X-ray Observations of the Warm-Hot Intergalactic Medium

We present Chandra observations that provide the most direct evidence to date for the pervasive, moderate density, shock-heated intergalactic medium predicted by leading cosmological scenarios. We also comment briefly on future observations with Constellation-X.Comment: To be published in the proceedings of the conference "IGM/Galaxy Connection- The Distribution of Baryons at z=0". 6 page

Andrzej Pekalski networks of scientific interests with internal degrees of freedom through self-citation analysis

Old and recent theoretical works by Andrzej Pekalski (APE) are recalled as possible sources of interest for describing network formation and clustering in complex (scientific) communities, through self-organisation and percolation processes. Emphasis is placed on APE self-citation network over four decades. The method is that used for detecting scientists field mobility by focusing on author's self-citation, co-authorships and article topics networks as in [1,2]. It is shown that APE's self-citation patterns reveal important information on APE interest for research topics over time as well as APE engagement on different scientific topics and in different networks of collaboration. Its interesting complexity results from "degrees of freedom" and external fields leading to so called internal shock resistance. It is found that APE network of scientific interests belongs to independent clusters and occurs through rare or drastic events as in irreversible "preferential attachment processes", similar to those found in usual mechanics and thermodynamics phase transitions.Comment: 7 pages, 1 table, 44 references, submitted to Int J Mod Phys

The Structure of Isothermal, Self-gravitating Gas Spheres for Softened Gravity

A theory for the structure of isothermal, self-gravitating gas spheres in pressure equilibrium in a softened gravitational field is developed. The one parameter spline softening proposed by Hernquist & Katz (1989) is used. We show that the addition of this extra scale parameter implies that the set of equilibrium solutions constitute a one-parameter family, rather than the one and only one isothermal sphere solution for Newtonian gravity. We demonstrate the perhaps somewhat surprising result that for any finite choice of softening length and temperature, it is possible to deposit an arbitrarily large mass of gas in pressure equilibrium and with a non-singular density distribution inside of r_0 for any r_0 > 0. The theoretical predictions of our models are compared with the properties of the small, massive, quasi-isothermal gas clumps which typically form in numerical Tree-SPH simulations of 'passive' galaxy formation of Milky Way sized galaxies. We find reasonable agreement despite the neglect of rotational support in the models. We comment on whether the hydrodynamical resolution in our numerical simulation of galaxy formation is sufficient, and finally we conclude that one should be cautious, when comparing results of numerical simulations involving gravitational softening and hydrodynamical smoothing, with reality.Comment: 22 pages Latex + 12 figure

Studying the WHIM Content of the Galaxy Large-Scale Structures along the Line of Sight to H 2356-309

We make use of a 500ks Chandra HRC-S/LETG spectrum of the blazar H2356-309, combined with a lower S/N spectrum of the same target, to search for the presence of warm-hot absorbing gas associated with two Large-Scale Structures (LSSs) crossed by this sightline at z=0.062 (the Pisces-Cetus Supercluster, PCS) and at z=0.128 ("Farther Sculptor Wall", FSW). No statistically significant (>=3sigma) individual absorption is detected from any of the strong He- or H-like transitions of C, O and Ne at the redshifts of the structures. However we are still able to constrain the physical and geometrical parameters of the associated putative absorbing gas, by performing joint spectral fit of marginal detections and upper limits of the strongest expected lines with our self-consistent hybrid ionization WHIM spectral model. At the redshift of the PCS we identify a warm phase with logT=5.35_-0.13^+0.07 K and log N_H =19.1+/-0.2 cm^-2 possibly coexisting with a hotter and less significant phase with logT=6.9^+0.1_-0.8 K and log N_H=20.1^+0.3_-1.7 cm^-2 (1sigma errors). For the FSW we estimate logT=6.6_-0.2^+0.1 K and log N_H=19.8_-0.8^+0.4 cm^-2. Our constraints allow us to estimate the cumulative number density per unit redshifts of OVII WHIM absorbers. We also estimate the cosmological mass density obtaining Omega_b(WHIM)=(0.021^+0.031_-0.018) (Z/Z_sun)^-1, consistent with the mass density of the intergalactic 'missing baryons' for high metallicities.Comment: 29 pages, 8 figures, 4 tables. Accepted for publication in Ap

