The amino acid composition of estuarine colloidal material     

A.C. Sigleo  P.E. Hare  G.R. Helz 《Estuarine, Coastal and Shelf Science》1983,17(1):87-96

Amino acids were determined in estuarine colloidal and particulate material from near surface waters ranging from fresh to brackish water (12 g kg^?1 salinity). The hydrolizable amino acids and associated ammonia account for an average of 80% of the nitrogen present in colloidal samples, and approximately 75% of the nitrogen from particulate fractions of the same samples. The relative proportions of these amino acids are similar to those of a cultured estuarine diatom. There are no significant amounts of D amino acids, or non-protein amino acids characteristic of bacteria in sediments and soil. A single deep-water sample (25 m) shows some evidence of bottom mud resuspension by the presence of a possible aspartic acid-hydroxyproline dimer.  相似文献   

A cosmological study of the star formation history in the solar neighbourhood     

X. Hernández  Vladimir Avila-Reese  Claudio Firmani 《Monthly notices of the Royal Astronomical Society》2001,327(1):329-338

We use a cosmological galactic evolutionary approach to model the Milky Way. A detailed treatment of the mass aggregation and dynamical history of the growing dark halo is included, together with a self-consistent physical treatment for the star formation processes within the growing galactic disc. This allows us to calculate the temporal evolution of star and gas surface densities at all galactic radii, in particular, the star formation history (SFH) at the solar radius. A large range of cosmological mass aggregation histories (MAHs) is capable of producing a galaxy with the present-day properties of the Milky Way. The resulting SFHs for the solar neighbourhood bracket the available observational data for this feature, the most probable MAH yielding the optimal comparison with these observations. We also find that the rotation curve for our Galaxy implies the presence of a constant density core in its dark-matter halo.  相似文献   

The angular momentum content of dwarf galaxies: new challenges for the theory of galaxy formation     

Frank C. van den Bosch  reas Burkert  Rob A. Swaters 《Monthly notices of the Royal Astronomical Society》2001,326(3):1205-1215

We compute the specific angular momentum distributions for a sample of low-mass disc galaxies observed by Swaters. We compare these distributions to those of dark matter haloes obtained by Bullock et al. from high-resolution N -body simulations of structure formation in a ΛCDM universe. We find that although the disc mass fractions are significantly smaller than the universal baryon fraction, the total specific angular momenta of the discs are in good agreement with those of dark matter haloes. This suggests that discs form out of only a small fraction of the available baryons, but yet manage to draw most of the available angular momentum. In addition we find that the angular momentum distributions of discs are clearly distinct from those of the dark matter; discs lack predominantly both low and high specific angular momenta. Understanding these findings in terms of a coherent picture for disc formation is challenging. Cooling, feedback and stripping, which are the main mechanisms to explain the small disc mass fractions found, seem unable to simultaneously explain the angular momentum distributions of the discs. In fact, it seems that the baryons that make up the discs must have been born out of angular momentum distributions that are clearly distinct from those of ΛCDM haloes. However, the dark and baryonic mass components experience the same tidal forces, and it is therefore expected that they should have similar angular momentum distributions. Therefore, understanding the angular momentum content of disc galaxies remains an important challenge for our picture of galaxy formation.  相似文献   

TRANSFERABLE PHOSPHORUS IN SEDIMENTS OF THE HUANGHE RIVER ESTUARY ＇S ADJACENT WATERS   总被引：1，自引：0，他引：1  

宋金明 《中国海洋湖沼学报》2001,19(1):81-86

Based on a new idea for research on cycling of marine biogenic elements, this study showed that only the leachable form phosphorus in natural grain sizes manne sediments constitutes the transferable phosphorous in the sediments. The transferable phosphorus content in the natural grain sizes surface sediments in the Huanghe River estuary adjacent waters ranges from 58.5-69.8 μg/g, accounting for only 9.1%-11.0% of the total phosphorus content, whereas the leachable form (“transferable” )phosphorus content in the sediment after it was totally ground into powder was found to be 454.8-529.2μg/g, accounting for 73.4%-89.1% of the total phosphorus. Analysis of the correlation between thebill,ass of benthos and the leachable form (“transferable” ) phosphorus showed that most of the leachableform (“transferable”) phosphorus in the totally ground sediment did not participate in the marine biogeo chemical cycling. Furthermore, a synchronous survey on benthos showed that the biornass of meio-and maero-benthos exhibited good positive correlation with the leachable form of phosphorus in the naturalgrain sizes sediment, but peorer correlation with the leachable form (“transferable”) phosphorus in the totally ground sediment, indicating that transferable phosphorus in marine sediment is the leachable form of phosphorus in the natural grain sizes sediments, and is not the previously known leachable form(“transferable”) phosphorus obtained from the totally ground sediment.  相似文献   

Convective cores in galactic cooling flows     

Alexei Kritsuk  Tomasz Plewa  Ewald Müller 《Monthly notices of the Royal Astronomical Society》2001,326(1):11-22

