Restoring the full velocity field in the gaseous disk ofthe spiral galaxy NGC 157

We analyse the line-of-sight velocity field of ionized gas in the spiral galaxy NGC 157 which has been obtained in the H\alpha emission at the 6m telescope of SAO RAS. The existence of systematic deviations of the observed gas velocities from pure circular motion is shown. A detailed investigation of these deviations is undertaken by applying a Fourier analysis of the azimuthal distributions of the line-of-sight velocities at different distances from the galactic center. As a result of the analysis, all the main parameters of the wave spiral pattern are determined: the corotation radius, the amplitudes and phases of the gas velocity perturbations at different radii, and the velocity of circular rotation of the disk corrected for the velocity perturbations due to spiral arms. At a high confidence level, the presence of the two giant anticyclones in the reference frame rotating with the spiral pattern is shown; their sizes and the localization of their centers are consistent with the results of the analytic theory and of numerical simulations. Besides the anticyclones, the existence of cyclones in residual velocity fields of spiral galaxies is predicted. In the reference frame rotating with the spiral pattern these cyclones have to reveal themselves in galaxies where a radial gradient of azimuthal residual velocity is steeper than that of the rotation velocity (abridged).Comment: 23 pages including 25 eps-figures. Accepted for publication in A&

Polarizable molecular interactions in condensed phase and their equivalent nonpolarizable models

Earlier, using phenomenological approach, we showed that in some cases polarizable models of condensed phase systems can be reduced to nonpolarizable equivalent models with scaled charges. Examples of such systems include ionic liquids, TIPnP-type models of water, protein force fields, and others, where interactions and dynamics of inherently polarizable species can be accurately described by nonpolarizable models. To describe electrostatic interactions, the effective charges of simple ionic liquids are obtained by scaling the actual charges of ions by a factor of 1/sqrt(eps_el), which is due to electronic polarization screening effect; the scaling factor of neutral species is more complicated. Here, using several theoretical models, we examine how exactly the scaling factors appear in theory, and how, and under what conditions, polarizable Hamiltonians are reduced to nonpolarizable ones. These models allow one to trace the origin of the scaling factors, determine their values, and obtain important insights on the nature of polarizable interactions in condensed matter systems.Comment: 43 pages, 3 figure

New Angles on Standard Force Fields: Toward a General Approach for Treating Atomic-Level Anisotropy

Nearly all standard force fields employ the “sum-of-spheres” approximation, which models intermolecular interactions purely in terms of interatomic distances. Nonetheless, atoms in molecules can have significantly nonspherical shapes, leading to interatomic interaction energies with strong orientation dependencies. Neglecting this “atomic-level anisotropy” can lead to significant errors in predicting interaction energies. Herein, we propose a simple, transferable, and computationally efficient model (MASTIFF) whereby atomic-level orientation dependence can be incorporated into ab initio intermolecular force fields. MASTIFF includes anisotropic exchange-repulsion, charge penetration, and dispersion effects, in conjunction with a standard treatment of anisotropic long-range (multipolar) electrostatics. To validate our approach, we benchmark MASTIFF against various sum-of-spheres models over a large library of intermolecular interactions between small organic molecules. MASTIFF achieves quantitative accuracy, with respect to both high-level electronic structure theory and experiment, thus showing promise as a basis for “next-generation” force field development

A Method for Reconstructing the Full Vector Velocity Field in the Gaseous Disks of Spiral Galaxies

this paper, the idea of galactic spirals as rigidly rotating eigenwave modes was first formulated. Lindblad further developed the wave theory of spiral galactic structure in his later works [2-5]. In 1941 [2], he examined the wave motion of a rotating stellar system more fully, and studied the effect of a density wave in the framework of his previously proposed model for a galactic system explaining the phenomenon of asymmetric motion of highvelocity stars. In [2], Lindblad did not neglect the effects of elasticity and self-gravitation, as he had in [1]. To describe the dynamics of density perturbations j ¸ (\Gammai!t + im') in a disk of finite thickness rigidly rotating with angular velocity \Omega\Gamma Lindblad [2] obtained the differential equatio

The orientation parameters and rotation curves of 15 spiral galaxies

We analyzed ionized gas motion and disk orientation parameters for 15 spiral galaxies. Their velocity fields were measured with the H-alpha emission line by using the Fabry-Perot interferometer at the 6m telescope of SAO RAS. Special attention is paid to the problem of estimating the position angle of the major axis (PA_0) and the inclination (i) of a disk, which strongly affect the derived circular rotation velocity. We discuss and compare different methods of obtaining these parameters from kinematic and photometric observations, taking into account the presence of regular velocity (brightness) perturbations caused by spiral density waves. It is shown that the commonly used method of tilted rings may lead to systematic errors in the estimation of orientation parameters (and hence of circular velocity) being applied to galaxies with an ordered spiral structure. Instead we recommend using an assumption of constancy of i and PA_0 along a radius, to estimate these parameters. For each galaxy of our sample we present monochromatic H-alpha- and continuum maps, velocity fields of ionized gas, and the mean rotation curves in the frame of a model of pure circular gas motion. Significant deviations from circular motion with amplitudes of several tens of km/s (or higher) are found in almost all galaxies. The character and possible nature of the non-circular motion are briefly discussed