The Deuterium Abundance in the z=0.7 absorber towards QSO PG1718+4807

We report a further analysis of the ratio of deuterium to hydrogen (D/H) using HST spectra of the z=0.701 Lyman limit system towards the QSO PG1718+481. Initial analyses of this absorber found it gave a high D/H value, 1.8 - 3.1 \times 10^{-4} (Webb et al. 1998), inconsistent with several higher redshift measurements. It is thus important to critically examine this measurement. By analysing the velocity widths of the DI, HI and metal lines present in this system, Kirkman et al. (2001) report that the additional absorption in the blue wing of the lya line can not be DI, with a confidence level of 98%. Here we present a more detailed analysis, taking into account possible wavelength shifts between the three sets of HST spectra used in the analysis. We find that the constraints on this system are not as strong as those claimed by Kirkman et al. The discrepancy between the parameters of the blue wing absorption and the parameters expected for DI is marginally worse than 1 sigma. Tytler et al.(1999) commented on the first analysis of Webb et al.(1997,1998), reporting the presence of a contaminating lower redshift Lyman limit system, with log[N(HI)] = 16.7 at z=0.602, which biases the N(HI) estimate for the main system. Here we show that this absorber actually has log[N(HI)] < 14.6 and does not impact on the estimate of N(HI) in the system of interest at z = 0.701. The purpose of the present paper is to highlight important aspects of the analysis which were not explored in previous studies, and hence help refine the methods used in future analyses of D/H in quasar spectra.Comment: 11 pages, 9 figures. Accepted by MNRA

Persistent Currents in Multichannel Interacting Systems

Persistent currents of disordered multichannel mesoscopic rings of spinless interacting fermions threaded by a magnetic flux are calculated using exact diagonalizations and self-consistent Hartree-Fock methods. The validity of the Hartree-Fock approximation is controled by a direct comparison with the exact results on small $4\times4$ clusters. For sufficiently large disorder (diffusive regime), the effect of repulsive interactions on the current distribution is to slightly decrease its width (mean square current) but to {\it increase} its mean value (mean current). This effect is stronger in the case of a long range repulsion. Our results suggest that the coupling between the chains is essential to understand the large currents observed experimentally.Comment: Revised version, uuencoded compressed file including fig

R.A.Fisher, design theory, and the Indian connection

Design Theory, a branch of mathematics, was born out of the experimental statistics research of the population geneticist R. A. Fisher and of Indian mathematical statisticians in the 1930s. The field combines elements of combinatorics, finite projective geometries, Latin squares, and a variety of further mathematical structures, brought together in surprising ways. This essay will present these structures and ideas as well as how the field came together, in itself an interesting story.Comment: 11 pages, 3 figure

Inclusive quasi-elastic electron-nucleus scattering

This article presents a review of the field of inclusive quasi-elastic electron-nucleus scattering. It discusses the approach used to measure the data and includes a compilation of data available in numerical form. The theoretical approaches used to interpret the data are presented. A number of results obtained from the comparison between experiment and calculation are then reviewed. The analogies and differences to other fields of physics exploiting quasi-elastic scattering from composite systems are pointed out.Comment: Accepted for publication in Reviews of Modern Physic

Three-dimensional effects on extended states in disordered models of polymers

We study electronic transport properties of disordered polymers in the presence of both uncorrelated and short-range correlated impurities. In our procedure, the actual physical potential acting upon the electrons is replaced by a set of nonlocal separable potentials, leading to a Schr\"odinger equation that is exactly solvable in the momentum representation. We then show that the reflection coefficient of a pair of impurities placed at neighboring sites (dimer defect) vanishes for a particular resonant energy. When there is a finite number of such defects randomly distributed over the whole lattice, we find that the transmission coefficient is almost unity for states close to the resonant energy, and that those states present a very large localization length. Multifractal analysis techniques applied to very long systems demonstrate that these states are truly extended in the thermodynamic limit. These results reinforce the possibility to verify experimentally theoretical predictions about absence of localization in quasi-one-dimensional disordered systems.Comment: 16 pages, REVTeX 3.0, 5 figures on request from FDA ([email protected]). Submitted to Phys. Rev. B. MA/UC3M/09/9

Haulout site selection by southern elephant seals at Marion Island

Using data from an ongoing mark-resight programme at Marion Island, we tested empirically whether southern elephant seals prefer certain terrestrial sites to others during the breeding, moulting and winter haulouts, and whether the pattern of site use is the same for different age and sex groups. Southern elephant seals preferred some sites, while discriminating against other sites, with different age and sex classes using different sites for certain haulout events. Wintering young animals did not show strong site selection. Some popular sites were used for all haulouts by all age and sex groups, and apparently have all the requirements of a good site for terrestrial haulout by southern elephant seals. Site selection becomes more apparent with age, suggesting the role of haulout experience in site selection

Gas Accretion via Lyman Limit Systems

In cosmological simulations, a large fraction of the partial Lyman limit systems (pLLSs; 16<log N(HI)<17.2) and LLSs (17.2log N(HI)<19) probes large-scale flows in and out of galaxies through their circumgalactic medium (CGM). The overall low metallicity of the cold gaseous streams feeding galaxies seen in these simulations is the key to differentiating them from metal rich gas that is either outflowing or being recycled. In recent years, several groups have empirically determined an entirely new wealth of information on the pLLSs and LLSs over a wide range of redshifts. A major focus of the recent research has been to empirically determine the metallicity distribution of the gas probed by pLLSs and LLSs in sizable and representative samples at both low (z2) redshifts. Here I discuss unambiguous evidence for metal-poor gas at all z probed by the pLLSs and LLSs. At z<1, all the pLLSs and LLSs so far studied are located in the CGM of galaxies with projected distances <100-200 kpc. Regardless of the exact origin of the low-metallicity pLLSs/LLSs, there is a significant mass of cool, dense, low-metallicity gas in the CGM that may be available as fuel for continuing star formation in galaxies over cosmic time. As such, the metal-poor pLLSs and LLSs are currently among the best observational evidence of cold, metal-poor gas accretion onto galaxies.Comment: Invited review to appear in Gas Accretion onto Galaxies, Astrophysics and Space Science Library, eds. A. J. Fox & R. Dav\'e, to be published by Springe

Detection of weak gravitational lensing distortions of distant galaxies by cosmic dark matter at large scales

Most of the matter in the universe is not luminous and can be observed directly only through its gravitational effect. An emerging technique called weak gravitational lensing uses background galaxies to reveal the foreground dark matter distribution on large scales. Light from very distant galaxies travels to us through many intervening overdensities which gravitationally distort their apparent shapes. The observed ellipticity pattern of these distant galaxies thus encodes information about the large-scale structure of the universe, but attempts to measure this effect have been inconclusive due to systematic errors. We report the first detection of this ``cosmic shear'' using 145,000 background galaxies to reveal the dark matter distribution on angular scales up to half a degree in three separate lines of sight. The observed angular dependence of this effect is consistent with that predicted by two leading cosmological models, providing new and independent support for these models.Comment: 18 pages, 5 figures: To appear in Nature. (This replacement fixes tex errors and typos.