Kurt Roettiger

The Observational Consequences of Merging Clusters of Galaxies

We present an observational analysis of the numerical simulations of galaxy custer mergers. We identify several observational signatures of recent merger activity and quantitatively assess the uncertainty introduced into cluster mass estimates when invoking the commonly held assumptions of hydrostatic equilibrium, virial equilibrium, spherical symmetry, and isothermality. We find that mergers result in multiple X-ray peaks, long-lived elongation of the X-ray emission, as well as isophotal twisting and centroid shifting to a degree consistent with recent observations. We also find an enlargement of the X-ray core relative to the dark matter core. Mergers result in nonisothermal clusters exhibiting observable inhomogeneities in the emission-weighted X-ray temperature of several keV on linear scales of less than 0.5 Mpc. The resulting gasdynamics are extremely complex, and we present an example of what might be observed by a high resolution X-ray spectograph. We further speculate that the gas dynamics, via shocks, bulk flows, and turbulence, play an important role in the evolution of cluster galaxies and associated radio sources, particularly wide-angle tailed (WAT) sources and radio halos. We find that X-ray based cluster mass estimates made under equilibrium assumptions can be uncertain by 50% or more in the first 2 Gyrs after a merger and by up to 25% after 2 Gyrs depending on the details of the analysis and projection effects. Uncertainties can be considerably larger if the temperature is not well constrained. Similar uncertainties are observed in the X-ray derived baryon mass fractions. Virial mass estimates are typically overestimated because the observed one-dimensional velocity dispersion can be severely contaminated by the infall velocity of the subcluster.

A cluster merger and the origin of the extended radio emission in abell 3667

We present a numerical model for the extended steep-spectrum radio sources and the elongated X-ray structure in A3667 based on new three-dimensional MHD/N-body simulations. The X-ray and optical analyses of A3667 indicate that it has undergone a recent subcluster merger event. We believe that the Mpc-scale radio sources identified in A3667 are also a consequence of the merger. Our previous numerical simulations show that mergers often produce large-scale shocks and turbulence capable of both magnetic field amplification and in situ reacceleration of relativistic particles. Our model suggests that these radio structures, separated by ~2.6 h-1100 Mpc, are in fact causally linked via a slightly off-axis merger that occurred nearly in the plane of the sky approximately 1 Gyr ago with a subcluster having a total mass equal to ~20% of the primary cluster.

Magnetic Field Evolution in Merging Clusters of Galaxies

We present initial results from the first three-dimensional numerical magnetohydrodynamical (MHD) simulations of magnetic field evolution in merging clusters of galaxies. Within the framework of idealized initial conditions similar to our previous work, we look at the gas dynamics and the magnetic field evolution during a major merger event in order to examine the suggestion that shocks and turbulence generated during a cluster/subcluster merger can produce magnetic field amplification and relativistic particle acceleration and, as such, may play a role in the formation and evolution of cluster-wide radio halos. The intracluster medium (ICM), as represented by the equations of ideal MHD, is evolved self-consistently within a changing gravitational potential defined largely by the collisionless dark matter component represented by an N-body particle distribution. The MHD equations are solved by the Eulerian, finite-difference code, ZEUS. The particles are evolved by a standard particle-mesh (PM) code. We find significant evolution of the magnetic field structure and strength during two distinct epochs of the merger evolution. In the first, the field becomes quite filamentary as a result of stretching and compression caused by shocks and bulk flows during infall, but only minimal amplification occurs. In the second, amplification of the field occurs more rapidly, particularly in localized regions, as the bulk flow is replaced by turbulent motions (i.e., eddies). The total magnetic field energy is seen to increase by nearly a factor of 3 over that seen in a nonmerging cluster. In localized regions (associated with high vorticity), the magnetic energy can increase by a factor of 20 or more. A power spectrum analysis of the magnetic energy shows the amplification is largely confined to scales comparable to and smaller than the cluster cores, indicating that the core dimensions define the injection scale. Although the cluster cores are numerically well-resolved, we cannot resolve the formation of eddies on scales smaller than approximately half a core radius. Consequently, the field amplification noted here likely represents a lower limit. We discuss the effects of anomalous resistivity associated with the finite numerical resolution of our simulations on the observed field amplification.

The coma cluster after lunch: Has a galaxcy group passed through the cluster core?

We propose that the Coma cluster has recently undergone a collision with the NGC 4839 galaxy group. The ROSAT X-ray morphology, the Coma radio halo, the presence of poststarburst galaxies in the bridge between Coma and NGC 4839, the usually high velocity dispersion for the NGC 4839 group, and the position of a large-scale galaxy filament to the NE of Coma are all used to argue that the NGC 4839 group passed through the core of Coma approximately 2 Gyr ago. We present a new Hydro/N-body simulation of the merger between a galaxy group and a rich cluster that reproduces many of the observed X-ray and optical properties of Coma/NGC 4839.

When clusters collide - A numerical Hydro/N-body simulation of merging galaxy clusters

A 3D numerical simulation of two merging clusters of galaxies, using a hybrid Hydro/N-body code, is presented. The hydrodynamics of the code is solved by an Eulerian finite difference method. Initial results disclose that the X-ray emission of the dominant cluster becomes elongated and broadened; heating occurs at the core of the dominant cluster as a result of multiple shocks, and high velocity gas motions within the intracluster medium. It is predicted that clusters which have undergone recent mergers and do not have cooling flows will have high peculiar gas velocities and that the shocks and turbulence generated during the merger may power cluster-wide radio halos. Prolonged high-velocity gas motions through the dominant cluster core possibly play a major role in the formation and shaping of wide-angle tailed radio sources associated with central dominant galaxies. The N-body component of the simulation reveals the subcluster to be dispersed as it passes through the dominant cluster.

