Relic Radio Bubbles and Cluster Cooling Flows

Recent suggestions that buoyant radio emitting cavities in the intracluster medium can cause significant reheating of cooling flows are re-examined when the effects of the intracluster magnetic field are included. Expansion of the cavity creates a tangential magnetic field in the ICM around the radio source, and this field can suppress instabilities that mix the ICM and the radio source. The onset of instability can be delayed for ~100 million years, and calculation of the actual reheating time shows that this may not occur until about 1Gy after creation of the cavity. These results may explain why the relic radio bubbles are still intact at such late times, and it may imply that the role of radio sources in reheating the ICM should be re-examined. In addition, the existence of relic radio cavities may also imply that the particle content of radio source lobes is primarily electrons and protons rather than electrons and positrons.Comment: 7 pages, 1 figure, to be published in MNRA

The Discovery of Extended Thermal X-ray Emission from PKS 2152-699: Evidence for a `Jet-cloud' Interaction

A Chandra ACIS-S observation of PKS 2152-699 reveals thermal emission from a diffuse region around the core and a hotspot located 10" northeast from the core. This is the first detection of thermal X-ray radiation on kiloparsec scales from an extragalactic radio source. Two other hotspots located 47" north-northeast and 26" southwest from the core were also detected. Using a Raymond-Smith model, the first hotspot can be characterized with a thermal plasma temperature of 2.6$\times10^6$ K and an electron number density of 0.17 cm$^{-3}$. These values correspond to a cooling time of about 1.6$\times10^7$ yr. In addition, an emission line from the hotspot, possibly Fe xxv, was detected at rest wavelength 10.04\AA. The thermal X-ray emission from the first hotspot is offset from the radio emission but is coincident with optical filaments detected with broadband filters of HST/WFPC2. The best explanation for the X-ray, radio, and optical emission is that of a `jet-cloud' interaction. The diffuse emission around the nucleus of PKS 2152-699 can be modeled as a thermal plasma with a temperature of 1.2$\times10^7$ K and a luminosity of 1.8$\times10^{41}$ erg s$^{-1}$. This emission appears to be asymmetric with a small extension toward Hotspot A, similar to a jet. An optical hotspot (EELR) is seen less than an arcsecond away from this extension in the direction of the core. This indicates that the extension may be caused by the jet interacting with an inner ISM cloud, but entrainment of hot gas is unavoidable. Future observations are discussed.Comment: To appear in the Astrophysical Journal 21 pages, 5 Postscript figures, 1 table, AASTeX v. 5.

Pluto's lower atmosphere structure and methane abundance from high-resolution spectroscopy and stellar occultations

Context: Pluto possesses a thin atmosphere, primarily composed of nitrogen, in which the detection of methane has been reported. Aims: The goal is to constrain essential but so far unknown parameters of Pluto's atmosphere such as the surface pressure, lower atmosphere thermal stucture, and methane mixing ratio. Methods: We use high-resolution spectroscopic observations of gaseous methane, and a novel analysis of occultation light-curves. Results: We show that (i) Pluto's surface pressure is currently in the 6.5-24 microbar range (ii) the methane mixing ratio is 0.5+/-0.1 %, adequate to explain Pluto's inverted thermal structure and ~100 K upper atmosphere temperature (iii) a troposphere is not required by our data, but if present, it has a depth of at most 17 km, i.e. less than one pressure scale height; in this case methane is supersaturated in most of it. The atmospheric and bulk surface abundance of methane are strikingly similar, a possible consequence of the presence of a CH4-rich top surface layer.Comment: AA vers. 6.1, LaTeX class for Astronomy & Astrophysics, 9 pages with 5 figures Astronomy and Astrophysics Letters, in pres

Dynamic and Stagnating Plasma Flow Leading to Magnetic Flux Tube Collimation

Highly collimated, plasma-filled magnetic flux tubes are frequently observed on galactic, stellar and laboratory scales. We propose that a single, universal magnetohydrodynamic pumping process explains why such collimated, plasma-filled magnetic flux tubes are ubiquitous. Experimental evidence from carefully diagnosed laboratory simulations of astrophysical jets confirms this assertion and is reported here. The magnetohydrodynamic process pumps plasma into a magnetic flux tube and the stagnation of the resulting flow causes this flux tube to become collimated.Comment: to be published in PRL; color figures on electronic versio

Local excitations in mean field spin glasses

We address the question of geometrical as well as energetic properties of local excitations in mean field Ising spin glasses. We study analytically the Random Energy Model and numerically a dilute mean field model, first on tree-like graphs, equivalent to a replica symmetric computation, and then directly on finite connectivity random lattices. In the first model, characterized by a discontinuous replica symmetry breaking, we found that the energy of finite volume excitation is infinite whereas in the dilute mean field model, described by a continuous replica symmetry breaking, it slowly decreases with sizes and saturates at a finite value, in contrast with what would be naively expected. The geometrical properties of these excitations are similar to those of lattice animals or branched polymers. We discuss the meaning of these results in terms of replica symmetry breaking and also possible relevance in finite dimensional systems.Comment: 7 pages, 4 figures, accepted for publicatio

Interplay of thermal and quantum spin fluctuations on the kagome lattice

We present a Raman spectroscopic investigation of the Herbertsmithite ZnCu3(OH)6Cl2, the first realization of a Heisenberg s=1/2 antiferromagnet on a perfect kagome lattice. The magnetic excitation spectrum of this compound is dominated by two components, a high temperature quasi elastic signal and a low temperature, broad maximum. The latter has a linear low energy slope and extends to high energy. We have investigated the temperature dependence and symmetry properties of both signals. Our data agree with previous calculations and point to a spin liquid ground state.Comment: 5 figure

Location of the Multicritical Point for the Ising Spin Glass on the Triangular and Hexagonal Lattices

A conjecture is given for the exact location of the multicritical point in the phase diagram of the +/- J Ising model on the triangular lattice. The result p_c=0.8358058 agrees well with a recent numerical estimate. From this value, it is possible to derive a comparable conjecture for the exact location of the multicritical point for the hexagonal lattice, p_c=0.9327041, again in excellent agreement with a numerical study. The method is a variant of duality transformation to relate the triangular lattice directly with its dual triangular lattice without recourse to the hexagonal lattice, in conjunction with the replica method.Comment: 9 pages, 1 figure; Minor corrections in notatio

Simulations of Metal Enrichment in Galaxy Clusters by AGN Outflows

We assess the importance of AGN outflows with respect to the metal enrichment of the intracluster medium (ICM) in galaxy clusters. We use combined N-body and hydrodynamic simulations, along with a semi-numerical galaxy formation and evolution model. Using assumptions based on observations, we attribute outflows of metal-rich gas initiated by AGN activity to a certain fraction of our model galaxies. The gas is added to the model ICM, where the evolution of the metallicity distribution is calculated by the hydrodynamic simulations. For the parameters describing the AGN content of clusters and their outflow properties, we use the observationally most favorable values. We find that AGNs have the potential to contribute significantly to the metal content of the ICM or even explain the complete abundance, which is typically ~0.5 Z_sun in core regions. Furthermore, the metals end up being inhomogeneously distributed, in accordance with observations.Comment: 7 pages, 6 figures, accepted for publication in A&