A deep Chandra observation of Abell 4059: a new face to radio-mode AGN feedback?

A deep Chandra observation of the cooling core cluster Abell 4059 (A4059) is presented. Previous studies have found two X-ray cavities in the central regions of A4059 together with a ridge of X-ray emission 20kpc south-west of the cluster center. These features are clearly related to the radio galaxy PKS2354-35 which resides in the cD galaxy. Our new data confirm these previous findings and strengthen previous suggestions that the south-western ridge is colder and denser than, but in approximate pressure equilibrium with, the surrounding ICM atmosphere. In addition, we find evidence for a weak shock that wraps around the north and east sides of the cavity structure. Our data allow us to map the 2-dimensional distribution of metals in the ICM of A4059 for the first time. We find that the SW ridge possesses an anomalously high (super-solar) metalicity. The unusual morphology, temperature structure and metal distribution all point to significant asymmetry in the ICM atmosphere prior to the onset of radio-galaxy activity. Motivated by the very high metalicity of the SW ridge, we hypothesize that the ICM asymmetry was caused by the extremely rapid stripping of metal enriched gas from a starburst galaxy that plunged through the core of A4059. Furthermore, we suggest that the onset of powerful radio-galaxy activity in the cD galaxy may have been initiated by this starburst/stripping event, either via the tidal-shocking of cold gas native to the cD galaxy, or the accretion of cold gas that had been stripped from the starburst galaxy.Comment: Accepted for publication in the Astrophysical Journal. 12 pages, 11 figures. A version of this paper including full resolution figures can be found at http://www.astro.umd.edu/~chris/publications/papers/a4059_2008.pd

Simulations of MHD Instabilities in Intracluster Medium Including Anisotropic Thermal Conduction

We perform a suite of simulations of cooling cores in clusters of galaxies in order to investigate the effect of the recently discovered heat flux buoyancy instability (HBI) on the evolution of cores. Our models follow the 3-dimensional magnetohydrodynamics (MHD) of cooling cluster cores and capture the effects of anisotropic heat conduction along the lines of magnetic field, but do not account for the cosmological setting of clusters or the presence of AGN. Our model clusters can be divided into three groups according to their final thermodynamical state: catastrophically collapsing cores, isothermal cores, and an intermediate group whose final state is determined by the initial configuration of magnetic field. Modeled cores that are reminiscent of real cluster cores show evolution towards thermal collapse on a time scale which is prolonged by a factor of ~2-10 compared with the zero-conduction cases. The principal effect of the HBI is to re-orient field lines to be perpendicular to the temperature gradient. Once the field has been wrapped up onto spherical surfaces surrounding the core, the core is insulated from further conductive heating (with the effective thermal conduction suppressed to less than 1/100th of the Spitzer value) and proceeds to collapse. We speculate that, in real clusters, the central AGN and possibly mergers play the role of "stirrers," periodically disrupting the azimuthal field structure and allowing thermal conduction to sporadically heat the core.Comment: 16 pages, 3 tables, 17 figures, accepted to ApJ with minor revisions, to appear in Volume 704, Oct 20, 2009 issu

Energetic Impact of Jet Inflated Cocoons in Relaxed Galaxy Clusters

Jets from active galactic nuclei (AGN) in the cores of galaxy clusters have the potential to be a major contributor to the energy budget of the intracluster medium (ICM). To study the dependence of the interaction between the AGN jets and the ICM on the parameters of the jets themselves, we present a parameter survey of two-dimensional (axisymmetric) ideal hydrodynamic models of back-to-back jets injected into a cluster atmosphere (with varying Mach numbers and kinetic luminosities). We follow the passive evolution of the resulting structures for several times longer than the active lifetime of the jet. The simulations fall into roughly two classes, cocoon-bounded and non-cocoon bounded sources. We suggest a correspondence between these two classes and the Faranoff-Riley types. We find that the cocoon-bounded sources inject significantly more entropy into the core regions of the ICM atmosphere, even though the efficiency with which energy is thermalized is independent of the morphological class. In all cases, a large fraction (50--80%) of the energy injected by the jet ends up as gravitational potential energy due to the expansion of the atmosphere.Comment: 12 pages, Accepted for publication in Ap

