Galaxy shear estimation from stacked images

Statistics of the weak lensing of galaxies can be used to constrain cosmology if the galaxy shear can be estimated accurately. In general, this requires accurate modelling of unlensed galaxy shapes and the point spread function (PSF). I discuss suboptimal but potentially robust methods for estimating galaxy shear by stacking images such that the stacked-image distribution is closely Gaussian by the central limit theorem. The shear can then be determined by radial fitting, requiring only an accurate model of the PSF rather than also needing to model each galaxy accurately. When noise is significant, asymmetric errors in the centroid must be corrected, but the method may ultimately be able to give accurate unbiased results when there is a high galaxy density with constant shear. It provides a useful baseline for more optimal methods, and a test case for estimating biases, though the method is not directly applicable to realistic data. I test stacking methods on the simple toy simulations with constant PSF and shear provided by the GREAT08 project, on which most other existing methods perform significantly more poorly, and briefly discuss generalizations to more realistic cases. In the Appendix , I discuss a simple analytic galaxy population model where stacking gives optimal errors in a perfect ideal case.     

Early Tertiary fluid flow and pressure–temperature conditions in the Shimanto accretionary complex of south-west Japan: constraints from fluid inclusions

The pressure and temperature history of the Tertiary Shimanto belt of south-west Japan has been elucidated by analysing fluids trapped in quartz crystals which grew syn-kinematically along late-stage brittle faults. The samples come from three areas that span the Paleogene exposures on the Muroto Peninsula of Shikoku Island. Applying microthermometric and laser Raman microsampling techniques to coeval water-rich and carbonic fluid inclusions, we have constrained the pressure and temperature conditions that accompanied a widespread and kinematically distinct phase of deformation. The results suggest elevated geothermal gradients during late-stage deformation, conditions that are in disaccord with previous plate reconstructions that have depicted old, thermally mature Pacific crust subducting beneath Eurasia during the early to middle Tertiary. These conditions can most easily be accounted for by including an additional plate boundary in the western Pacific during Paleogene time. Plate reconstructions that include the Kula plate in this region are therefore consistent with our findings. In addition, our results provide clues to the conditions that likely accompany seismogenic deformation at active convergent plate boundaries.     

A finite element solution of a fully coupled implicit formulation for reservoir simulation

An iterative method is presented for solving a fully coupled and implicit formulation of fluid flow in a porous medium. The mathematical model describes a set of fully coupled three-phase flow of compressible and immiscible fluids in a saturated oil reservoir. The finite element method is applied to obtain the simultaneous solution (SS) for the resulting highly non-linear partial differential equations where fluid pressures are the primary unknowns. The final discretized equations are solved iteratively by using a fully implicit numerical scheme. Several examples, illustrating the use of the present model, are described. The increased stability achieved with this scheme has permitted the use of larger time steps with smaller material balance errors.     

Radio sources in the 2dF Galaxy Redshift Survey – II. Local radio luminosity functions for AGN and star-forming galaxies at 1.4 GHz

We have cross-matched the 1.4-GHz NRAO VLA Sky Survey (NVSS) with the first 210 fields observed in the 2dF Galaxy Redshift Survey (2dFGRS), covering an effective area of 325 deg² (about 20 per cent of the final 2dFGRS area). This yields a set of optical spectra of 912 candidate NVSS counterparts, of which we identify 757 as genuine radio identifications – the largest and most homogeneous set of radio source spectra ever obtained. The 2dFGRS radio sources span the redshift range     to 0.438, and are a mixture of active galaxies (60 per cent) and star-forming galaxies (40 per cent). About 25 per cent of the 2dFGRS radio sources are spatially resolved by NVSS, and the sample includes three giant radio galaxies with projected linear size greater than 1 Mpc. The high quality of the 2dF spectra means we can usually distinguish unambiguously between AGN and star-forming galaxies. We make a new determination of the local radio luminosity function at 1.4 GHz for both active and star-forming galaxies, and derive a local star formation density of         .     

