12,285 research outputs found

    Scattering functions of knotted ring polymers

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    We discuss the scattering function of a Gaussian random polygon with N nodes under a given topological constraint through simulation. We obtain the Kratky plot of a Gaussian polygon of N=200 having a fixed knot for some different knots such as the trivial, trefoil and figure-eight knots. We find that some characteristic properties of the different Kratky plots are consistent with the distinct values of the mean square radius of gyration for Gaussian polygons with the different knots.Comment: 4pages, 3figures, 3table

    Ram pressure stripping and galaxy orbits: The case of the Virgo cluster

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    We investigate the role of ram pressure stripping in the Virgo cluster using N-body simulations. Radial orbits within the Virgo cluster's gravitational potential are modeled and analyzed with respect to ram pressure stripping. The N-body model consists of 10000 gas cloud complexes which can have inelastic collisions. Ram pressure is modeled as an additional acceleration on the clouds located at the surface of the gas distribution in the direction of the galaxy's motion within the cluster. We made several simulations changing the orbital parameters in order to recover different stripping scenarios using realistic temporal ram pressure profiles. We investigate systematically the influence of the inclination angle between the disk and the orbital plane of the galaxy on the gas dynamics. We show that ram pressure can lead to a temporary increase of the central gas surface density. In some cases a considerable part of the total atomic gas mass (several 10^8 M_solar) can fall back onto the galactic disk after the stripping event. A quantitative relation between the orbit parameters and the resulting HI deficiency is derived containing explicitly the inclination angle between the disk and the orbital plane. The comparison between existing HI observations and the results of our simulations shows that the HI deficiency depends strongly on galaxy orbits. It is concluded that the scenario where ram pressure stripping is responsible for the observed HI deficiency is consistent with all HI 21cm observations in the Virgo cluster.Comment: 29 pages with 21 figures. Accepted for publication in Ap

    Does the Fornax dwarf spheroidal have a central cusp or core?

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    The dark matter dominated Fornax dwarf spheroidal has five globular clusters orbiting at ~1 kpc from its centre. In a cuspy CDM halo the globulars would sink to the centre from their current positions within a few Gyrs, presenting a puzzle as to why they survive undigested at the present epoch. We show that a solution to this timing problem is to adopt a cored dark matter halo. We use numerical simulations and analytic calculations to show that, under these conditions, the sinking time becomes many Hubble times; the globulars effectively stall at the dark matter core radius. We conclude that the Fornax dwarf spheroidal has a shallow inner density profile with a core radius constrained by the observed positions of its globular clusters. If the phase space density of the core is primordial then it implies a warm dark matter particle and gives an upper limit to its mass of ~0.5 keV, consistent with that required to significantly alleviate the substructure problem.Comment: 6 pages, 5 figures, accepted for publication in MNRAS, high resolution simulations include

    Velocity Structure of Self-Similar Spherically Collapsed Halos

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    Using a generalized self-similar secondary infall model, which accounts for tidal torques acting on the halo, we analyze the velocity profiles of halos in order to gain intuition for N-body simulation results. We analytically calculate the asymptotic behavior of the internal radial and tangential kinetic energy profiles in different radial regimes. We then numerically compute the velocity anisotropy and pseudo-phase-space density profiles and compare them to recent N-body simulations. For cosmological initial conditions, we find both numerically and analytically that the anisotropy profile asymptotes at small radii to a constant set by model parameters. It rises on intermediate scales as the velocity dispersion becomes more radially dominated and then drops off at radii larger than the virial radius where the radial velocity dispersion vanishes in our model. The pseudo-phase-space density is universal on intermediate and large scales. However, its asymptotic slope on small scales depends on the halo mass and on how mass shells are torqued after turnaround. The results largely confirm N-body simulations but show some differences that are likely due to our assumption of a one-dimensional phase space manifold.Comment: 11 pages, 4 figures. Accepted by PR

    GHASP : an H alpha kinematic survey of spiral and irregular galaxies. V. Dark matter distribution in 36 nearby spiral galaxies

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    The results obtained from a study of the mass distribution of 36 spiral galaxies are presented. The galaxies were observed using Fabry-Perot interferometry as part of the GHASP survey. The main aim of obtaining high resolution H alpha 2D velocity fields is to define more accurately the rising part of the rotation curves which should allow to better constrain the parameters of the mass distribution. The H alpha velocities were combined with low resolution HI data from the literature, when available. Combining the kinematical data with photometric data, mass models were derived from these rotation curves using two different functional forms for the halo: an isothermal sphere and an NFW profile. For the galaxies already modeled by other authors, the results tend to agree. Our results point at the existence of a constant density core in the center of the dark matter halos rather than a cuspy core, whatever the type of the galaxy from Sab to Im. This extends to all types the result already obtained by other authors studying dwarf and LSB galaxies but would necessitate a larger sample of galaxies to conclude more strongly. Whatever model is used (ISO or NFW), small core radius halos have higher central densities, again for all morphological types. We confirm different halo scaling laws, such as the correlations between the core radius and the central density of the halo with the absolute magnitude of a galaxy: low luminosity galaxies have small core radius and high central density. We find that the product of the central density with the core radius of the dark matter halo is nearly constant, whatever the model and whatever the absolute magnitude of the galaxy. This suggests that the halo surface density is independent from the galaxy type.Comment: 21 pages, 14 figures. MNRAS (accepted october 3rd 2007

    Greedy Forwarding in Dynamic Scale-Free Networks Embedded in Hyperbolic Metric Spaces

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    We show that complex (scale-free) network topologies naturally emerge from hyperbolic metric spaces. Hyperbolic geometry facilitates maximally efficient greedy forwarding in these networks. Greedy forwarding is topology-oblivious. Nevertheless, greedy packets find their destinations with 100% probability following almost optimal shortest paths. This remarkable efficiency sustains even in highly dynamic networks. Our findings suggest that forwarding information through complex networks, such as the Internet, is possible without the overhead of existing routing protocols, and may also find practical applications in overlay networks for tasks such as application-level routing, information sharing, and data distribution

    Excitons in type-II quantum dots: Finite offsets

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    Quantum size effects for an exciton attached to a spherical quantum dot are calculated by a variational approach. The band line-ups are assumed to be type-II with finite offsets. The dependence of the exciton binding energy upon the dot radius and the offsets is studied for different sets of electron and hole effective masses

    On the equilibrium morphology of systems drawn from spherical collapse experiments

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    We present a purely theoretical study of the morphological evolution of self-gravitating systems formed through the dissipationless collapse of N-point sources. We explore the effects of resolution in mass and length on the growth of triaxial structures formed by an instability triggered by an excess of radial orbits. We point out that as resolution increases, the equilibria shift, from mildly prolate, to oblate. A number of particles N ~= 100000 or larger is required for convergence of axial aspect ratios. An upper bound for the softening, e ~ 1/256, is also identified. We then study the properties of a set of equilibria formed from scale-free cold initial mass distributions, ro ~ r^-g with 0 <= g <= 2. Oblateness is enhanced for initially more peaked structures (larger values of g). We map the run of density in space and find no evidence for a power-law inner structure when g <= 3/2 down to a mass fraction <~0.1 per cent of the total. However, when 3/2 < g <= 2, the mass profile in equilibrium is well matched by a power law of index ~g out to a mass fraction ~ 10 per cent. We interpret this in terms of less-effective violent relaxation for more peaked profiles when more phase mixing takes place at the centre. We map out the velocity field of the equilibria and note that at small radii the velocity coarse-grained distribution function (DF) is Maxwellian to a very good approximation.Comment: 16 page
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