The origin of the light distribution in spiral galaxies

We analyse a high-resolution, fully cosmological, hydrodynamical disc galaxy simulation, to study the source of the double-exponential light profiles seen in many stellar discs, and the effects of stellar radial migration upon the spatiotemporal evolution of both the disc age and metallicity distributions. We find a 'break' in the pure exponential stellar surface brightness profile, and trace its origin to a sharp decrease in the star formation per unit surface area, itself produced by a decrease in the gas volume density due to a warping of the gas disc. Star formation in the disc continues well beyond the break. We find that the break is more pronounced in bluer wavebands. By contrast, we find little or no break in the mass density profile. This is, in part, due to the net radial migration of stars towards the external parts of the disc. Beyond the break radius, we find that ∼60 per cent of the resident stars migrated from the inner disc, while ∼25 per cent formed in situ . Our simulated galaxy also has a minimum in the age profile at the break radius but, in disagreement with some previous studies, migration is not the main mechanism producing this shape. In our simulation, the disc metallicity gradient flattens with time, consistent with an 'inside-out' formation scenario. We do not find any difference in the intensity or the position of the break with inclination, suggesting that perhaps the differences found in empirical studies are driven by dust extinction.     

Burrow distributions and population estimates for the fiddler crabsUca spinicarpa andUca longisignalis in a Gulf of Mexico salt marsh

The distribution of fiddler crab (Uca spinicarpa andU. longisignalis) burrows in coastal marsh habitats is described, and the influence of substratum characteristics on the distributions of each is examined. Burrow densities were higher in vegetated areas than in sparsely vegetated and nonvegetated areas. Elevation and landward distance from shoreline (water's edge) also co-varied with burrow densities along transects. Burrow aperture diameters and burrow numbers m^?2 exhibited seasonal variations for both species ofUca. The depth of burrows increased with increased distance from the water's edge (approximate mean sea level) along transects into the marsh. A significant correlation between burrow aperture diameter and carapace width of the inhabitant enabled subsequent estimates of size and biomass in populations. Mean burrow densities forU. longisignalis ranged from a high of 182 m^?2 in September 1992 to a low of 26 m^?2 in May–June 1992. Mean burrow densities forU. spinicarpa ranged from a high of 46 m^?2 in March April 1993 to a low of 29 m^?2 in May–June 1992. Estimates of mean biomass ranged from 17.6 g AFDW m^?2 to 13.2 g AFDW m^?2 for aU. longisignalis-dominated site and from 6.5 g AFDW m^?2 to 4.9 g AFDW m^?2 for aU. spinicarpa dominated site. Field observations and laboratory soil-choice tests demonstrated that, in mixed populations,U. spinicarpa preferred to burrow in substrates of higher percent clay than didU. longisignalis.     

Strength of solid pressure media and implications for high pressure apparatus

The stress-strain properties of talc, pyrophyllite, silver chloride, sodium chloride, boron nitride and graphite have been measured under confining pressures up to 8 or 10 kb at room temperature, and, in the case of talc, also at temperatures up to 900° C. The extrapolation and application of these results to solid medium high pressure apparatus of pistoncylinder type is discussed and a calculation made of the correction to nominal pressure (friction correction), taking into account the stress gradients in the medium and the shearing between the medium and the cylinder wall. Correction to the nominal differential stress measured in solid medium stress-strain apparatus is also discussed.     

Geomorphology and hydrology of upper sinking cove,cumberland plateau,Tennessee

Upper Sinking Cove, dissecting the eastern escarpment of the Cumberland Plateau, is characterized by a multiple aquifer, predominantly vadose hydrologic system with minor surface components. There is a central trunk channel along the axis of the cove and a network of independent tributaries. Aquitards within the limestones, particularly Hartselle Formation shales, have influenced both cave and surface landform development by perching ground waters and slowing the vertical growth of closed depressions. Long-term solutional denudation in the portion of the cove underlain by limestones (40 per cent) is an estimated 56 mm per 1000 years, suggesting that karst development began 15–16 million years ago. Despite lower soil CO₂ and spring water hardness, 61 per cent of annual denudation occurs in the six winter months when 76 per cent of yearly runoff occurs. Landform development in Upper Sinking Cove appears to have begun as stream erosion carved a valley first in the sandstone caprock of the escarpment and later in the underlying Pennington Formation limestones containing numerous shale layers which promoted surface stream flow. Eventually stream erosion exposed the massive Bangor limestones which allowed deep ground water flow. Surface streams were pirated underground with the eventual formation of the chain of three closed depressions which constitute Upper Sinking Cove.     

