The graininess of dark matter haloes     

Marcel Zemp  Jürg Diemand †  Michael Kuhlen  Piero Madau  Ben Moore  Doug Potter  Joachim Stadel  Lawrence Widrow 《Monthly notices of the Royal Astronomical Society》2009,394(2):641-659

We use the recently completed one billion particle Via Lactea II Λ cold dark matter simulation to investigate local properties like density, mean velocity, velocity dispersion, anisotropy, orientation and shape of the velocity dispersion ellipsoid, as well as the structure in velocity space of dark matter haloes. We show that at the same radial distance from the halo centre, these properties can deviate by orders of magnitude from the canonical, spherically averaged values, a variation that can only be partly explained by triaxiality and the presence of subhaloes. The mass density appears smooth in the central relaxed regions but spans four orders of magnitude in the outskirts, both because of the presence of subhaloes as well as of underdense regions and holes in the matter distribution. In the inner regions, the local velocity dispersion ellipsoid is aligned with the shape ellipsoid of the halo. This is not true in the outer parts where the orientation becomes more isotropic. The clumpy structure in local velocity space of the outer halo cannot be well described by a smooth multivariate normal distribution. Via Lactea II also shows the presence of cold streams made visible by their high 6D phase space density. Generally, the structure of dark matter haloes shows a high degree of graininess in phase space that cannot be described by a smooth distribution function.  相似文献   

Multimass spherical structure models for N-body simulations     

Marcel Zemp  Ben Moore  Joachim Stadel  C. Marcella Carollo  Piero Madau 《Monthly notices of the Royal Astronomical Society》2008,386(3):1543-1556

We present a simple and efficient method to set up spherical structure models for N -body simulations with a multimass technique. This technique reduces by a substantial factor the computer run time needed in order to resolve a given scale as compared to single-mass models. It therefore allows to resolve smaller scales in N -body simulations for a given computer run time. Here, we present several models with an effective resolution of up to  1.68 × 10⁹  particles within their virial radius which are stable over cosmologically relevant time-scales. As an application, we confirm the theoretical prediction by Dehnen that in mergers of collisionless structures like dark matter haloes always the cusp of the steepest progenitor is preserved. We model each merger progenitor with an effective number of particles of approximately 10⁸ particles. We also find that in a core–core merger the central density approximately doubles whereas in the cusp–cusp case the central density only increases by approximately 50 per cent. This may suggest that the central regions of flat structures are better protected and get less energy input through the merger process.  相似文献   

Sediment accumulation rates in European lakes since AD 1850: trends,reference conditions and exceedence     

Neil L. Rose  David Morley  Peter G. Appleby  Richard W. Battarbee  Tiiu Alliksaar  Piero Guilizzoni  Erik Jeppesen  Atte Korhola  Jaan-Mati Punning 《Journal of Paleolimnology》2011,45(4):447-468

Sediment accumulation rate (SAR) is an important physical parameter in all lakes and increases have been observed in many over the last c.100 years. This has been ascribed to changes in land-use and land-management causing accelerated catchment soil erosion and an increase in autochthonous organic matter production. The EU Water Framework Directive requires that assessment of biological, hydromorphological and chemical elements of water quality should be based on the degree to which present day conditions deviate from those expected in the absence of significant anthropogenic influence, termed reference conditions. Currently however, the reference condition for sediment accumulation rate for lakes of different types is undefined. To improve our understanding of the controls on SARs we compiled SAR and lake typology data for 207 European lakes derived from ²¹⁰Pb dated cores to assess how rates have changed through time (in 25 year classes) both overall and for lakes of different types. Seventy-one percent of these sediment cores showed surface SARs higher than “basal” (mainly nineteenth century) rates, 11% showed no change while 18% showed a decline. Lakes were then classified into lake-types using four variables: alkalinity (3 classes), altitude (3 classes), maximum depth (2 classes) and lake area (2 classes). This generated a possible 36 lake classes of which 25 were represented in the dataset. Nine lake-types contained >10 lakes. Little change in SAR occurred prior to 1900 and most increases occurred in more recent periods, in particular 1950–1975 and post-1975. This indicates a general acceleration in SAR in European lakes during the second half of the twentieth century. Reference SARs were estimated for six lake-types with the highest number of sites. European mountain lakes had the lowest reference SAR (0.005 ± 0.003 g cm⁻² yr⁻¹) while lowland, high alkalinity sites had the highest (0.03–0.04 g cm⁻² yr⁻¹). SARs for other lake-types ranged between 0.012 and 0.024 g cm⁻² yr⁻¹. Using the mountain lake-type as an example, the 1850 reference SAR appears to show good agreement with available data for lakes beyond Europe indicating these values may be more broadly applicable. Contemporary SARs in lakes of all classes showed exceedence over their defined reference SAR. This may be partly due to diagenetic processes. Greatest exceedences were found in shallow, low altitude lakes and these are considered to be the ones under the greatest threat from continued elevation of SAR. It is considered that climate change may play a progressively more important role in driving SAR in the future.  相似文献   

