Heating, conduction and minimum temperatures in cooling flows

There is mounting observational evidence from Chandra for strong interaction between keV gas and AGN in cooling flows. It is now widely accepted that the temperatures of cluster cores are maintained at a level of 1 keV and that the mass deposition rates are lower than earlier ROSAT/Einstein values. Recent theoretical results suggest that thermal conduction can be very efficient even in magnetized plasmas. Motivated by these discoveries, we consider a ``double heating model'' which incorporates the effects of simultaneous heating by both the central AGN and thermal conduction from the hot outer layers of clusters. Using hydrodynamical simulations, we demonstrate that there exists a family of solutions that does not suffer from the cooling catastrophe. In these cases, clusters relax to a stable final state, which is characterized by minimum temperatures of order 1 keV and density and temperature profiles consistent with observations. Moreover, the accretion rates are much reduced, thereby reducing the need for excessive mass deposition rates required by the standard cooling flow models.Comment: 7 pages, 2 figures, minor changes, accepted for The Astrophysical Journa

Draping of Cluster Magnetic Fields over Bullets and Bubbles -- Morphology and Dynamic Effects

High-resolution X-ray observations have revealed cavities and `cold fronts' with sharp edges in temperature, density, and metallicity within galaxy clusters. Their presence poses a puzzle since these features are not expected to be hydrodynamically stable, or to remain sharp in the presence of diffusion. However, a moving core or bubble in even a very weakly magnetized plasma necessarily sweeps up enough magnetic field to build up a dynamically important sheath around the object; the layer's strength is set by a competition between `plowing up' of field and field lines slipping around the core. We show that a two-dimensional approach to the problem is quite generally not possible. In three dimensions, we show with analytic arguments and in numerical experiments, that this magnetic layer modifies the dynamics of a plunging core, greatly modifies the effects of hydrodynamic instabilities on the core, modifies the geometry of stripped material, and even slows the fall of the core through magnetic tension. We derive an expression for the maximum magnetic field strength, the thickness of the layer, and the opening angle of the magnetic wake. The morphology of the magnetic draping layer implies the suppression of thermal conduction across the layer, thus conserving strong temperature gradients over the contact surface. The intermittent amplification of the magnetic field as well as the injection of MHD turbulence in the wake of the core is identified to be due to vorticity generation within the magnetic draping layer. These results have important consequences for understanding the physical properties and the complex gasdynamical processes of the intra-cluster medium, and apply quite generally to motions through other magnetized environments, e.g., the ISM.Comment: For version of this paper with interactive 3D graphics and full-resolution figures, see http://www.cita.utoronto.ca/~ljdursi/draping/ . 19p, 26 figures, emulateapj format. Version accepted by ApJ - new references, improved figure

AGN heating and dissipative processes in galaxy clusters

Recent X-ray observations reveal growing evidence for heating by active galactic nuclei (AGN) in clusters and groups of galaxies. AGN outflows play a crucial role in explaining the riddle of cooling flows and the entropy problem in clusters. Here we study the effect of AGN on the intra-cluster medium in a cosmological simulation using the adaptive mesh refinement FLASH code. We pay particular attention to the effects of conductivity and viscosity on the dissipation of weak shocks generated by the AGN activity in a realistic galaxy cluster. Our 3D simulations demonstrate that both viscous and conductive dissipation play an important role in distributing the mechanical energy injected by the AGN, offsetting radiative cooling and injecting entropy to the gas. These processes are important even when the transport coefficients are at a level of 10% of the Spitzer value. Provided that both conductivity and viscosity are suppressed by a comparable amount, conductive dissipation is likely to dominate over viscous dissipation. Nevertheless, viscous effects may still affect the dynamics of the gas and contribute a significant amount of dissipation compared to radiative cooling. We also present synthetic Chandra observations. We show that the simulated buoyant bubbles inflated by the AGN, and weak shocks associated with them, are detectable with the Chandra observatory.Comment: accepted to ApJ, minor change

ATD-2 Integrated Arrival/Departure/Surface (IADS) System Software Version 3.1.x Releases and Notes

This document summarizes the change reports for the ATD-2 V3.1.x series of software releases to Charlotte-Douglas International Airport (CLT). These list all the changes since the previous release. The Release Notes for Distribution are meant to be more readable by stakeholders. [Includes updates to changes for RTC (Ramp Traffic Console), Scheduler, STBO (Surface Trajectory-Based Operations), and Surface Metering Display (graphical-user interface, GUI) clients.

