Hierarchical clustering and the baryon distribution in galaxy clusters

The baryon fraction of galaxy clusters in numerical simulations is found to be dependant on the cluster formation method. In all cases, the gas is anti-biased compared with the dark matter. However, clusters formed hierarchically are found to be more depleted in baryons than clusters formed non-hierarchically. There is a depletion of 10 to 15% for hierarchically formed clusters while the depletion is less than 10% for those clusters formed non-hierarchically. This difference is dependent on the mass of the clusters. The mean baryon enrichment profile for the hierarchically formed clusters shows an appreciable baryon enhancement around the virial radius not seen in the clusters formed without substructure. If this phenomenon applies to real clusters, it implies that determinations of the baryon fractions in clusters of galaxies require data extending beyond the virial radius of the clusters in order to achieve an unbiased value.Comment: 13 pages including 2 tables and 2 figures. Submitted to MNRA

Cluster mergers, core oscillations, and cold fronts

We use numerical simulations with hydrodynamics to demonstrate that a class of cold fronts in galaxy clusters can be attributed to oscillations of the dark matter distribution. The oscillations are initiated by the off-axis passage of a low-mass substructure. From the simulations, we derive three observable morphological features indicative of oscillations: 1) The existence of compressed isophotes; 2) The regions of compression must be alternate (opposite and staggered) and lie on an axis passing through the center of the cluster; 3) The gradient of each compression region must pass through the center of the cluster. Four of six clusters reported in the literature to have cold fronts have morphologies consistent with the presence of oscillations. The clusters with oscillations are A496, A1795, A2142, and RX J1720.1+2638. Galaxy clusters A2256 and A3667 are not consistent so the cold fronts are interpreted as group remnants. The oscillations may be able to provide sufficient energy to solve the cooling-flow problem and, importantly, provide it over an extended duration.Comment: Submitted to ApJ. 11 pages, 9 figure

Cold fronts in cool core clusters

Cold fronts have been detected both in merging and in cool core clusters, where little or no sign of a merging event is present. A systematic search of sharp surface brightness discontinuities performed on a sample of 62 galaxy clusters observed with XMM-Newton shows that cold fronts are a common feature in galaxy clusters. Indeed most (if not all) of the nearby clusters (z < 0.04) host a cold front. Understanding the origin and the nature of a such frequent phenomenon is clearly important. To gain insight on the nature of cold fronts in cool core clusters we have undertaken a systematic study of all contact discontinuities detected in our sample, measuring surface brightness, temperature and when possible abundance profiles across the fronts. We measure the Mach numbers for the cold fronts finding values which range from 0.2 to 0.9; we also detect a discontinuities in the metal profile of some clusters.Comment: 6 pages, 3 figures, for proceedings of "Heating vs. Cooling in Galaxies and Clusters of Galaxies," eds H. Boehringer, P. Schuecker, G. W. Pratt & A. Finoguenov, in Springer-Verlag series "ESO Astrophysics Symposia.

The marine algal (seaweed) flora of the Azores: additions and amendments.

Copyright © 2005 by Walter de Gruyter.Thirteen species of benthic marine algae are reported new for the isolated mid-Atlantic Azores archipelago and five older records are confirmed. Seven (Bangia atropurpurea, Codium fragile spip. tomentosoides, Ectocarous fasciculatus, E siliculosus, Porphyrostromium ciliare, Ulothrix flacca and Ulvaria oxysperma) are widespread amphi-Atlantic species some of which extend in distributional range south to the Canary Islands. Cryptonemia seminervis, Ctenosiphonia hypnoides and Phyllophora sicula are known from the northeastern Atlantic Ocean only and also occur in the Canary Islands. The Azores archipelago is the western limit of distribution of Codium fragile ssp. atlanticum, Bostrychia scorpioides, Bryopsis cupressina, Erythroglossum laciniatum, Haliptilon squamatum and Phyllophora sicula, and the southern limit of C. fragile ssp. atlanticum and E laciniatum. The tropical species Phyllodictyon anastomosans, Valonia macrophysa and Wurdemannia miniata reach their northern limit of distribution there. Asparagopsis taxiformis is now less common possibly due to competition with the non-native A. armata; subspecies of the non-native C. fragile now occur widely in the Azores. The status of Fucus vesiculosus, Ulva (Enteromorpha) clathrata and Rhizoclonium tortuosum is clarified and other species have been removed from the flora. The continuing addition of new species records for the Azores shows the archipelago to be floristically richer than other Atlantic islands except for the Canary Islands

