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

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

Chemical Gradients in Galaxy Clusters and the Multiple Ways of Making a Cold Front

Cold fronts were originally interpreted as being the result of subsonic/transonic motions of head-on merging substructures. This merger core remnant model is theoretically justified and hold relatively well for clusters that have clear signs of merging, such as 1E0657-56, but they do not work well for the increasing number of cold fronts found in clusters that do not show clear merging signs, such as A496. Here we report the results of a deeper observation of that cluster that allowed us to produce high quality maps of the gas parameters and to compare more closely the observations with the predictions given by different models for cold front formation. We found for the first time a ``cold arm'' characteristic of a flyby of a massive DM halo near the core of the cluster. The cold arm is accompanied by an enhanced SN II Fe mass fraction, inconsistent with the merger core remnant scenario.Comment: 3 pages, 1 figures, to appear in the Proceedings of "Heating vs. Cooling in Galaxies and Clusters of Galaxies", August 2006, Garching (Germany

Structure and zonation of algal communities in the bay of São Vicente (São Miguel, Azores)

An appraisal of the benthic marine algal communities of the bay of São Vicente (São Miguel island. Azores) was undertaken in July 1996 and July 1997 as part of the project "Biodiversity of the archipelago of the Azores". A permanent transect was laid down across the subtidal zone. Quadrates were sampled at pre-determined intervals along the transect from the low water level, down to 30m depth. Qualitative collections were made along the transect. The relative importance of each species was determined using a semi-quantitative scale. The transect revealed the occurrence of relatively large and frondose algae establishing the transition between the intertidal and the subtidal zones. Pterocladiella capillacea was the dominant species in this transition zone and extended its presence down to 12m depth. A depth-related change in the algal flora was present in both years. Pteroclardiella, Ulva spp., Stypocaulon scoparia, Hypnea musciformis and Asparagopsis armata dominated the shallow levels; Zonaria tournefortii, together with Sphaeroroccus coronopifolius and Dictyota dichotoma dominated the deep ones

Gas around galaxy haloes – II. Hydrogen absorption signatures from the environments of galaxies at redshifts 2

We compare predictions of large-scale cosmological hydrodynamical simulations for neutral hydrogen absorption signatures in the vicinity of 1011–1012.5 M⊙ haloes with observational measurements. Two different hydrodynamical techniques and a variety of prescriptions for gas removal in high-density regions are examined. Star formation and wind feedback play only secondary roles in the H i absorption signatures outside the virial radius, but play important roles within. Accordingly, we identify three distinct gaseous regions around a halo: the virialized region, the mesogalactic medium outside the virial radius arising from the extended haloes of galaxies out to about two turnaround radii and the intergalactic medium beyond. Predictions for the amount of absorption from the mesogalactic and intergalactic media are robust across different methodologies, and the predictions agree with the amount of absorption observed around star-forming galaxies and quasi-stellar object host galaxies. Recovering the measured amount of absorption within the virialized region, however, requires either a higher dynamic range in the simulations, additional physics, or both

Subtidal Rocky Shore Communities of the Azores: Developing a Biotope Survey Method

Copyright © 2008 Coastal Education and Research Foundation.At 23 sites selected randomly around the island of S o Miguel (Azores), video records were made at the depths of 5, 10, 15, 20, 25, and 30 m where a rocky substratum was present. Semiquantitative abundance data of the most common benthic organisms (algae and fixed/sedentary macroinvertebrates) were recorded in the vicinity of each depth reference point. Qualitative samples of the more conspicuous organisms were taken for confirmation of identification in the laboratory. At each site, substratum type, geographical orientation, and depth level were recorded. Multivariate analysis (nonmetric multidimensional scaling; analysis of similarity Anosim tests; similarity percentages Simper analysis) on the collected data enabled patterns of community distribution to be identified, as well as their relationship to abiotic factors. Since depth was found to be the sole determining factor for communities, it was the only one considered in the subtidal biotope survey protocol proposed here