Do Cooling Flows Survive Cluster Mergers

We report the results of recent numerical simulations of the head-on merger of a cooling flow cluster with an infalling subcluster of galaxies. The objective of these simulations was to examine the effects of different types of cluster mergers (with 16 : 1 and 4 : 1 mass ratios) on the evolution of cluster cooling flows (with mass accretion rates of 100 and 400 M☉ yr-1). The two-dimensional simulations were performed with a combined hydrodynamics/N-body code on a uniform grid with a resolution of 20 kpc (~12 zones per core radius). In our simulations, cooling flow disruption is indicated by a dramatic increase (by a factor of 10-40) in the central cooling time of the primary cluster. We find that the ram pressure of the infalling gas is crucial in determining the fate of the cooling flow, because disruption occurs when a substantial amount of subcluster gas reaches the primarys core. In such cases, the subcluster gas can increase the central cooling time by displacing the high-density cooling gas and by heating it via shocks and turbulent gas motions. However, the fate of a merging cooling flow is also dependent on its initial cooling time. In cases where the initial cooling time is very short (i.e., 10-40 times shorter than the Hubble time), then even if the flow is disrupted, the central cooling time will remain less than a Hubble time, and the flow will likely reestablish itself. This has an important observational consequence, because such clusters will be classified as cooling flows on the basis of their cooling times, even though they have undergone a significant merger. In addition, we find that there is a time delay between core crossing and the point at which the central cooling time of a disrupted flow becomes of order a Hubble time. Thus, even in the case of disruption, a cluster can be classified as a cooling flow and exhibit substructure (indicative of a merger) for 1-2 Gyr after merging with a subcluster. We argue that our results make it possible to reconcile the high cooling flow frequency inferred by some observations with both high merger rates and a high frequency of substructure.

A PREDICTION OF OBSERVABLE ROTATION IN THE INTRACLUSTER MEDIUM OF ABELL 3266

We present a numerical, hydrodynamical/N-body model of A3266, whose X-ray surface brightness, temperature distribution, and galaxy spatial- and velocity-distribution data are consistent with the A3266 data. The model is an old (D3 Gyr) oU-axis merger having a mass ratio of D 2.5:1. The less massive subcluster in the model is moving on a trajectory from southwest to northeast passing on the western side of the dominant cluster while moving into the plane of the sky at D45i. OU-axis mergers such as this one are an eUective mechanism for transferring angular momentum to the intracluster medium (ICM), making possible a large-scale rotation of the ICM. We demonstrate here that the ICM rotation predicted by our fully three-dimensional model of A3266 is observable with Astro-E. Subject headings: galaxies: clusters: individual (Abell 3266) ¨ hydrodynamicsintergalactic medium ¨ methods: numericaltechniques: spectroscopicX-rays: galaxies

SYSTEMATIC ERRORS IN THE HUBBLE CONSTANT BASED UPON MEASUREMENT OF THE SUNYAEV-ZELDOVICH EFFECT

Values of the Hubble constant reported to date that are based upon measurement of the Sunyaev-Zeldovich (SZ) effect in clusters of galaxies are systematically lower than those derived by other methods (e.g., Cepheid variable stars or the Tully-Fisher relation). We investigate the possibility that systematic errors may be introduced into the analysis by the generally adopted assumptions that observed clusters are in hydrostatic equilibrium, are spherically symmetric, and are isothermal. We construct self-consistent theoretical models of merging clusters of galaxies, using hydrodynamic/N-body simulations. We then compute the magnitude of H0 derived from the SZ effect at different times and at different projection angles, both from first principles and by applying each of the standard assumptions used in the interpretation of observations. Our results indicate that the assumption of isothermality in the evolving clusters can result in H0 being underestimated by 10%-30%, depending upon both epoch and projection angle. Moreover, use of the projected, emission-weighted temperature profile under the assumption of spherical symmetry does not significantly improve the situation except in the case of more extreme mergers (i.e., those involving relatively gas-rich subclusters). Although less significant, we find that asphericity in the gas density can also result in a 15% error in H0. If the cluster is prolate (as is generally the case for on-axis, or nearly on-axis, mergers) and viewed along its major axis, H0 will be systematically underestimated. More extreme off-axis mergers may result in oblate merger remnants, which, when viewed nearly face-on, may result in an overestimation of H0. A similar effect is noted when viewing a prolate distribution along a line of sight that is nearly perpendicular to its major axis. In both cases the potential overestimation occurs only when the remnant is viewed within 15°-30° of face-on. Bulk gas motions and the kinematic SZ effect do not appear to be significant except for a brief period during the very early stages of a merger. Our study shows that the most meaningful SZ measurement will be accompanied by high-resolution temperature data and a detailed dynamical modeling of the observed system. In lieu of this, a large sample selected to avoid dynamically evolving systems is preferred.

Rossi X-Ray Timing Explorer Hard X-Ray Observation of A754: Constraining the Hottest Temperature Component and the Intracluster Magnetic Field

Abell 754, a cluster undergoing merging, was observed in hard X-rays with the Rossi X-ray Timing Explorer (RXTE) in order to constrain its hottest temperature component and search for evidence of nonthermal emission. Simultaneous modeling of RXTE data and those taken with previous missions yields an average intracluster temperature of

Stormy Weather and Cluster Radio Galaxies

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