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排序方式: 共有362条查询结果,搜索用时 31 毫秒
231.
Vivienne Wild John A. Peacock Ofer Lahav Edward Conway Steve Maddox Ivan K. Baldry Carlton M. Baugh Joss Bland-Hawthorn Terry Bridges Russell Cannon Shaun Cole Matthew Colless Chris Collins Warrick Couch Gavin Dalton Roberto De Propris Simon P. Driver George Efstathiou Richard S. Ellis Carlos S. Frenk Karl Glazebrook Carole Jackson Ian Lewis Stuart Lumsden Darren Madgwick Peder Norberg Bruce A. Peterson Will Sutherland Keith Taylor 《Monthly notices of the Royal Astronomical Society》2005,356(1):247-269
232.
Noam I. Libeskind Shaun Cole Carlos S. Frenk Takashi Okamoto Adrian Jenkins 《Monthly notices of the Royal Astronomical Society》2007,374(1):16-28
We investigate the properties of satellite galaxies formed in N -body/SPH simulations of galaxy formation in the ΛCDM cosmology. The simulations include the main physical effects thought to be important in galaxy formation and, in several cases, produce realistic spiral discs. In total, a sample of nine galaxies of luminosity comparable to the Milky Way was obtained. At magnitudes brighter than the resolution limit, MV =−12 , the luminosity function of the satellite galaxies in the simulations is in excellent agreement with data for the Local Group. The radial number density profile of the model satellites, as well as their gas fractions also match observations very well. In agreement with previous N -body studies, we find that the satellites tend to be distributed in highly flattened configurations whose major axis is aligned with the major axis of the (generally triaxial) dark halo. In two out of three systems with sufficiently large satellite populations, the satellite system is nearly perpendicular to the plane of the galactic disc, a configuration analogous to that observed in the Milk Way. The discs themselves are perpendicular to the minor axis of their host haloes in the inner parts, and the correlation between the orientation of the galaxy and the shape of the halo persists even out to the virial radius. However, in one case the disc's minor axis ends up, at the virial radius, perpendicular to the minor axis of the halo. The angular momenta of the galaxies and their host halo tend to be well aligned. 相似文献
233.
A. J. Benson C. G. Lacey C. M. Baugh S. Cole C. S. Frenk 《Monthly notices of the Royal Astronomical Society》2002,333(1):156-176
We develop a coupled model for the evolution of the global properties of the intergalactic medium (IGM) and the formation of galaxies, in the presence of a photoionizing background due to stars and quasars. We use this model to predict the thermodynamic history of the IGM when photoionized by galaxies forming in a cold dark matter (CDM) universe. The evolution of the galaxies is calculated using a semi-analytical model, including a detailed treatment of the effects of tidal stripping and dynamical friction on satellite galaxies orbiting inside larger dark matter haloes. We include in the model the negative feedback on galaxy formation from the photoionizing background. Photoionization inhibits galaxy formation in low-mass dark matter haloes in two ways: (i) heating of the IGM and inhibition of the collapse of gas into dark haloes by the IGM pressure, and (ii) reduction in the rate of radiative cooling of gas within haloes. The result of our method is a self-consistent model of galaxy formation and the IGM. The IGM is reheated twice (during reionization of H i and He ii ), and we find that the star formation rate per unit volume is slightly suppressed after each episode of reheating. We find that galaxies brighter than L ★ are mostly unaffected by reionization, while the abundance of faint galaxies is significantly reduced, leading to present-day galaxy luminosity functions with shallow faint-end slopes, in good agreement with recent observational data. Reionization also affects other properties of these faint galaxies, in a readily understandable way. 相似文献
234.
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237.
Y.K. Kharaka D.R. Cole J.J. Thordsen E. Kakouros H.S. Nance 《Journal of Geochemical Exploration》2006,89(1-3):183
To investigate the potential for the geologic storage of CO2 in saline sedimentary aquifers, 1600 ton of CO2 were injected at 1500 m depth into a 24-m sandstone section of the Frio Formation — a regional reservoir in the US Gulf Coast. Fluid samples obtained from the injection and observation wells before, during and after CO2 injection show a Na–Ca–Cl type brine with 93,000 mg/L TDS and near saturation of CH4 at reservoir conditions. As injected CO2 gas reached the observation well, results showed sharp drops in pH (6.5 to 5.7), pronounced increases in alkalinity (100 to 3000 mg/L as HCO3) and Fe (30 to 1100 mg/L), and significant shifts in the isotopic compositions of H2O and DIC. Geochemical modeling indicates that brine pH would have dropped lower, but for buffering by dissolution of calcite and Fe oxyhydroxides. Post-injection results show the brine gradually returning to its pre-injection composition. 相似文献
238.
