Optimal softening for force calculations in collisionless N-body simulations

In N-body simulations the force calculated between particles representing a given mass distribution is usually softened, to diminish the effect of graininess. In this paper we study the effect of such a smoothing, with the aim of finding an optimal value of the softening parameter. As already shown by Merritt (1996), for too small a softening the estimates of the forces will be too noisy, while for too large a softening the force estimates are systematically misrepresented. In between there is an optimal softening, for which the forces in the configuration approach best the true forces. The value of this optimal softening depends both on the mass distribution and on the number of particles used to represent it. For higher number of particles the optimal softening is smaller. More concentrated mass distributions necessitate smaller softening, but the softened forces are never as good an approximation of the true forces as for not centrally concentrated configurations. We give good estimates of the optimal softening for homogeneous spheres, Plummer spheres, and Dehnen spheres. We also give a rough estimate of this quantity for other mass distributions, based on the harmonic mean distance to the $k$th neighbour ($k$ = 1, .., 12), the mean being taken over all particles in the configuration. Comparing homogeneous Ferrers ellipsoids of different shapes we show that the axial ratios do not influence the value of the optimal softening. Finally we compare two different types of softening, a spline softening (Hernquist & Katz 1989) and a generalisation of the standard Plummer softening to higher values of the exponent. We find that the spline softening fares roughly as well as the higher powers of the power-law softening and both give a better representation of the forces than the standard Plummer softening.Comment: 16 pages Latex, 19 figures, accepted for publication in MNRAS, corrected typos, minor changes mainly in sec.

Forming disk galaxies in wet major mergers. I. Three fiducial examples

Using three fiducial Nbody+SPH simulations, we follow the merging of two disk galaxies with a hot gaseous halo component each, and examine whether the merger remnant can be a spiral galaxy. The stellar progenitor disks are destroyed by violent relaxation during the merging and most of their stars form a classical bulge, while the remaining form a thick disk and its bar. A new stellar disk forms subsequently and gradually in the remnant from the gas accreted mainly from the halo. It is vertically thin and well extended in its equatorial plane. A bar starts forming before the disk is fully in place, contrary to what is assumed in idealised simulations of isolated bar-forming galaxies. It has morphological features such as ansae and boxy/peanut bulges. Stars of different ages populate different parts of the box/peanut. A disky pseudobulge forms also, so that by the end of the simulation, all three types of bulges coexist. The oldest stars are found in the classical bulge, followed by those of the thick disk, then by those in the thin disk. The youngest stars are in the spiral arms and the disky pseudobulge. The disk surface density profiles are of type II (exponential with downbending), and the circular velocity curves are flat and show that the disks are submaximum in these examples: two clearly so and one near-borderline between maximum and submaximum. On average, only roughly between 10 and 20% of the stellar mass is in the classical bulge of the final models, i.e. much less than in previous simulations.Comment: 17 pages, 8 figures, accepted for publication in ApJ. V2: replaced Figure 4 with correct versio

Les Dactylogyridae (Plathelminthes, Monogenea) des Cyprinidae du genre Barbus du Sud Cameroun

Nous décrivons sur cinq espèces de #Barbus (Téléostéens, Cyprinidae), du sud Cameroun, 14 espèces nouvelles de #Monogenea Dactylogyridae. Ce sont : chez #B. jae : Dactylogyrus kii n. sp.; #D. jaei n. sp. et #Dactylogyroides biradius n. sp.; chez #B. camptacanthus : Dactylogyrus amieti, n. sp.; #D. valeti n. sp. et #Dogielius njinei n. sp.; chez #B. martorelli : Dactylogyrus bopeleti n. sp., #D. maillardi n.sp.; #D. insolitus n.sp., #D. simplex n. sp. et #Dogielus martorellii n. sp.; chez #B. aspilus : Dactylogyrus aspili n. sp., #D. mendehei n.sp. et #D. nyongensis n. sp. Ces deux dernières espèces parasitent également #B. guirali. D'après les types morphologiques de ses parasites, #Barbus jae se distingue nettement des autres espèces de #Barbus$. Nous discutons des problèmes de spéficité parasitaire observés et posons le problème de la définition générique de ces Monogènes en fonction de la notion de type morphologique. (Résumé d'auteur

One step multiderivative methods for first order ordinary differential equations

A family of one-step multiderivative methods based on Padé approximants to the exponential function is developed. The methods are extrapolated and analysed for use in PECE mode. Error constants and stability intervals are calculated and the combinations compared with well known linear multi-step combinations and combinations using high accuracy Newton-Cotes quadrature formulas as correctors. w926020

