Contribution of galaxies to the background hydrogen-ionizing flux     

Julien E. G. Devriendt  Shiv K. Sethi  Bruno Guiderdoni  & Biman B. Nath 《Monthly notices of the Royal Astronomical Society》1998,298(3):708-718

We estimate the evolution of the contribution of galaxies to the cosmic background flux at 912 Å by means of a semi-analytic model of galaxy formation and evolution. Such modelling has been quite successful in reproducing the optical properties of galaxies. We assume that high-redshift damped Lyman α systems are the progenitors of present-day galaxies, and we design a series of models that are consistent with the evolution of cosmic comoving emissivities in the available near-infrared, optical, ultraviolet and far-infrared bands along with the evolution of the neutral hydrogen content and average metallicity of damped Lyman α systems. We use these models to compute the galactic contribution to the Lyman-limit emissivity and background flux for 0 ≃  z  ≤ 4. We take into account the absorption of Lyman-limit photons by H  I and dust in the interstellar medium of the galaxies. We find that the background Lyman-limit flux due to galaxies might dominate (or be comparable to) the contribution from quasars at almost all redshifts if the absorption by H  I in the interstellar medium is neglected. Such H  I absorption would result in a severe diminishing of this flux — by almost three orders of magnitude at high redshifts and by one to two orders at z  ≃ 0. Though the resulting galaxy flux is completely negligible at high redshifts, it is comparable to the quasar flux at z  ≃ 0.  相似文献   

CDM models with a BSI step-like primordial spectrum and a cosmological constant     

Julien Lesgourgues  D. Polarski  & A. A. Starobinsky 《Monthly notices of the Royal Astronomical Society》1998,297(3):769-776

A class of spatially flat models with cold dark matter (CDM), a cosmological constant and a broken-scale-invariant (BSI) step-like primordial (initial) spectrum of adiabatic perturbations, generated in an exactly solvable inflationary model where the inflaton potential has a rapid change of its first derivative at some point, is confronted with existing observational data on angular fluctuations of the CMB temperature, galaxy clustering and peculiar velocities of galaxies. If we locate the step in the initial spectrum at k  ≃ 0.05  h Mpc⁻¹, where a feature in the spectrum of Abell clusters of galaxies was found that could reflect a property of the initial spectrum, and if the large-scale flat plateau of the spectrum is normalized according to the COBE data, the only remaining parameter of the spectrum is p — the ratio of amplitudes of the metric perturbations between the small-scale and large-scale flat plateaux. Allowed regions in the plane of parameters (Ω = 1 − Ω_Λ,  H ₀) satisfying all data have been found for p lying in the region (0.8–1.7). Especially good agreement of the form of the present power spectrum in this model with the form of the cluster power spectrum is obtained for the inverted step ( p  < 1,  p  = 0.7–0.8), when the initial spectrum has slightly more power on small scales.  相似文献   

Magnetic susceptibility of the rock matrix related to magnetic fabric studies     

Pierre Rochette 《Journal of Structural Geology》1987,9(8)

Non-ferromagnetic minerals constitute what is called the rock matrix, whose susceptibility (K₁) is directly accessible using only high magnetic fields. Measurements on minerals and a wide range of rock types show that K₁ is mainly due to paramagnetism and hardly exceeds 10⁻³ SI, with an anisotropy degree (P) less than 1.35. Different methods to estimate the role of the matrix component in low-field susceptibility (K) and its anisotropy include petrological and chemical analysis, evolution of P vs K, low temperature studies and comparison of K with remanent magnetizations.  相似文献   

Metamorphic control of the magnetic mineralogy of black shales in the Swiss Alps: toward the use of “magnetic isogrades”     

Pierre Rochette 《Earth and Planetary Science Letters》1987,84(4)

The magnetic susceptibility of the Helvetic Jurassic black shales, sampled around the Aar and Gothard massifs in the Swiss Alps, is due to iron-bearing silicates, magnetite or pyrrhotite. It exhibits a clear correlation with the Alpine metamorphism, from zeolite to amphibolite facies: it firstly decreases due to the breakdown of original magnetite, then sharply increases when pyrite is transformed into magnetic pyrrhotite. Using numerous field measurements of the susceptibility it was possible to carry out a fast and precise mapping of the pyrrhotite-in isograde, which appears to correspond closely to the stilpnomelane-out isograde. The matrix susceptibility, obtained by means of high-field measurements with a cryogenic magnetometer, is due to iron-bearing silicates and shows only smooth variations with metamorphism. Both monoclinic and hexagonal pyrrhotites are observed in the high-grade area.  相似文献   

Tertiary remagnetization of normal polarity in Mesozoic marly limestones from SE France     

Frdric Kechra  Didier Vandamme  Pierre Rochette 《Tectonophysics》2003,362(1-4):219-238

The results of our new paleomagnetic investigations on 21 sites in the Cévennes and Lure regions as well as previous studies demonstrate that all Mesozoic marly limestones of SE France exhibit similar paleomagnetic behavior with remagnetization disputed in age. The studied areas have the particularity to have been folded before (Late Eocene), the Alpine folding (Oligo–Miocene). Samples (201 marly limestones) dated from Lower Jurassic to Lower Cretaceous have been demagnetized by thermal treatment. They all present a well-defined component with a normal polarity which was mostly obtained between 200 and 350 °C. Numerous arguments lead from pretectonic to syntectonic widespread remagnetization related to orogenic fluid circulation affecting the whole basin. An Eocene age (between 35 and 40 Ma) is obtained for this remagnetization thanks both to the comparison of the average inclination of all regional paleomagnetic studies (+54.9°/−1.5°) with the expected paleomagnetic inclination and the syntectonic character of remagnetization.  相似文献   