Regulation of thermal conductivity in hot galaxy clusters by MHD turbulence

The role of thermal conduction in regulating the thermal behavior of cooling flows in galaxy clusters is reexamined. Recent investigations have shown that the anisotropic Coulomb heat flux caused by a magnetic field in a dilute plasma drives a dynamical instability. A long standing problem of cooling flow theory has been to understand how thermal conduction can offset radiative core losses without completely preventing them. In this Letter we propose that magnetohydrodynamic turbulence driven by the heat flux instability regulates field-line insulation and drives a reverse convective thermal flux, both of which may mediate the stabilization of the cooling cores of hot clusters. This model suggests that turbulent mixing should accompany strong thermal gradients in cooling flows. This prediction seems to be supported by the spatial distribution of metals in the central galaxies of clusters, which shows a much stronger correlation with the ambient hot gas temperature gradient than with the parent stellar population.Comment: 4 Pages. Accepted for publication in Astrophysical Journal Letter

Connections Between Local and Global Turbulence in Accretion Disks

We analyze a suite of global magnetohydrodynamic (MHD) accretion disk simulations in order to determine whether scaling laws for turbulence driven by the magnetorotational instability, discovered via local shearing box studies, are globally robust. The simulations model geometrically-thin disks with zero net magnetic flux and no explicit resistivity or viscosity. We show that the local Maxwell stress is correlated with the self-generated local vertical magnetic field in a manner that is similar to that found in local simulations. Moreover, local patches of vertical field are strong enough to stimulate and control the strength of angular momentum transport across much of the disk. We demonstrate the importance of magnetic linkages (through the low-density corona) between different regions of the disk in determining the local field, and suggest a new convergence requirement for global simulations -- the vertical extent of the corona must be fully captured and resolved. Finally, we examine the temporal convergence of the average stress, and show that an initial long-term secular drift in the local flux-stress relation dies away on a time scale that is consistent with turbulent mixing of the initial magnetic field.Comment: 8 Pages, 7 Figures ApJ, In Pres

XMM-Newton Archival Study of the ULX Population in Nearby Galaxies

We present the results of an archival XMM-Newton study of the bright X-ray point sources (L_X > 10^38 erg/s) in 32 nearby galaxies. From our list of approximately 100 point sources, we attempt to determine if there is a low-state counterpart to the Ultraluminous X-ray (ULX) population, searching for a soft-hard state dichotomy similar to that known for Galactic X-ray binaries and testing the specific predictions of the IMBH hypothesis. To this end, we searched for "low-state" objects, which we defined as objects within our sample which had a spectrum well fit by a simple absorbed power law, and "high-state" objects, which we defined as objects better fit by a combined blackbody and a power law. Assuming that ``low-state'' objects accrete at approximately 10% of the Eddington luminosity (Done & Gierlinski 2003) and that "high-state" objects accrete near the Eddington luminosity we further divided our sample of sources into low and high state ULX sources. We classify 16 sources as low-state ULXs and 26 objects as high-state ULXs. As in Galactic black hole systems, the spectral indices, Gamma, of the low-state objects, as well as the luminosities, tend to be lower than those of the high-state objects. The observed range of blackbody temperatures for the high state is 0.1-1 keV, with the most luminous systems tending toward the lowest temperatures. We therefore divide our high-state ULXs into candidate IMBHs (with blackbody temperatures of approximately 0.1 keV) and candidate stellar mass BHs (with blackbody temperatures of approximately 1.0 keV). A subset of the candidate stellar mass BHs have spectra that are well-fit by a Comptonization model, a property similar of Galactic BHs radiating in the "very-high" state near the Eddington limit.Comment: 54 pages, submitted to ApJ (March 2005), accepted (May 2006); changes to organization of pape

A Supervised Molecular Dynamics Approach to Unbiased Ligand–Protein Unbinding

The recent paradigm shift toward the use of the kinetics parameters in place of thermodynamic constants is leading the computational chemistry community to develop methods for studying the mechanisms of drug binding and unbinding. From this standpoint, molecular dynamics (MD) plays an important role in delivering insight at the molecular scale. However, a known limitation of MD is that the time scales are usually far from those involved in ligand–receptor unbinding events. Here, we show that the algorithm behind supervised MD (SuMD) can simulate the dissociation mechanism of druglike small molecules while avoiding the input of any energy bias to facilitate the transition. SuMD was tested on seven different intermolecular complexes, covering four G protein-coupled receptors: the A2A and A1 adenosine receptors, the orexin 2 and the muscarinic 2 receptors, and the soluble globular enzyme epoxide hydrolase. SuMD well-described the multistep nature of ligand–receptor dissociation, rationalized previous experimental data and produced valuable working hypotheses for structure–kinetics relationships