Rocky shore marine flora of the Azores

“[…]. O principal objectivo deste livro é fornecer informação útil aos visitantes da zona das marés nos Açores e chamar a sua atenção para estas plantas muitas vezes ignoradas. Existem cerca de 400 espécies de macroalgas marinhas nos Açores. As maiores são fáceis de encontrar, mas muitas só são visíveis à lupa ou microscópio. Neste livro incluem-se descrições e ilustrações de 46 espécies (44 algas, 1 cianobactéria, 1 líquen) relativamente fáceis de encontrar e identificar. Os leitores são elucidados sobre a melhor forma de as encontrar, bem como sobre aspectos básicos da respectiva morfologia e valor natural e/ou comercial. Para os leitores interessados, são ainda sugeridas publicações mais especializadas sobre o assunto. O livro inclui um glossário com os termos técnicos utilizados nas descrições apresentadas.”ABSTRACT: “[…]. The aim of this booklet is to make some of this new information available to the interested visitor to the sea-shore, and to provide an introduction to these often overlooked plants. Not all the species that occur in the Azores (approximately 400) are considered here; many are small in size and seen only with the aid of a microscope, others may be larger but are inconspicuous forms difficult to recognise. We present a synopsis of the seaweed flora of the Azores through pictures and brief descriptions of 46 species (44 seaweeds, 1 cyanobacterium, 1 lichen) that can be found without too much difficulty. We also present some basic facts as to what seaweeds are and what they look like, where they grow, and their value to the natural world as well as to mankind. For those wanting more information some references to other publications are given.”Secretaria Regional do Ambiente e do Mar / Universidade dos Açores / Natural History Museum / CRUP - Conselho de Reitores das Universidades Portuguesas

Smoothed Particle Hydrodynamics in cosmology: a comparative study of implementations

We analyse the performance of twelve different implementations of Smoothed Particle Hydrodynamics (SPH) using seven tests designed to isolate key hydrodynamic elements of cosmological simulations which are known to cause the SPH algorithm problems. In order, we consider a shock tube, spherical adiabatic collapse, cooling flow model, drag, a cosmological simulation, rotating cloud-collapse and disc stability. In the implementations special attention is given to the way in which force symmetry is enforced in the equations of motion. We study in detail how the hydrodynamics are affected by different implementations of the artificial viscosity including those with a shear-correction modification. We present an improved first-order smoothing-length update algorithm that is designed to remove instabilities that are present in the Hernquist and Katz (1989) algorithm. For all tests we find that the artificial viscosity is the most important factor distinguishing the results from the various implementations. The second most important factor is the way force symmetry is achieved in the equation of motion. Most results favour a kernel symmetrization approach. The exact method by which SPH pressure forces are included has comparatively little effect on the results. Combining the equation of motion presented in Thomas and Couchman (1992) with a modification of the Monaghan and Gingold (1983) artificial viscosity leads to an SPH scheme that is both fast and reliable.Comment: 30 pages, 26 figures and 9 tables included. Submitted to MNRAS. Postscript version available at ftp://phobos.astro.uwo.ca/pub/etittley/papers/sphtest.ps.g

Dynamics and Magnetization in Galaxy Cluster Cores Traced by X-ray Cold Fronts

Cold fronts (CFs) - density and temperature plasma discontinuities - are ubiquitous in cool cores of galaxy clusters, where they appear as X-ray brightness edges in the intracluster medium, nearly concentric with the cluster center. We analyze the thermodynamic profiles deprojected across core CFs found in the literature. While the pressure appears continuous across these CFs, we find that all of them require significant centripetal acceleration beneath the front. This is naturally explained by a tangential, nearly sonic bulk flow just below the CF, and a tangential shear flow involving a fair fraction of the plasma beneath the front. Such shear should generate near-equipartition magnetic fields on scales ~<50 pc from the front, and could magnetize the entire core. Such fields would explain the apparent stability of cool-core CFs and the recently reported CF-radio minihalo association.Comment: Revised version to appear in Astrophys.J.Let

Gas around galaxy haloes: methodology comparisons using hydrodynamical simulations of the intergalactic medium

We perform cosmological simulations of the intergalactic medium (IGM) at redshift z ∼ 3 using the numerical gravity-hydrodynamics codes gadget-3 and enzo for the purpose of modelling the gaseous environments of galaxies. We identify haloes in the simulations using three different algorithms. Different rank orderings of the haloes by mass result, introducing a limiting factor, in identifying haloes with observed galaxies. We also compare the physical properties of the gas between the two codes, focusing primarily on the gas outside the virial radius, motivated by recent H i absorption measurements of the gas around z ∼ 2–3 galaxies. The internal dispersion velocities of the gas in the haloes have converged for a box size of 30 comoving Mpc, but the centre-of-mass peculiar velocities of the haloes have not up to a box size of 60 comoving Mpc. The density and temperature of the gas within the instantaneous turn-around radii of the haloes are adequately captured for box sizes of 30 Mpc on a side, but the results are highly sensitive to the treatment of unresolved, rapidly cooling gas, with the gas mass fraction within the virial radius severely depleted by star formation in the gadget-3 simulations. Convergence of the gas peculiar velocity field on large scales requires a box size of at least 60 Mpc. Outside the turn-around radius, the physical state of the gas agrees to 30 per cent or better both with box size and between simulation methods. We conclude that generic IGM simulations make accurate predictions for the intergalactic gas properties beyond the halo turn-around radii, but the gas properties on smaller scales are highly dependent on star formation and feedback implementations