Femtosecond x rays from laser-plasma accelerators

Relativistic interaction of short-pulse lasers with underdense plasmas has recently led to the emergence of a novel generation of femtosecond x-ray sources. Based on radiation from electrons accelerated in plasma, these sources have the common properties to be compact and to deliver collimated, incoherent and femtosecond radiation. In this article we review, within a unified formalism, the betatron radiation of trapped and accelerated electrons in the so-called bubble regime, the synchrotron radiation of laser-accelerated electrons in usual meter-scale undulators, the nonlinear Thomson scattering from relativistic electrons oscillating in an intense laser field, and the Thomson backscattered radiation of a laser beam by laser-accelerated electrons. The underlying physics is presented using ideal models, the relevant parameters are defined, and analytical expressions providing the features of the sources are given. Numerical simulations and a summary of recent experimental results on the different mechanisms are also presented. Each section ends with the foreseen development of each scheme. Finally, one of the most promising applications of laser-plasma accelerators is discussed: the realization of a compact free-electron laser in the x-ray range of the spectrum. In the conclusion, the relevant parameters characterizing each sources are summarized. Considering typical laser-plasma interaction parameters obtained with currently available lasers, examples of the source features are given. The sources are then compared to each other in order to define their field of applications.Comment: 58 pages, 41 figure

DNA Vaccines Encoding Antigen Targeted to MHC Class II Induce Influenza-Specific CD8+ T Cell Responses, Enabling Faster Resolution of Influenza Disease

Current influenza vaccines are effective but imperfect, failing to cover against emerging strains of virus and requiring seasonal administration to protect against new strains. A key step to improving influenza vaccines is to improve our understanding of vaccine induced protection. Whilst it is clear that antibodies play a protective role, vaccine induced CD8+ T cells can improve protection. To further explore the role of CD8+ T cells we used a DNA vaccine that encodes antigen dimerised to an immune cell targeting module. Immunising CB6F1 mice with the DNA vaccine in a heterologous prime boost regime with the seasonal protein vaccine improved the resolution of influenza disease compared to protein alone. This improved disease resolution was dependent on CD8+ T cells. However, DNA vaccine regimes that induced CD8+ T cells alone were not protective and did not boost the protection provided by protein. The MHC targeting module used was an anti-I-Ed single chain antibody specific to the BALB/c strain of mice. To test the role of MHC targeting we compared the response between BALB/c, C57BL/6 mice and an F1 cross of the two strains (CB6F1). BALB/c mice were protected, C57BL/6 were not and the F1 had an intermediate phenotype; showing that the targeting of antigen is important in the response. Based on these findings, and in agreement with other studies using different vaccines, we conclude that in addition to antibody, inducing a protective CD8 response is important in future influenza vaccines

The Case for Doing Nothing about Institutional Investors\u27 Common Ownership of Small Stakes in Competing Firms

Recent empirical research purports to demonstrate that institutional investors\u27 common ownership of small stakes in competing firms causes those firms to compete less aggressively, injuring consumers. A number of prominent antitrust scholars have cited this research as grounds for limiting the degree to which institutional investors may hold stakes in multiple firms that compete in any concentrated market. This Article contends that the purported competitive problem is overblown and that the proposed solutions would reduce overall social welfare. With respect to the purported problem, we show that the theory of anti-competitive harm from institutional investors\u27 common ownership is implausible and that the empirical studies supporting the theory are methodologically unsound. The theory fails to account for the fact that intra-industry diversified institutional investors are also inter-industry diversified, and it rests upon unrealistic assumptions about managerial decision-making. The empirical studies purporting to demonstrate anti-competitive harm from common ownership are deficient because the inaccurately assess institutional investors\u27 economic interests and employ an endogenous measure that precludes causal inferences

Homeland Security and the Police Mission

The terrorist attacks on America are seen as a pivotal period for the nation and for policing. They have thrust policing into a new mission that will have a wide ranging impact on the police role, organizational strategies, staffing, training and policy. State, local and tribal police are now at the forefront of Homeland Security activities, yet there remains a great deal of uncertainty about their mission.The ramp-up efforts by federal and state government are reminiscent of the staging for civil defense in the 1960\u27s. As a result of 9/11, there has been an invigoration for cities and towns to develop response plans for any localized terrorist incidents. The safety of the public is important and falls to government agencies.Community policing was intended to encourage community input and involvement. As communities and our country attempt to put safeguards in place and raise awareness, community and business leaders, and neighborhood groups should be enlisted to assist. The tenets of community policing can be utilized to plan and engage the community as police agencies endeavor to respond to a new mission.Using a sample of New England police agencies and police practitioners, this research will query whether police agencies are changing their organizational mission to integrate Homeland Security activities. The researchers used several methods to analyze police agencies and their role in Homeland Security. First, examining police agency mission statements through content analysis, the authors found little evidence of a formal Homeland Security focus. This research utilized a web-based survey tool to elicit officer perspectives on Homeland Security objectives

Singular tachyon kinks from regular profiles

We demonstrate how Sen's singular kink solution of the Born-Infeld tachyon action can be constructed by taking the appropriate limit of initially regular profiles. It is shown that the order in which different limits are taken plays an important role in determining whether or not such a solution is obtained for a wide class of potentials. Indeed, by introducing a small parameter into the action, we are able circumvent the results of a recent paper which derived two conditions on the asymptotic tachyon potential such that the singular kink could be recovered in the large amplitude limit of periodic solutions. We show that this is explained by the non-commuting nature of two limits, and that Sen's solution is recovered if the order of the limits is chosen appropriately.Comment: 7 pages, 3 figures. References adde