A fully coupled hydro‐thermo‐poro‐mechanical model for black oil reservoir simulation

A fully coupled formulation of a hydro‐thermo‐poro‐mechanical model for a three‐phase black oil reservoir model is presented. The model is based upon the approach proposed by one of the authors which fully couples geomechanical effects to multiphase flow. Their work is extended here to include non‐isothermal effects. The gas phase contribution to the energy equation has been neglected based on a set of assumptions. The coupled formulation given herein differs in several ways when compared to the earlier work and an attempt is made to link the flow based formulation and mixture theory. The Finite Element Method is employed for the numerical treatment and essential algorithmic implementation is discussed. Numerical examples are presented to provide further understanding of the current methodology. Copyright © 2001 John Wiley & Sons, Ltd.     

Small faults and kink bands in the Nankai accretionary complex: Textural observations from Site 808 of ODP Leg 131

Abstract We present backscattered scanning electron microscope and petrographic microscope observations of deformed sediments from Ocean Drilling Program (ODP) Site 808 in order to better understand the dewatering and deformation history of the Nankai accretionary complex. This synthesis of deformation textures has three implications. First, the early structures that dominate the Nankai prism, small faults and kink bands, have very different electron microscope versus optical microscopic expressions. This observation is important to investigations of fine-grained sediment in both stable and active tectonic settings, in part, because these materials have often been studied almost exclusively by electron microscope methods. In sediments of this type, investigators often forego petrographic analysis because of the relative opacity of samples at normal (i.e. 30 pm) thin section thicknesses. Second, the textural observations we have compiled suggest that these deformation structures acted as 'single-event' pathways that contributed to diffusive dewatering of the prism. Third, our observations serve as a reference frame for the early tectonic structures that are important to the dewatering history of a 'sandy' accretionary prism.     

Variations in structures and distributions of C25 highly branched isoprenoid (HBI) alkenes in cultures of the diatom, Haslea ostrearia (Simonsen)

The highly branched isoprenoid (HBI) alkenes of three batch cultures of the diatom Haslea ostrearia have been examined and the occurrence of C₂₅ tri-, tetra- and pentaenes confirmed. Growth of cultures outside at ambient temperature, under natural sunlight, in May (6 days) and June (10 days) 1995 and monitoring of HBI concentrations in samples collected daily and stored frozen, revealed that in May greater concentrations of HBIs (ca 6500 fg cell⁻¹) were produced than in June and at a much earlier stage of growth. Furthermore, in May the HBIs were more unsaturated (predominantly a tetraene rather than a triene). The reasons for these differences are at present unknown and future investigations of the effect of irradiance, temperature, salinity and other environmental variables are required. Such experiments may allow the various HBI distributions in the alga and in sediments to be better understood and, like some other polyunsaturated lipids, HBI alkenes may become useful environmental and palaeoenvironmental indicators. Two previously unreported alkenes, 2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)pentadec-2,5,9,13-ene (GC retention index, 2201_DB-1) and 2,6,10,14-tetramethyl-7-(3-methylene pent-4-enyl)pentadec-2,5,9,13-ene (GC RI, 2248_DB-1) from a batch culture of Haslea ostrearia grown in October/November 1993 and stored frozen for over a year, have also been isolated and characterised by ¹³C and ¹H NMR, epoxide derivatisation and mass spectrometry. Such structural characterisation studies of HBI alkenes should also aid our understanding of the geochemical fate of these widely distributed hydrocarbons.     

Oxidation state of the atmosphere and crust of Venus from pioneer Venus results

Recent spacecraft observations of Venus permit a detailed model of sulfur chemistry in the atmosphere-lithosphere system. Pioneer Venus experiments confirm that, as predicted, COS and H₂S are dominant over SO₂ in the lower atmosphere, and that the equilibrium concentrations of S₂ and S₃ are significant. Many criteria serve to bracket the oxidation state of the crust: it is nearly certain that the S₂^2?/SO₄^2? buffer regulates the oxygen fucagity, and that FeO is at least as abundant as Fe₂O₃ in crustal silicates. A highly oxidized crust (as, for example, would result from O₂ absorption complementary to escape of vast amounts of H₂) is incompatible with the gas-phase sulfur chemistry. If the Pioneer Venus mass spectrometer estimates of the abundance of sulfur gases are correct, Earth-like models for the bulk composition of Venus are seriously in error, and a far lower FeO content is required for Venus.