The Launching of Cold Clouds by Galaxy Outflows II: The Role of Thermal Conduction

We explore the impact of electron thermal conduction on the evolution of radiatively-cooled cold clouds embedded in flows of hot and fast material, as occur in outflowing galaxies. Performing a parameter study of three-dimensional adaptive mesh refinement hydrodynamical simulations, we show that electron thermal conduction causes cold clouds to evaporate, but it can also extend their lifetimes by compressing them into dense filaments. We distinguish between low column-density clouds, which are disrupted on very short times, and high-column density clouds with much-longer disruption times that are set by a balance between impinging thermal energy and evaporation. We provide fits to the cloud lifetimes and velocities that can be used in galaxy-scale simulations of outflows, in which the evolution of individual clouds cannot be modeled with the required resolution. Moreover, we show that the clouds are only accelerated to a small fraction of the ambient velocity because compression by evaporation causes the clouds to present a small cross-section to the ambient flow. This means that either magnetic fields must suppress thermal conduction, or that the cold clouds observed in galaxy outflows are not formed of cold material carried out from the galaxy.Comment: accepted by Ap

The Role of Turbulence in AGN Self-Regulation in Galaxy Clusters

Cool cores of galaxy clusters are thought to be heated by low-power active galactic nuclei (AGN), whose accretion is regulated by feedback. However, the interaction between the hot gas ejected by the AGN and the ambient intracluster medium is extremely difficult to simulate, as it involves a wide range of spatial scales and gas that is Rayleigh-Taylor (RT) unstable. Here we use a subgrid model for RT-driven turbulence to overcome these problems and present the first observationally-consistent hydrodynamical simulations of AGN self-regulation in galaxy clusters. For a wide range of parameter choices the cluster in our three-dimensional simulations regulates itself for at least several Gyrs years. Heating balances cooling through a string of outbreaks with a typical recurrence time of approximately 80 Myrs, a timescale that depends only on the global cluster properties.Comment: 4 pages, 1 figure, To appear in proceedings of The Monster's Fiery Breath: Feedback in Galaxies, Groups, and Clusters (AIP conference series

3D simulations of shear instabilities in magnetized flows

We present results of three-dimensional (3D) simulations of the magnetohydrodynamic Kelvin-Helmholtz instability in a stratified shear layer. The magnetic field is taken to be uniform and parallel to the shear flow. We describe the evolution of the fluid flow and the magnetic field for a range of initial conditions. In particular, we investigate how the mixing rate of the fluid depends on the Richardson number and the magnetic field strength. It was found that the magnetic field can enhance as well as suppress mixing. Moreover, we have performed two-dimensional (2D) simulations and discuss some interesting differences between the 2D and 3D results.Comment: submitted to MNRAS, figures in colour and higher quality at http://www.mpa-garching.mpg.de/~maria/greenreports/mpa00/reports_00.htm

Self-similar collapse with cooling and heating in an expanding universe

We derive self-similar solutions including cooling and heating in an Einstein de-Sitter universe, and investigate the effects of cooling and heating on the gas density and temperature distributions. We assume that the cooling rate has a power-law dependence on the gas density and temperature, $\Lambda$$\propto$$\rho^{A}T^{B}$, and the heating rate is $\Gamma$$\propto$$\rho T$. The values of $A$ and $B$ are chosen by requiring that the cooling time is proportional to the Hubble time in order to obtain similarity solutions. In the region where the cooling rate is greater than the heating rate, a cooling inflow is established, and the gas is compressed and heats up. Because the compression is greater in the inner region than in the outer region, the temperature becomes an increasing profile toward the center. In particular, when a large infall velocity is produced due to an enormous energy loss, the slope of the density approaches a value that depends on $A$, $B$, and the velocity slope, and the slope of the temperature approaches $-$1. On the other hand, in the region where the heating rate is greater than the cooling rate, the infall velocity is suppressed, compression of the gas is weakened, and the gas cools down. The slope of the density becomes shallow due to suppression of the contraction, and the temperature is lower than that without heating. The self-similar collapse presented here gives insights to the effects of cooling and heating on the gas distributions in galaxies and clusters of galaxies.Comment: 12pages, 29figures. Accepted for publication in MNRA

Testing cosmic-ray acceleration with radio relics: a high-resolution study using MHD and tracers

Weak shocks in the intracluster medium may accelerate cosmic-ray protons and cosmic-ray electrons differently depending on the angle between the upstream magnetic field and the shock normal. In this work, we investigate how shock obliquity affects the production of cosmic rays in high-resolution simulations of galaxy clusters. For this purpose, we performed a magneto-hydrodynamical simulation of a galaxy cluster using the mesh refinement code \enzo. We use Lagrangian tracers to follow the properties of the thermal gas, the cosmic rays and the magnetic fields over time. We tested a number of different acceleration scenarios by varying the obliquity-dependent acceleration efficiencies of protons and electrons, and by examining the resulting hadronic $\gamma$-ray and radio emission. We find that the radio emission does not change significantly if only quasi-perpendicular shocks are able to accelerate cosmic-ray electrons. Our analysis suggests that radio emitting electrons found in relics have been typically shocked many times before $z=0$. On the other hand, the hadronic $\gamma$-ray emission from clusters is found to decrease significantly if only quasi-parallel shocks are allowed to accelerate cosmic-ray protons. This might reduce the tension with the low upper limits on $\gamma$-ray emission from clusters set by the \textit{Fermi}-satellite.Comment: 16 pages, 17 Figures, accepted for publication by MNRA