Sediment partitioning and winnowing in a mixed eolian-marine system (Mauritanian shelf)     

Julien Michel  Hildegard Westphal  Till J. J. Hanebuth 《Geo-Marine Letters》2009,29(4):221-232

Continental shelf systems are highly dynamic sedimentary environments, where sediments from biogenic production as well as from terrigenous sources are redistributed in the shelf depositional system, and partly exported off the shelf to the slope and the deep sea. The Golfe d’Arguin (Mauritania, NW Africa) is dominated by such redistribution processes, involving clastic silt imported as dust from the Sahara desert and biogenic carbonates of marine origin. Indeed, surface-sediment grain size and mineralogy show a clear north–south partitioning of sediment type. Fine material is winnowed from the northern part of the gulf, and transported toward the southern part off the Banc d’Arguin, where coarse silt settles on the outer shelf and upper slope, at least down to 600 m water depth. Particles of the fine silt fraction, estimated in terms of eolian material collected aboard the research vessel, are thought to be exported further offshore as they correspond to grain sizes previously reported from adjacent deep-sea sediments. These findings suggest that the interpretation of dust records from the continental slope and rise off NW Africa must consider reworking and partitioning processes active on the Mauritanian shelf.  相似文献   

Jet disc coupling in black hole binaries     

Julien Malzac 《Astrophysics and Space Science》2007,311(1-3):149-159

In the last decade multi-wavelength observations have demonstrated the importance of jets in the energy output of accreting black hole binaries. The observed correlations between the presence of a jet and the state of the accretion flow provide important information on the coupling between accretion and ejection processes. After a brief review of the properties of black hole binaries, I illustrate the connection between accretion and ejection through two particularly interesting examples. First, an INTEGRAL observation of Cygnus X-1 during a ‘mini-’ state transition reveals disc jet coupling on time scales of orders of hours. Second, the black hole XTEJ1118+480 shows complex correlations between the X-ray and optical emission. Those correlations are interpreted in terms of coupling between disc and jet on time scales of seconds or less. Those observations are discussed in the framework of current models.  相似文献   

lemomaf: Lensed Mock Map Facility     

Jaime E. Forero-Romero  Jérémy Blaizot  Julien Devriendt  Ludovic Van Waerbeke  Bruno Guiderdoni 《Monthly notices of the Royal Astronomical Society》2007,379(4):1507-1518

We present the Lensed Mock Map Facility ( lemomaf ), a tool designed to perform mock weak-lensing measurements on numerically simulated chunks of the Universe. Coupling N -body simulations to a semi-analytical model of galaxy formation, lemomaf can create realistic lensed images and mock catalogues of galaxies, at wavelengths ranging from the ultraviolet to the submillimetre. To demonstrate the power of such a tool, we compute predictions of the source–lens clustering (SLC) effect on the convergence statistics, and quantify the impact of weak lensing on galaxy counts in two different filters. We find that the SLC effect skews the probability density function of the convergence towards low values, with an intensity which strongly depends on the redshift distribution of galaxies. On the other hand, the degree of enhancement or depletion in galaxy counts due to weak lensing is independent of the SLC effect. We discuss the impact on the two-point shear statistics to be measured by future missions like SNAP and LSST . The SLC effect would bias the estimation of σ₈ from two-point statistics up to 5 per cent for a narrow redshift distribution of mean   z ∼ 0.5  , and up to 2 per cent in small angular scales for a redshift distribution of mean   z ∼ 1.5  . We conclude that accurate photometric redshifts for individual galaxies are necessary in order to quantify and isolate the SLC effect.  相似文献   

PaleoNet: new software for building,evaluating and applying neural network based transfer functions in paleoecology     

Julien M. J. Racca  Robert Racca  Reinhard Pienitz  Yves T. Prairie 《Journal of Paleolimnology》2007,38(3):467-472

Transfer functions that implement organism–environment relationships are now commonly used for inferring past environmental conditions in paleoecology. Specific software for developing and evaluating commonly used modelling techniques such as Weighted averaging (WA), Weighted averaging partial least square (WA-PLS), Maximum likelihood (ML), and Modern analog technique (MAT) are available. A new software programme, PaleoNet, is now available for modelling organism–environment relationships which is specifically designed for the development and the evaluation of artificial neural network (ANN) based transfer functions in paleoecology. Here we present the main characteristics of this new software PaleoNet (User guide version 1.01) and discuss in more detail one of its specific features: the pruning.  相似文献   

Detecting thermal boundary control in surface flows from numerical dynamos     

Julien Aubert  Hagay Amit 《Physics of the Earth and Planetary Interiors》2007,160(2):143-156

The geomagnetic field and secular variation exhibit asymmetrical spatial features which are possibly originating from an heterogeneous thermal control of the Earth's lower mantle on the core. The identification of this control in magnetic data is subject to several difficulties, some of which can be alleviated by the use of core surface flow models. Using numerical dynamos driven by heterogeneous boundary heat flux, we confirm that within the parameter space accessible to simulations, time average surface flows obey a simple thermal wind equilibrium between the Coriolis and buoyancy forces, the Lorentz, inertial and viscous forces playing only a secondary role, even for Elsasser numbers significantly larger than 1. Furthermore, we average the models over the duration of three vortex turnovers, and correlate them with a longer time average which fully reveals the signature of boundary heterogeneity. This allows us to quantify the possibility of observing mantle control in core surface flows averaged over a short time period. A scaling analysis is performed in order to apply the results to the Earth's core. We find that three vortex turnovers could represent between 100 and 360 years of Earth time, and that the heat flux heterogeneity at the core-mantle boundary could be large enough to yield an observable signature of thermal mantle control in a time average core surface flow within reach of the available geomagnetic data.  相似文献