Parker/buoyancy instabilities with anisotropic thermal conduction, cosmic rays, and arbitrary magnetic field strength

We report the results of a local stability analysis for a magnetized, gravitationally stratified plasma containing cosmic rays. We account for cosmic-ray diffusion and thermal conduction parallel to the magnetic field and allow beta to take any value, where p is the plasma pressure and B is the magnetic field strength. We take the gravitational acceleration to be in the -z-direction and the equilibrium magnetic field to be in the y-direction, and we derive the dispersion relation for small-amplitude instabilities and waves in the large-|k_x| limit. We use the Routh-Hurwitz criterion to show analytically that the necessary and sufficient criterion for stability in this limit is n k_B dT/dz + dp_cr/dz + (1/8pi)dB^2/dz > 0, where T is the temperature, n is the number density of thermal particles, and p_cr is the cosmic-ray pressure. We present approximate analytical solutions for the normal modes in the low- and high-diffusivity limits, show that they are consistent with the derived stability criterion, and compare them to numerical results obtained from the full, unapproximated, dispersion relation. Our results extend earlier analyses of buoyancy instabilities in galaxy-cluster plasmas to the beta <= 1 regime. Our results also extend earlier analyses of the Parker instability to account for anisotropic thermal conduction, and show that the interstellar medium is more unstable to the Parker instability than was predicted by previous studies in which the thermal plasma was treated as adiabatic.Comment: 36 pages, 2 figures, Accepted for publication in Ap

Swimming against the current: simulations of central AGN evolution in dynamic galaxy clusters

We present a series of three-dimensional hydrodynamical simulations of central active galactic nuclei (AGN)-driven jets in a dynamic, cosmologically evolved galaxy cluster. Extending previous work, we study jet powers ranging from L jet = 10 44  erg s −1 to L jet = 10 46  erg s −1 and in duration from 30 to 200 Myr. We find that large-scale motions of cluster gas disrupt the AGN jets, causing energy to be distributed throughout the centre of the cluster, rather than confined to a narrow angle around the jet axis. Disruption of the jet also leads to the appearance of multiple disconnected X-ray bubbles from a long-duration AGN with a constant luminosity. This implies that observations of multiple bubbles in a cluster are not necessarily an expression of the AGN duty cycle. We find that the ‘sphere of influence’ of the AGN, the radial scale within which the cluster is strongly affected by the jet, scales as R ∝ L 1/3 jet . Increasing the duration of AGN activity does not increase the radius affected by the AGN significantly, but does change the magnitude of the AGN's effects. How an AGN delivers energy to a cluster will determine where that energy is deposited: a high luminosity is needed to heat material outside the core of the cluster, while a low-luminosity, long-duration AGN is more efficient at heating the inner few tens of kpc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/79339/1/j.1365-2966.2010.17059.x.pd

Web-Based Surface Metering Display (SMD) User Manual

This document serves as a user manual for the ATD-2 Web-Based Surface Metering Display (SMD). It describes the elements of the full SMD interface and provides explanations for how to interact with SMD. The document provides instructions for selecting the type of metering, entering specific metering parameters, and setting excess queue time variables. There are instructions for submitting system feedback and bug reports, as well.This document also includes information to guide a user in how to check the current metering state on other components of the ATD-2 system, via either the STBO Client or the RTC/RMTC

Deep Chandra observation and numerical studies of the nearest cluster cold front in the sky

We present the results of a very deep (500 ks) Chandra observation, along with tailored numerical simulations, of the nearest, best resolved cluster cold front in the sky, which lies 90 kpc (19 arcmin) to the north-west of M 87. The northern part of the front appears the sharpest, with a width smaller than 2.5 kpc (1.5 Coulomb mean free paths; at 99 per cent confidence). Everywhere along the front, the temperature discontinuity is narrower than 4–8 kpc and the metallicity gradient is narrower than 6 kpc, indicating that diffusion, conduction and mixing are suppressed across the interface. Such transport processes can be naturally suppressed by magnetic fields aligned with the cold front. Interestingly, comparison to magnetohydrodynamic simulations indicates that in order to maintain the observed sharp density and temperature discontinuities, conduction must also be suppressed along the magnetic field lines. However, the northwestern part of the cold front is observed to have a non-zero width. While other explanations are possible, the broadening is consistent with the presence of Kelvin–Helmholtz instabilities (KHI) on length-scales of a few kpc. Based on comparison with simulations, the presence of KHI would imply that the effective viscosity of the intracluster medium is suppressed by more than an order of magnitude with respect to the isotropic Spitzer-like temperature dependent viscosity. Underneath the cold front, we observe quasi-linear features that are ∼10 per cent brighter than the surrounding gas and are separated by ∼15 kpc from each other in projection. Comparison to tailored numerical simulations suggests that the observed phenomena may be due to the amplification of magnetic fields by gas sloshing in wide layers below the cold front, where the magnetic pressure reaches ∼5–10 per cent of the thermal pressure, reducing the gas density between the bright features