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Dark-matter Haloes (dark-matter + halo)

Distribution by Scientific Domains

Physics and Astronomy	100%

Selected Abstracts

Two measures of the shape of the dark halo of the Milky Way

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2000
Rob P. Olling
In order to test the reliability of determinations of the shapes of dark-matter haloes of the galaxies, we have made such measurements for the Milky Way by two independent methods. First, we have combined the measurements of the overall mass distribution of the Milky Way derived from its rotation curve and the measurements of the amount of dark matter in the solar neighbourhood obtained from stellar kinematics to determine the flattening of the dark halo. Secondly, we have used the established technique based on the variation in thickness of the Milky Way's H i layer with radius: by assuming that the H i gas is in hydrostatic equilibrium in the gravitational potential of a galaxy, one can use the observed flaring of the gas layer to determine the shape of the dark halo. These techniques are found to produce a consistent estimate for the flattening of the dark-matter halo, with a shortest-to-longest axis ratio of q,0.8, but only if one adopts somewhat non-standard values for the distance to the Galactic centre, R0, and the local Galactic rotation speed, ,0. For consistency, one requires values of R0,7.6 kpc and ,0,190 km s,1. The results depend on the Galactic constants because the adopted values affect both distance measurements within the Milky Way and the shape of the rotation curve, which, in turn, alter the inferred halo shape. Although differing significantly from the current IAU-sanctioned values, these upper limits are consistent with all existing observational constraints. If future measurements confirm these lower values for the Galactic constants, then the validity of the gas-layer-flaring method will be confirmed. Further, dark-matter candidates such as cold molecular gas and massive decaying neutrinos, which predict very flat dark haloes with q,0.2, will be ruled out. Conversely, if the Galactic constants were found to be close to the more conventional values, then there would have to be some systematic error in the methods for measuring dark halo shapes, so the existing modelling techniques would have to be viewed with some scepticism. [source]

Simulations of the formation and evolution of dwarf galaxies

ASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009
S. Valcke
Abstract We present models of the formation and evolution of isolated dwarf galaxies. The models follow the evolution of an initially homogeneous gas cloud collapsing in a pre-existing dark-matter halo. These simplified initial conditions are supported by the merger trees of isolated dwarf galaxies extracted from the milli-Millennium Simulation. An extensive comparison of the models to observations was made. The models' surface brightness profiles are well fitted by Sérsic profiles and the correlations between the models' Sérsic parameters and luminosity agree with the observations. We have also compared model predictions for the half-light radius Re, central velocity dispersion ,c, broad band colour B , V, metallicity [Z/Z,] versus luminosity relations and for the location relative to the fundamental plane with the available data. In all cases the models give the correct slope, in most cases we also get the zero-point right (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Modelling angular-momentum history in dark-matter haloes

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2002
Ariyeh H. Maller
We model the acquisition of spin by dark-matter haloes in semi-analytic merger trees. We explore two different algorithms: one in which halo spin is acquired from the orbital angular momentum of merging satellites, and another in which halo spin is gained via tidal torquing on shells of material while still in the linear regime. We find that both scenarios produce the characteristic spin distribution of haloes found in N -body simulations, namely, a log-normal distribution with mean , 0.04 and standard deviation , 0.5 in the log. A perfect match requires fine-tuning of two free parameters. Both algorithms also reproduce the general insensitivity of the spin distribution to halo mass, redshift and cosmology seen in N -body simulations. The spin distribution can be made strictly constant by physically motivated scalings of the free parameters. In addition, both schemes predict that haloes that have had recent major mergers have systematically larger spin values. These algorithms can be implemented within semi-analytic models of galaxy formation based on merger trees. They yield detailed predictions of galaxy properties that strongly depend on angular momentum (such as size and surface brightness) as a function of merger history and environment. [source]

Testing the modified Press,Schechter model against N -body simulations

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2001
Andreu Raig
A modified version of the extended Press,Schechter model for the growth of dark-matter haloes was introduced in two previous papers, with the aim of explaining the mass,density relation shown by haloes in high-resolution cosmological simulations. In this model, major mergers are well separated from accretion, thereby allowing a natural definition of halo formation and destruction. This makes it possible to derive analytic expressions for halo formation and destruction rates, the mass accretion rate and the probability distribution functions of halo formation times and progenitor masses. The stochastic merger histories of haloes can be readily derived and easily incorporated into semi-analytical models of galaxy formation, thus avoiding the usual problems encountered in the construction of Monte Carlo merger trees from the original extended Press,Schechter formalism. Here we show that the predictions of the modified Press,Schechter model are in good agreement with the results of N -body simulations for several scale-free cosmologies. [source]