Equilibration processes in the Warm-Hot Intergalactic Medium

The Warm-Hot Intergalactic Medium (WHIM) is thought to contribute about 40-50 % to the baryonic budget at the present evolution stage of the universe. The observed large scale structure is likely to be due to gravitational growth of density fluctuations in the post-inflation era. The evolving cosmic web is governed by non-linear gravitational growth of the initially weak density fluctuations in the dark energy dominated cosmology. Non-linear structure formation, accretion and merging processes, star forming and AGN activity produce gas shocks in the WHIM. Shock waves are converting a fraction of the gravitation power to thermal and non-thermal emission of baryonic/leptonic matter. They provide the most likely way to power the luminous matter in the WHIM. The plasma shocks in the WHIM are expected to be collisionless. Collisionless shocks produce a highly non-equilibrium state with anisotropic temperatures and a large differences in ion and electron temperatures. We discuss the ion and electron heating by the collisionless shocks and then review the plasma processes responsible for the Coulomb equilibration and collisional ionisation equilibrium of oxygen ions in the WHIM. MHD-turbulence produced by the strong collisionless shocks could provide a sizeable non-thermal contribution to the observed Doppler parameter of the UV line spectra of the WHIM.Comment: 13 pages, 4 figures, accepted for publication in Space Science Reviews, special issue "Clusters of galaxies: beyond the thermal view", Editor J.S. Kaastra, Chapter 8; work done by an international team at the International Space Science Institute (ISSI), Bern, organised by J.S. Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke

Toward detailed prominence seismology - II. Charting the continuous magnetohydrodynamic spectrum

Starting from accurate MHD flux rope equilibria containing prominence condensations, we initiate a systematic survey of their linear eigenoscillations. To quantify the full spectrum of linear MHD eigenmodes, we require knowledge of all flux-surface localized modes, charting out the continuous parts of the MHD spectrum. We combine analytical and numerical findings for the continuous spectrum for realistic prominence configurations. The equations governing all eigenmodes for translationally symmetric, gravitating equilibria containing an axial shear flow, are analyzed, along with their flux-surface localized limit. The analysis is valid for general 2.5D equilibria, where either density, entropy, or temperature vary from one flux surface to another. We analyze the mode couplings caused by the poloidal variation in the flux rope equilibria, by performing a small gravity parameter expansion. We contrast the analytical results with continuous spectra obtained numerically. For equilibria where the density is a flux function, we show that continuum modes can be overstable, and we present the stability criterion for these convective continuum instabilities. Furthermore, for all equilibria, a four-mode coupling scheme between an Alfvenic mode of poloidal mode number m and three neighboring (m-1, m, m+1) slow modes is identified, occurring in the vicinity of rational flux surfaces. For realistically prominence equilibria, this coupling is shown to play an important role, from weak to stronger gravity parameter g values. The analytic predictions for small g are compared with numerical spectra, and progressive deviations for larger g are identified. The unstable continuum modes could be relevant for short-lived prominence configurations. The gaps created by poloidal mode coupling in the continuous spectrum need further analysis, as they form preferred frequency ranges for global eigenoscillations.Comment: Accepted by Astronmy & Astrophysics, 21 pages, 15 figure

Unstable magnetohydrodynamical continuous spectrum of accretion disks. A new route to magnetohydrodynamical turbulence in accretion disks