A spatial relationship between high capacity municipal production wells (>5,000 m3/day), completed in a deep bedrock aquifer, and a buried bedrock valley was recognized in the city of Guelph, southwestern Ontario, Canada. Most production wells are completed in a discrete zone, ~60 m below ground surface, within flat-lying dolostones of the Silurian Amabel Formation. Thick overburden and limited subsurface data make characterization of the karstic aquifer difficult. This study integrates hydrogeologic data with models of karst formation, deriving a conceptual model of porosity development as it relates to valley incision. Bedrock valley incision likely occurred prior to the early Wisconsinan age (>60–75 ka). Incision created steep hydraulic gradients within the flat-lying bedrock, and provided the driving force required to integrate regional groundwater flow into karst conduits that drained at the base of the valley. Dissolution in production zone dolostones was favoured over dissolution in shallower bedrock due to abundant bedding plane partings and fossiliferous facies with high intercrystalline porosity. Burial of the valley during subsequent ice advances reduced the valley’s hydraulic influence and the efficacy of the flow system to cause dissolution. The high capacity municipal wells near the buried bedrock valley tap into the now dormant karst aquifer system. 相似文献
239.
Claudia I. Mora Lee R. Riciputi David R. Cole Karen D. Walker 《Contributions to Mineralogy and Petrology》2009,157(6):781-795
The Boehls Butte anorthosite consists predominantly of an unusual bimodal assemblage of andesine and bytownite–anorthite.
Oxygen isotope compositions of the anorthosite were profoundly altered by high temperature, retrograde interaction with meteoric-hydrothermal
fluids that varied in composition from isotopically evolved to nearly pristine meteoric water. Oxygen isotope ratios of bulk
plagioclase separates are in the range +7.0 to −6.2‰ V-SMOW, however, secondary ion mass spectrometry indicates spot-sized
isotope values as low as −16‰. Typical inter- and intra-plagioclase grain variability is 3–6‰, and extreme heterogeneity of
up to 20‰ is noted in a few samples. High-temperature hydrothermal alteration of intermediate plagioclase is proposed to explain
the origin of bytownite–anorthite in the anorthosite and creation of its unusual bimodal plagioclase assemblage. The anorthite-forming
reaction created retrograde reaction-enhanced permeability which, together with rapid decompression, extension, and unroofing
of the anorthosite complex, helped to accommodated influx of significant volumes of meteoric-hydrothermal fluids into the
anorthosite. 相似文献
240.
Capture of high-speed (hypervelocity) particles in aerogel at ambient temperatures of 175-763 K is reported. This extends previous work which has mostly focussed on conducting experiments at ambient laboratory temperatures, even though aerogels are intended for use in cosmic dust capture cells in space environments which may experience a range of temperatures (e.g., the NASA Stardust mission which collected dust at 1.81 AU and putative Mars atmospheric sampling missions). No significant change in track length (normalised to impactor size) was found over the range 175-600 K, although at 763 K a significant reduction (30%) was found. By contrast, entrance hole diameter remained constant only up to 400 K, above this sudden changes of up to 50% were observed. Experiments were also carried out at normal laboratory temperature using a wide range of aerogel densities and particle sizes. It was found that track length normalised to particle size varies inversely with aerogel density. This is a power law dependence and not linear as previously reported, with longer tracks at lower densities. Glass projectiles (up to 100 μm size) were found to undergo a variety of degrees of damage during capture. In addition to the well known acquisition of a coating (partial or complete) of molten aerogel the mechanical damage includes pitting and meridian fractures. Larger (500 μm diameter) stainless steel spheres also showed damage during capture. In this case melting and ablation occurs, suggesting surficial temperatures during impact in excess of 1400 °C. The response of the aerogel itself to passage of particles through it is reported. The presence of fan-like fractures around the tracks is attributed to cone cracking similar to that in glasses of normal density, with the difference that here it is a repetitive process as the particles pass through the aerogel. 相似文献