Modelling the merging history of Binary SMBHs in Hierarchical Models of Galaxy Formation     

Marta Volonteri  Francesco Haardt  Piero Madau 《Astrophysics and Space Science》2002,281(1-2):501-504

We present a model describing the evolution of a Super Massive Black Holes (SMBHs) population within the framework of hierarchical models of galaxy formation. The occurrence of SMBHs binaries at different redshifts and the influence of the interacting SMBHS on the host galaxies are studied by means of Montecarlo realizations of halo merging histories, coupled with a semi-analytical treatment of the interactions between the systems. We find that the main parameters governing the coalescence timescale, and hence the fraction of binary systems, are the initial mass and the accretion history of the SMBHs. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

Dynamics and mass balance of the 2007 Cima Una rockfall (Eastern Alps, Italy)     

Alessia Viero  Sandro Furlanis  Cristina Squarzoni  Giordano Teza  Antonio Galgaro  Piero Gianolla 《Landslides》2013,10(4):393-408

On October 12th, 2007 about 40,000 m³ of dolomitic rock detached from the northern wall of the peak known as “Cima Una” (Val Fiscalina, Sesto Dolomites, Bolzano, Italy), and fell 900 m to Fiscalina Valley below. The event generated a dense dust cloud, which traveled up to 4 km from the source area. The failure surface was formed by two near-vertical surfaces, almost perpendicular to each other. The orientation of these surfaces is consistent with two of the main regional tectonic sets. Only a small portion of the fallen material appeared to be preserved as blocks deposited at the base of the rock wall. About a fifth of the fallen mass was deposited on a colluvial cone. The missing mass, estimated to be about 80 %, may be represented by highly fragmented rock in part deposited as sand on the valley floor and in part dispersed as a dense dust cloud generated during the rockfall. There appears to be a deficit of deposited material, which could lead underestimation in the calculation of rock–cliff recession rates. The dynamics of the rockfall, strongly conditioned by the local topography, partially explains the intense rock breakage and the generation of the dust cloud. The rockfall was not caused by an external trigger, such as an earthquake or heavy rainfall; the failure was most likely progressive due to mechanical and physical degradation along highly stressed failure surfaces, possibly promoted by permafrost degradation and freeze and thaw processes.  相似文献   

Dust emission structure in comets with type II tails     

Piero Benvenuti 《Astrophysics and Space Science》1974,27(1):203-209

Short exposure plates of Comet Arend-Roland (1956h) are examined and compared with similar photographs of Comet Bennett (1969i), Comet Halley (1910 II) and Comet Mrkos (1957d). It is found that the emission structure of the dust near the nucleus in the first comet is different from that in the others, although in all cases we have the formation of parabolic envelopes. Close resemblances with configurations described by Wurm and Mammano (1972) have been found for Comet Halley and Comet Mrkos.  相似文献   

Occurrence of IRGAROL 1051 in coastal waters from Biscayne Bay,Florida, USA     

Gardinali PR  Plasencia M  Mack S  Poppell C 《Marine pollution bulletin》2002,44(8):781-788

