Projectionalongthelineof sightleads tosignificant uncertainty in the structure of individual clusters: less than 50% of clusters which appear relaxed in projection based on our structure measures are truly relaxed. Subject headingg galaxies: clusters: general — hydrodynamics — large-scale structure of universe — methods: numerical — X-rays: galaxies: clusters

Numerical Simulations of the Onset and Stability of Dynamical Mass Transfer in Binaries

Hydrodynamical simulations of semidetached, polytropic binary stars are presented in an effort to study the onset and stability of dynamical mass transfer events. Initial, synchronously rotating equilibrium models are constructed using a self-consistent field technique and then evolved with an Eulerian hydrodynamics code in a fully self-consistent manner. We describe code improvements introduced over the past few years that permit us to follow dynamical mass transfer events through more than 30 orbits. Mass transfer evolutions are presented for two different initial configurations: a dynamically unstable binary with initial mass ratio (donor/accretor) q0 = 1.3 that leads to a complete merger in ~10 orbits, and a double-degenerate binary with initial mass ratio q0 = 0.5 that, after some initial unstable growth of mass transfer, tends to separate as the mass transfer rate levels off.

Magnetized Neutron-Star Mergers and Gravitational-Wave Signals

We investigate the influence of magnetic fields upon the dynamics of, and resulting gravitational waves from, a binary neutron-star merger in full general relativity coupled to ideal magnetohydrodynamics. We consider two merger scenarios: one where the stars have aligned poloidal magnetic fields and one without. Both mergers result in a strongly differentially rotating object. In comparison to the nonmagnetized scenario, the aligned magnetic fields delay the full merger of the stars. During and after merger we observe phenomena driven by the magnetic field, including Kelvin-Helmholtz instabilities in shear layers, winding of the field lines, and transition from poloidal to toroidal magnetic fields. These effects not only mediate the production of electromagnetic radiation, but also can have a strong influence on the gravitational waves. Thus, there are promising prospects for studying such systems with both types of waves.

Simulating binary neutron stars: Dynamics and gravitational waves

We model two mergers of orbiting binary neutron stars, the first forming a black hole and the second a differentially rotating neutron star. We extract gravitational waveforms in the wave zone. Comparisons to a post-Newtonian analysis allow us to compute the orbital kinematics, including trajectories and orbital eccentricities. We verify our code by evolving single stars and extracting radial perturbative modes, which compare very well to results from perturbation theory. The Einstein equations are solved in a first-order reduction of the generalized harmonic formulation, and the fluid equations are solved using a modified convex essentially non-oscillatory method. All calculations are done in three spatial dimensions without symmetry assumptions. We use the had computational infrastructure for distributed adaptive mesh refinement.

The Stability of Double White Dwarf Binaries Undergoing Direct-Impact Accretion

We present numerical simulations of dynamically unstable mass transfer in a double white dwarf binary with initial mass ratio q = 0.4. The binary components are approximated as polytropes of index n = 3/2, and the initially synchronously rotating, semidetached equilibrium binary is evolved hydrodynamically, with the gravitational potential being computed through the solution of Poissons equation. Upon initiating deep contact in our baseline simulation, the mass transfer rate grows by more than an order of magnitude over approximately 10 orbits, as would be expected for dynamically unstable mass transfer. However, the mass transfer rate then reaches a peak value, the binary expands, and the mass transfer event subsides. The binary must therefore have crossed the critical mass ratio for stability against dynamical mass transfer. Despite the initial loss of orbital angular momentum into the spin of the accreting star, we find that the accretors spin saturates and that angular momentum is returned to the orbit more efficiently than has been previously suspected for binaries in the direct-impact accretion mode. To explore this surprising result, we directly measure the critical mass ratio for stability by imposing artificial angular momentum loss at various rates to drive the binary to an equilibrium mass transfer rate. For one of these driven evolutions, we attain equilibrium mass transfer and deduce that, effectively, qcrit has evolved to approximately 2/3. Despite the absence of a fully developed disk, tidal interactions appear to be effective in returning excess spin angular momentum to the orbit.

The β-Model Problem: The Incompatibility of X-Ray and Sunyaev-Zeldovich Effect Model Fitting for Galaxy Clusters

We have analyzed a large sample of numerically simulated clusters to demonstrate the adverse effects resulting from the use of X-ray-fitted β-model parameters with Sunyaev-Zeldovich effect (SZE) data. There is a fundamental incompatibility between β-model fits to X-ray surface brightness profiles and those done with SZE profiles. Since observational SZE radial profiles are in short supply, the X-ray parameters are often used in SZE analysis. We show that this leads to biased estimates of the integrated Compton y-parameter inside r500 calculated from clusters. We suggest a simple correction of the method, using a nonisothermal β-model modified by a universal temperature profile, which brings these calculated quantities into closer agreement with the true values.

Numerical Methods for the Simulation of Dynamical Mass Transfer in Binaries

We describe computational tools that have been developed to simulate dynamical mass transfer in semidetached, polytropic binaries that are initially executing synchronous rotation upon circular orbits. Initial equilibrium models are generated with a self-consistent field algorithm; models are then evolved in time with a parallel, explicit, Eulerian hydrodynamics code with no assumptions made about the symmetry of the system. Poissons equation is solved along with the equations of ideal fluid mechanics to allow us to treat the nonlinear tidal distortion of the components in a fully self-consistent manner. We present results from several standard numerical experiments that have been conducted to assess the general viability and validity of our tools, and from benchmark simulations that follow the evolution of two detached systems through five full orbits (up to approximately 90 stellar dynamical times). These benchmark runs allow us to gauge the level of quantitative accuracy with which simulations of semidetached systems can be performed using presently available computing resources. We find that we should be able to resolve mass transfer at levels /M > few × 10-5 per orbit through approximately 20 orbits with each orbit taking about 30 hours of computing time on parallel computing platforms.

Challenges for Precision Cosmology with X-Ray and Sunyaev-Zeldovich Effect Gas Mass Measurements of Galaxy Clusters

We critically analyze the measurement of galaxy cluster gas masses, which is central to cosmological studies that rely on the galaxy cluster gas mass fraction. Using synthetic observations of numerically simulated clusters viewed through their X-ray emission and thermal Sunyaev-Zeldovich effect (SZE), we reduce the observations to obtain measurements of the cluster gas mass. We quantify the possible sources of uncertainty and systematic bias associated with the common simplifying assumptions used in reducing real cluster observations, including isothermality and hydrostatic equilibrium. We find that intrinsic variations in clusters limit the precision of observational gas mass estimation to ~10% to 1 σ confidence, excluding instrumental effects. Gas mass estimates show surprisingly little trending in the scatter as a function of cluster redshift. For the full cluster sample, methods that use SZE profiles out to roughly the virial radius are the simplest, most accurate, and unbiased way to estimate cluster mass. X-ray methods are systematically more precise mass estimators than are SZE methods if merger and cool-core systems are removed, but slightly overestimate (5%-10%) the cluster gas mass on average. We find that cool-core clusters in our samples are particularly poor candidates for observational mass estimation, even when excluding emission from the core region. The effects of cooling in the cluster gas alter the radial profile of the X-ray and SZE surface brightness even outside the cool-core region. Finally, we find that methods using a universal temperature profile estimate cluster masses to higher precision than those assuming isothermality.