We present a detailed study of localised magnetohydrodynamical (MHD) instabilities occuring in two--dimensional magnetized accretion disks. We model axisymmetric MHD disk tori, and solve the equations governing a two--dimensional magnetized accretion disk equilibrium and linear wave modes about this equilibrium. We show the existence of novel MHD instabilities in these two--dimensional equilibria which do not occur in an accretion disk in the cylindrical limit. The disk equilibria are numerically computed by the FINESSE code. The stability of accretion disks is investigated analytically as well as numerically. We use the PHOENIX code to compute all the waves and instabilities accessible to the computed disk equilibrium. We concentrate on strongly magnetized disks and sub--Keplerian rotation in a large part of the disk. These disk equilibria show that the thermal pressure of the disk can only decrease outwards if there is a strong gravitational potential. Our theoretical stability analysis shows that convective continuum instabilities can only appear if the density contours coincide with the poloidal magnetic flux contours. Our numerical results confirm and complement this theoretical analysis. Furthermore, these results show that the influence of gravity can either be stabilizing or destabilizing on this new kind of MHD instability. In the likely case of a non--constant density, the height of the disk should exceed a threshold before this type of instability can play a role. This localised MHD instability provides an ideal, linear route to MHD turbulence in strongly magnetized accretion disk tori.Comment: 20 pages, 10 figures, accepted for publication in Astronomy & Astrophysic

Numerical simulations of the Warm-Hot Intergalactic Medium

In this paper we review the current predictions of numerical simulations for the origin and observability of the warm hot intergalactic medium (WHIM), the diffuse gas that contains up to 50 per cent of the baryons at z~0. During structure formation, gravitational accretion shocks emerging from collapsing regions gradually heat the intergalactic medium (IGM) to temperatures in the range T~10^5-10^7 K. The WHIM is predicted to radiate most of its energy in the ultraviolet (UV) and X-ray bands and to contribute a significant fraction of the soft X-ray background emission. While O VI and C IV absorption systems arising in the cooler fraction of the WHIM with T~10^5-10^5.5 K are seen in FUSE and HST observations, models agree that current X-ray telescopes such as Chandra and XMM-Newton do not have enough sensitivity to detect the hotter WHIM. However, future missions such as Constellation-X and XEUS might be able to detect both emission lines and absorption systems from highly ionised atoms such as O VII, O VIII and Fe XVII.Comment: 18 pages, 5 figures, accepted for publication in Space Science Reviews, special issue "Clusters of galaxies: beyond the thermal view", Editor J.S. Kaastra, Chapter 14; work done by an international team at the International Space Science Institute (ISSI), Bern, organised by J.S. Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke

Genome resequencing reveals multiscale geographic structure and extensive linkage disequilibrium in the forest tree Populus trichocarpa

This is the publisher’s final pdf. The article is copyrighted by the New Phytologist Trust and published by John Wiley & Sons, Inc. It can be found at: http://onlinelibrary.wiley.com/journal/10.1111/%28ISSN%291469-8137. To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work.•Plant population genomics informs evolutionary biology, breeding, conservation and bioenergy feedstock development. For example, the detection of reliable phenotype–genotype associations and molecular signatures of selection requires a detailed knowledge about genome-wide patterns of allele frequency variation, linkage disequilibrium and recombination.\ud •We resequenced 16 genomes of the model tree Populus trichocarpa and genotyped 120 trees from 10 subpopulations using 29 213 single-nucleotide polymorphisms.\ud •Significant geographic differentiation was present at multiple spatial scales, and range-wide latitudinal allele frequency gradients were strikingly common across the genome. The decay of linkage disequilibrium with physical distance was slower than expected from previous studies in Populus, with r² dropping below 0.2 within 3–6 kb. Consistent with this, estimates of recent effective population size from linkage disequilibrium (N[subscript e] ≈ 4000–6000) were remarkably low relative to the large census sizes of P. trichocarpa stands. Fine-scale rates of recombination varied widely across the genome, but were largely predictable on the basis of DNA sequence and methylation features.\ud •Our results suggest that genetic drift has played a significant role in the recent evolutionary history of P. trichocarpa. Most importantly, the extensive linkage disequilibrium detected suggests that genome-wide association studies and genomic selection in undomesticated populations may be more feasible in Populus than previously assumed