Surface water samples from marinas, commercial ports and open bay areas collected from Biscayne Bay and the Miami River, Florida, USA, were analyzed for the occurrence of IRGAROL 1051 by GC/MS. The anifouling boosting herbicide was found in 80% (46/57) of the samples collected between March 1999 and September 2000. Concentrations within the bay range between non-detected (<1 ppt) and 61 ppt (ng/L) and were generally low compared with levels reported in European or Japanese waters. Aside from the elevated concentrations observed along the Miami River South Fork (61 ppt), the highest concentrations observed in the bay corresponded to marinas with high density of pleasure craft and restricted water circulation. In contrast, occurrence of IRGAROL 1051 along the commercial port or the cruise line terminal was generally lower (<1-2.2 ppt). Concentrations around Coconut Grove Marina were consistently higher (5-12 ppt) than the rest of the bay waters during the whole period of time surveyed.  相似文献   

A conceptual and numerical model for groundwater management: a case study on a coastal aquifer in southern Tuscany, Italy   总被引：5，自引：3，他引：2  

Piero Barazzuoli  Monica Nocchi  Roberto Rigati  Massimo Salleolini 《Hydrogeology Journal》2008,16(8):1557-1576

Ongoing hydrogeological research aims to develop a correct management model for the Plio-Pleistocene multi-aquifer system of the Albegna River coastal plain (southern Tuscany, Italy); overexploitation of this aquifer for irrigation and tourism has caused seawater intrusion. The conceptual model is based on field and laboratory data collected during the 1995–2003 period. Meteoric infiltration and flows from the adjoining carbonate aquifer recharge the aquifer. Natural outflow occurs through a diffuse flow into the sea and river; artificial outflow occurs through intensive extraction of groundwater from wells. Water exchanges in the aquifer occur naturally (leakage, closing of aquitard) and artificially (multiscreened wells). The aquifer was represented by a three-dimensional finite element model using the FEFLOW numerical code. The model was calibrated for steady-state and transient conditions by matching computed and measured piezometric levels (February 1995–February 1996). The model helped establish that seawater intrusion is essentially due to withdrawals near the coast during the irrigation season and that it occurs above all in the Osa-Albegna sector, as well as along the river that at times feeds the aquifer. The effects of hypothetical aquifer exploitation were assessed in terms of water budget and hydraulic head evolution.  相似文献   

Relations between climatic–geomorphological parameters and sediment yield in a mediterranean semi-arid area (Sicily,southern Italy)     

Sergio Grauso  Andrea Pagano  Grazia Fattoruso  Piero De Bonis  Filippo Onori  Pasquale Regina  Carlo Tebano 《Environmental Geology》2008,54(2):219-234

In the present work on-going research is discussed whose primary goal is to test some statistical methods to estimate the average yearly area-specific sediment yield (SSY). Using geomorphological and climatic parameters various multiple regression formulae have been set up based on SSY observed data from 16 catchments of Sicily. Three distinct techniques were adopted to select the parameters to be used in the equations: simple correlation, stepwise regression analysis and a supervised geomorphological–statistical correlation. The comparison of the results showed the effectiveness of the stepwise analysis, which led to a regression equation with a coefficient of determination (r ²) of 0.87. Nevertheless, even this methodology showed some elements of uncertainty that have caused, in some cases, appreciable differences between observed and predicted values (mean percentage error equal to 26%). These differences are likely either due to the hydraulic regime of most of Sicilian watercourses (typical of semi-arid regions) or to hidden factors (e.g., topography, human impact and concentrated erosion), which can greatly affect the processes of sediment production and transport. The study has pointed out the need to take into account a larger number of observations and to perform the analysis of relations between suitable variables and SSY at a more detailed time resolution.  相似文献   

Shear strength degradation due to flexural ductility demand in circular RC columns     

Piero Colajanni  Antonino Recupero  Nino Spinella 《Bulletin of Earthquake Engineering》2015,13(6):1795-1807

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