Vladimir I. Korchagin

The Dynamics of Heavy Gaseous Disks

This paper seeks to develop a fuller understanding of the behavior of thin, polytropic, self-gravitating disks. Our approach consists of performing a detailed analysis of a representative model disk that is known to be susceptible to a single, rapidly growing, two-armed spiral mode. Numerical simulations of the evolution of this disk are presented; these new simulations have both an improved radial dynamic range and more realistic boundary conditions. This numerical work indicates that the primary unstable two-armed spiral mode is robustly endemic to the disk itself and is not a pathological product of artificially reflecting disk edges. The numerical simulations are then used to motivate (1) a linear modal analysis of both the representative disk and other related disks. In describing the linear analysis, we include a discussion of the relative merits of matrix and direct integration methods for solving the linear problem. The numerical simulations are also used to motivate (2) second-order and third-order perturbative analyses of the hydrodynamic governing equations, including the derivation of a higher order analogue to the energy integral constraint. We show that the development of mass and angular momentum transport through the disk is very well explained by a second-order nonlinear process involving self-interaction of the dominant (m = 2) linear mode. We then demonstrate that the saturation phenomena seen in earlier studies of this (and other) disks is the result of a third-order nonlinear effect. This definitive explanation of saturation provides a key for understanding the quasi-steady behavior that is often seen in self-gravitating disks.

Spiral Mode Saturation in Self-Gravitating Disks

This paper examines a second-order nonlinear mechanism that appears to bear responsibility for (1) eliciting transport of mass and angular momentum in self-gravitating gaseous disks and (2) inducing mode saturation that can preclude the onset of disk fragmentation. Our analysis indicates in quantitative detail how torques arising from gravitationally unstable spiral modes can lead to disk accretion. We begin by performing a linear global stability analysis on an idealized model equilibrium disk that is prone to a single , rapidly growing two-armed spiral. We compare the linearized predictions with the full hydrodynamical evolution of the disk provided by numerical simulations. We then retain second-order terms in a perturbative reanalysis of the hydrodynamic governing equations. We derive equations that describe how mass and angular momentum are redistributed in the disk. Then we solve these equations numerically and compare the results with the simulations. We conclude with a discussion of how nonlinear mode interactions and self-interactions are responsible for mode saturation in the disk and the development of steady mass accretion.

THE THREE-DIMENSIONAL VELOCITY STRUCTURE OF THE THICK DISK FROM SPM4 AND RAVE DR2

We analyze the three-dimensional kinematics of a sample of ~4400 red clump stars ranging between 5 and 10?kpc from the Galactic center and up to 3 kpc from the Galactic plane. This sample is representative for the metal-rich ([Fe/H] = ?0.6 ? +0.5) thick disk. Absolute proper motions are from the fourth release of the Southern Proper Motion Program and radial velocities from the second release of the Radial Velocity Experiment. The derived kinematical properties of the thick disk include the rotational velocity gradient ?V ?/?z = ?25.2 ? 2.1?km?s?1?kpc?1, velocity dispersions ?km?s?1, and velocity-ellipsoid tilt angle ? Rz = 86 ? 18. Our dynamical estimate of the thin-disk scale length is R thin = 2.0 ? 0.4 kpc and of the thick-disk scale height is z thick = 0.7 ? 0.1 kpc. The observed orbital eccentricity distribution compared with those from four different models of the formation of the thick disk from Sales et?al. favors the gas-rich merger model and the minor merger heating model. Interestingly, when referred to the currently accepted value of the LSR, stars more distant than 0.7 kpc from the Sun show a net average radial velocity of 13 ? 3?km?s?1. This result is seen in previous kinematical studies using other tracers at distances larger than ~1 kpc. We suggest this motion reflects an inward perturbation of the locally defined LSR induced by the spiral density wave.

PROPER-MOTION STUDY OF THE MAGELLANIC CLOUDS USING SPM MATERIAL

Resumen en: Absolute proper motions are determined for stars and galaxies to V = 17.5 over a 450 square degree area that encloses both Magellanic Clouds. The proper ...

Local Surface Density of the Galactic Disk from a Three-Dimensional Stellar Velocity Sample

We have reestimated the surface density of the Galactic disk in the solar neighborhood within ±0.4 kpc of the Sun using the parallaxes and proper motions of a kinematically and spatially unbiased sample of 1476 old bright red giant stars from the Hipparcos catalog with measured radial velocities from Barbier-Brossat & Figon. We determine the vertical distribution of the red giants as well as the vertical velocity dispersion of the sample (14.4 ± 0.3 km s-1) and combine these to derive the surface density of the gravitating matter in the Galactic disk as a function of the Galactic coordinate z. The surface density of the disk increases from 10.5 ± 0.5 M⊙ pc-2 within ±50 pc to 42 ± 6 M⊙ pc-2 within ±350 pc. The estimated volume density of the Galactic disk within ±50 pc is about 0.1 M⊙ pc-3 which is close to the volume density estimates of the observed baryonic matter in the solar neighborhood.