Radium and barium at GEOSECS stations in the Atlantic and Pacific

²²⁶Ra and Ba show a general linear correlation in the oceanic water column within the uncertainties of the data: the slope of the line is about 4.6 nanomoles (nmoles) Ra/mole Ba, the intercept being at about 4 nmoles Ba/kg. This demonstrates the usefulness of Ba as a “chemical analogue” of Ra. Box-model calculations indicate that the average deep-water excess of Ra over Ba should be about 10% relative to the surface. This is consistent with the observations outside the deep northeast Pacific. However, the uncertainties in the data are such that the regional variation in the primary input cannot be resolved. In the deep waters of the North Pacific there is in fact a large excess of Ra relative to Ba. The one detailed profile presently available (204) can be explained consistently by a simple vertical advection-diffusion model.     

The geochemical cycle of 9Be: a reconnaissance

An initial survey has been made of the geochemistry of ⁹Be in natural waters with a view to utilizing the isotope with the cosmogenic species ¹⁰Be (t_1/2 = 1.5 m.y.) in geochronological studies of sedimentary accumulation. The mobility of⁹Be in continental waters is a strong function of pH with acid streams (pH < 6) being strongly enriched as compared to alkaline carbonate rivers. Large-scale scavenging of beryllium occurs in the estuaries of rivers containing more than about 200 pM of the element. In alkaline systems of lower concentration the element appears to behave conservatively. The net flux of beryllium to the ocean is estimated as that equivalent to an average effective river concentration of about 150 pM. Beryllium is strongly enriched in ridge crest hydrothermal solutions over ambient seawater. The flux is approximately 70% of that from the continents. The deep water concentrations of ⁹Be vary by only 20% between the North Atlantic and North Pacific demonstrating that the “chemical saltation” produced by scavenging from the deep water column and release from the sediments predominates over simple advective concentration along flow lines. Isotopic homogenization of ¹⁰Be and ⁹Be is probably achieved in the ocean. The situation in continental waters appears much less promising.     

On the formation of metal-rich deposits at ridge crests

Data from the hot springs at the Galapagos spreading center (T = 3–13°C) show depletions of the exiting waters in Cu, Ni, Cd, Se, Cr and U relative to ambient seawater. Manganese is strongly enriched. Iron shows highly variable behavior between vent fields but is in general low. The data confirm the occurrence of extensive subsurface mixing between the primary high-temperature, acid, reducing hydrothermal fluids and “groundwater”. The composition of the latter is indistinguishable from that of the free water column adjacent to the ridge axis. The final solutions are on the boundary between those forming MnO₂ crusts and those producing iron-manganese rich sediments. The suite of metal rich deposits observed at ridge crests — Mn-O, Fe-Mn-O, Fe-S — can be explained as the manifestation of the degree of subsurface mixing, decreasing from 100 : <1 to <1 : 1 across the series (assuming an end-member temperature of 350°C).     

Restoring landscapes: the authenticity problem

Philosophical concerns about restoring landscapes often revolve around two, connected, issues. First is the idea that a restored landscape, even if it is a perfect replica, has lost some of its value. The claim might appeal to a break in the continuity of the landscape and that continuity is part of what is valuable. Alternatively, often in the case of natural landscapes, the appeal is that any human manipulation is inauthentic; here the analogy is sometimes made with the art world and the restoration is deemed a fake. The second problem highlighted in philosophical debates is that the greater the success of restoration projects, the more threatened natural landscapes become: any claim that something must be preserved in its pristine or historically layered state is undermined by the claim that it could be put back again. Initially I discuss two opposing potential responses to these claims: (1) that humans are part of nature and thus cannot be an alien dominating force outside of nature; and (2) that nature is itself a social construct. Neither of these positions is entirely satisfactory, but what they jointly reveal is the reality of our fluid and multifaceted relationship with the world. I then show that Elliot's claim of the additional value of pristine nature is actually not based on an inherent value, but is dependent on the human valuation of it. I propose an alternative that places the source of value in the thing itself and thus arrive at a positive role for restoration as the setting in train and guiding of positive relationships above and beyond their social or public amenity value to us. Copyright © 2006 John Wiley & Sons, Ltd.