We use hydrodynamic simulations with adaptive grid refinement to study the dependence of hot gas flows in X-ray luminous giant elliptical galaxies on the efficiency of heat supply to the gas. We consider a number of potential heating mechanisms including Type Ia supernovae and sporadic nuclear activity of a central supermassive black hole. As a starting point for this research we use an equilibrium hydrostatic recycling model. We show that a compact cooling inflow develops, if the heating is slightly insufficient to counterbalance radiative cooling of the hot gas in the central few kiloparsecs. An excessive heating in the centre, instead, drives a convectively unstable outflow. We model the onset of the instability and a quasi-steady convective regime in the core of the galaxy in two dimensions assuming axial symmetry.
Provided the power of net energy supply in the core is not too high, the convection remains subsonic. The convective pattern is dominated by buoyancy driven large-scale mushroom-like structures. Unlike in the case of a cooling inflow, the X-ray surface brightness of an (on average) isentropic convective core does not display a sharp maximum at the centre. A hybrid model, which combines a subsonic peripheral cooling inflow with an inner convective core, appears to be stable. We also discuss observational implications of these results.  相似文献   

The build-up of haloes within Press–Schechter theory     

Will J. Percival 《Monthly notices of the Royal Astronomical Society》2001,327(4):1313-1322

Modelling the build-up of haloes is important for linking the formation of galaxies with cosmological models. A simple model of halo growth is provided by Press–Schechter (PS) theory, where the initial field of density fluctuations is smoothed using spherically symmetric filters centred on a given position to obtain information about the likelihood of later collapse on varying scales. In this paper the predicted halo mass growth is compared for three filter shapes: Gaussian, top-hat and sharp k -space. Preliminary work is also presented analysing the build-up of haloes within numerical simulations using a friends-of-friends group finder. The best-fit to the simulation mass function was obtained using PS theory with a top-hat filter. By comparing both the backwards conditional mass function, which gives the distribution of halo progenitors, and the distribution of halo mergers in time, the build-up of haloes in the simulations is shown to be better fitted by PS theory with a sharp k -space filter. This strengthens previous work, which also found the build-up of haloes in simulations to be well matched to PS theory with a sharp k -space filter by providing a direct comparison of different filters and by extending the statistical tools used to analyse halo mass growth. The usefulness of this work is illustrated by showing that the cosmological evolution in the proportion of haloes that have undergone recent merger is predicted to be independent of mass and power spectrum and to only depend upon cosmology. Recent results from observations of field galaxies are shown to match the evolution expected, but are not sufficiently accurate to distinguish usefully between cosmological parameters.  相似文献   

Cuspy dark matter haloes and the Galaxy     

J.J. Binney  N.W. Evans 《Monthly notices of the Royal Astronomical Society》2001,327(2):L27-L31

The microlensing optical depth to Baade's Window constrains the minimum total mass in baryonic matter within the Solar circle to be greater than ∼     , assuming the inner Galaxy is barred with viewing angle ∼20°. From the kinematics of solar neighbourhood stars, the local surface density of dark matter is ∼     . We construct cuspy haloes normalized to the local dark matter density and calculate the circular-speed curve of the halo in the inner Galaxy. This is added in quadrature to the rotation curve provided by the stellar and ISM discs, together with a bar sufficiently massive so that the baryonic matter in the inner Galaxy reproduces the microlensing optical depth. Such models violate the observational constraint provided by the tangent-velocity data in the inner Galaxy (typically at radii     . The high baryonic contribution required by the microlensing is consistent with implications from hydrodynamical modelling and the pattern speed of the Galactic bar. We conclude that the cuspy haloes favoured by the cold dark matter cosmology (and its variants) are inconsistent with the observational data on the Galaxy.  相似文献   

Sharp images of young stellar objects     

N. Ageorges  A. Eckart  R. Hofmann 《Astrophysics and Space Science》1995,224(1-2):411-414

We present high resolution infrared images of the pre-main sequence star V536 Aql and polarization maps of the reflection nebula IRN Cha observed with the SHARP camera on the ESO NTT. Extended structures are observed near V536 Aql. The polarization pattern of IRN Cha is typical of a centrally illuminated source but with a central band of anomalously oriented polarization believed to occur in an optically thick disk.  相似文献   

Transfeer of organic pollutants during sewage-seawater mixing     

Chen Song  Liao Wenzhuo  Fu Tianbao 《中国海洋湖沼学报》1992,10(2):178-183

The transfer processes of organic pollutants during sewage-seawater mixing were simulated in three different mixing systems. The release rate of organic matter from sewage particles to seawater was found to be about 0.02–0.04 d⁻¹. The release processes can be simulated by an exchange-equilibrium kinetic model. Degradation of COD was observed in all three mixing systems and can be described by a first order kinetic equation. The degradation rate constantk ranged between 0.04 and 0.07 d⁻¹. These results have provided important parameters for estimation of environmental capacity.  相似文献   

Wolf-Rayet stars     

K. A. van der Hucht 《Astronomy and Astrophysics Review》1992,4(2):123-159

Summary Recent literature on Population I Wolf-Rayet star research extending from the Milky Way to blue compact dwarf galaxies is reviewed, broken down into inventory, basic parameters and galactic distribution, atmospheres, binaries, intrinsic variability, mass loss, enrichment and evolution. Also the incidence of Wolf-Rayet stars with variable non-thermal radio emission, excess X-ray fluxes, and episodic/periodic IR excesses is reviewed. These phenomena appear to be associated with wind-wind interaction in wide long-period WR+OB binaries and with wind-compact object interaction in WR+c binaries, with orbit sizes of the order of magnitude of the WR radio photosphere sizes or larger.  相似文献