Velocity Shear of the Thick Disk from SPM3 Proper Motions at the South Galactic Pole

The kinematical properties of the Galactic thick disk are studied using absolute proper motions from the Third Yale/San Juan Southern Proper Motion Catalog and Two Micron All Sky Survey near-infrared photometry for a sample of ~1200 red giants in the direction of the south Galactic pole. The photometrically selected sample is dominated by thick-disk stars, as indicated by the number-density distribution that varies with distance from the Galactic plane as a single-valued exponential over the range 1 kpc < z < 4 kpc. The inferred scale height of the thick disk is 0.783 ± 0.048 kpc. The kinematics of the sample are also consistent with disklike motion. The U-velocity component is roughly constant, reflecting the Suns peculiar motion, while a considerable shear is seen in the mean rotational velocity, V. The V-velocity profiles dependence on z is linear, with a gradient of dV/dz = -30 ± 3 km s-1 kpc-1. The velocity dispersions in both U and V show a lesser gradient of about 9 ± 3 km s-1 kpc-1. We demonstrate that the derived velocity and velocity-dispersion profiles are consistent with the assumptions of dynamical equilibrium and reasonable models of the overall Galactic potential.

SPACE VELOCITIES OF SOUTHERN GLOBULAR CLUSTERS. VI. NINE CLUSTERS IN THE INNER MILKY WAY

(abridged) We have measured the absolute proper motions of nine low-latitude, inner Galaxy globular clusters, namely NGC 6273 (M 19), NGC 6284, NGC 6287, NGC 6293, NGC 6333 (M 9), NGC 6342, NGC 6356, NGC 6388 and NGC 6441. These are the first determinations ever made for these clusters. The proper motions are on the ICRS via Hipparcos. The proper-motion errors range between 0.4 and 0.9 mas/yr, and are dominated by the number of measurable cluster members in these regions which are very crowded by the bulge/bar and the thick disk. This samle contains five metal poor ([Fe/H < -1.0) and four metal rich clusters; seven clusters are located within 4 kpc from the Galactic center, while the remaining two, namely NGC 6356 and NGC 6284 are in the background of the bulge at 7.5 kpc from the Galactic center. By combining proper motions with radial velocities and distances from the literature we derive 3D velocities. In a number of cases, distance uncertainties make the kinematical classification ambiguous. The two metal rich clusters NGC 6388 and NGC 6441 have velocities incompatibile with membership in the thick disk or the bar of the Milky Way. They can be though of as members of a kinematically hot system in the inner Galaxy. Curiously, both clusters have similar velocity components. Together with their similar Galactic location and peculiar but similar stellar-population characteristics, these two clusters may share a common origin. Their velocities are also very low indicating that the two clusters are now at/near apocenter, and they will not leave the inner ~4 kpc of the Galaxy.

A Capture Scenario for the Globular Cluster ω Centauri

We explore an accretion origin for ω Cen by N-body modeling of the orbital decay and disruption of a Milky Way dwarf satellite. This work is focused on studying a particular satellite model that aims to reproduce the present orbit of ω Cen, as recently determined from absolute proper motions. The model satellite is launched from 58 kpc from the Galactic center, on a radial low-inclination orbit. We find that a capture scenario can produce an ω Cen-like object with the current low-energy orbit of the cluster. Our best model is a nucleated dwarf galaxy with a Hernquist density profile that has a mass of 8 × 109 M☉ and a half-mass radius of 1.4 kpc.

Disruption of a dwarf galaxy under strong shocking: the origin of ω Centauri

We perform N-body simulations of the dynamical evolution of a dwarf galaxy falling into the Milky Way galaxy in order to understand the formation scenario of the peculiar globular cluster ω Centauri. We use self-consistent models of the bulge and the disc of the Milky Way, as well as of the dwarf galaxy, and explore a range of dwarf models with different density distributions. Namely, we use King and Hemquist density profiles to model the density distribution in the dwarf. The central region of our King model has a density profile approximately oc r -2 , while that of the Hernquist model is r -1 . The difference in the dwarfs density distributions leads to distinct evolutionary scenarios. The King model dwarf loses its mass exponentially as a function of apocentric distance, with the mass loss rate depending on the initial mass and size of the dwarf. Regardless of the initial mass and size, the King model dwarf remains more massive than 10 8 M ○. after a few gigayears of evolution. The Hemquist model dwarf experiences an accelerated mass loss, and the mass of the remnant falls below 10 8 M ○. within a few gigayears. By exploring an appropriate set of parameters, we find a Hernquist model that can attain the mass and orbital characteristics of co Cen after a few gigayears.

Global Spiral Modes in a Three-Phase Gravitating Disk

We analyze the global modal properties of self-gravitating disks with exponential density distributions both in one- and three-component approaches. We take into account the observational properties of galactic disks, namely, that the radial dependence of the stellar velocity dispersion cs is proportional to the square of the stellar surface density. The stability properties of a one-component disk are determined mainly by its central velocity dispersion and its maximal rotational velocity. Disks with the central velocity dispersion less than unity in units G = Rd = Md = 1 are unstable to tightly wound spirals, if the minimal value of Toomres Q-parameter is less than or close to unity. Here G is the gravitational constant, and Rd and Md are the radius and mass of the disk, respectively. Higher velocity dispersions increase the wavelength of unstable modes, as well as the probability of their generation. Disks with cs > 1.0 at the center can be unstable if the minimal value of the Q-parameter is less than 1.8. The stability of multicomponent disks is jointly determined by self-gravity and mass transformations between different phases. In this paper we discuss the situation in which the stability properties of the disk are primarily determined by self-gravity. If the admixture of clouds and gas are small, the shape and the growth rate of the principal unstable mode does not change significantly. However, a significant cold component in the system increases the value of the growth rate of an unstable mode. A new effect, in comparison to the one-component approach, is an angular phase separation between spirals of different components. Such displacements have been observed in the spiral arms of some nearby galaxies and can thus be considered as a confirmation of the validity of a global modal approach to self-gravitating multiphase galactic disks.