Instabilities of a family of oblate stellar spheroids

We have examined the stability of a sequence of oblate elliptical galaxy models having the Stackel form suggested by Kuz'min & Kutuzov. We have employed the 2-integral DFs given by Dejonghe & de Zeeuw for which flattened non-rotating models are characterized by counter-streaming motion and are radially cool; we introduce net rotation in some models by changing the sign of the z-component of angular momentum for a fraction of the particles. We have found that all non-rotating and slowly rotating members of this sequence rounder than E7 are stable, and that even maximally rotating models rounder than E4 are stable. In the absence of strong rotation, the most disruptive instability, and the last to be stabilized by increasing thickness, is a lopsided (m=1) mode. This instability appears to be driven by counter-rotation in radially cool models. Its vigour is lessened as rotation is increased, but it remains strong even in models with net angular momentum 90% of that of a maximally rotating model before finally disappearing in maximally rotating models. Strongly rotating models are more unstable to bar-forming modes which afflict maximally rotating models with c/a >~ 0.5, but this mode is quickly stabilized by moderate fractions of counter-rotating particles. Bending instabilities appear not to be very important; they are detectable in the inner parts of the flatter models, but are less vigorous and more easily stabilized than the lop-sided or bar modes in every case. We briefly discuss the possible relevance of the lop-sided instability to the existence of many lop-sided disk galaxies.Comment: 18 pages including 8 figures, TeX, macro file included, to appear in Monthly Notices of RA

Dynamical Friction and the Distribution of Dark Matter in Barred Galaxies

We use fully self-consistent N-body simulations of barred galaxies to show that dynamical friction from a dense dark matter halo dramatically slows the rotation rate of bars. Our result supports previous theoretical predictions for a bar rotating within a massive halo. On the other hand, low density halos, such as those required for maximum disks, allow the bar to continue to rotate at a high rate. There is somewhat meager observational evidence indicating that bars in real galaxies do rotate rapidly and we use our result to argue that dark matter halos must have a low central density in all high surface brightness disk galaxies, including the Milky Way. Bars in galaxies that have larger fractions of dark matter should rotate slowly, and we suggest that a promising place to look for such candidate objects is among galaxies of intermediate surface brightness.Comment: 6 pages, Latex, 3 figures, Accepted by Ap.J.L., revised copy, includes an added paragrap

Stability of disk galaxies in the modified dynamics

General analytic arguments lead us to expect that in the modified dynamics (MOND) self-gravitating disks are more stable than their like in Newtonian dynamics. We study this question numerically, using a particle-mesh code based on a multi-grid solver for the (nonlinear) MOND field equation. We start with equilibrium distribution functions for MOND disk models having a smoothly truncated, exponential surface-density profiles and a constant Toomre $Q$ parameter. We find that, indeed, disks of a given ``temperature'' are locally more stable in MOND than in Newtonian dynamics. As regards global instability to bar formation, we find that as the mean acceleration in the disk is lowered, the stability of the disk is increased as we cross from the Newtonian to the MOND regime. The degree of stability levels off deep in the MOND regime, as expected from scaling laws in MOND. For the disk model we use, this maximum degree of stability is similar to the one imparted to a Newtonian disk by a halo three times as massive at five disk scale lengths.Comment: 20 pages, Latex, 8 embedded figures, version to be published in The Astrophys.

Bar-Halo Friction in Galaxies II: Metastability

It is well-established that strong bars rotating in dense halos generally slow down as they lose angular momentum to the halo through dynamical friction. Angular momentum exchanges between the bar and halo particles take place at resonances. While some particles gain and others lose, friction arises when there is an excess of gainers over losers. This imbalance results from the generally decreasing numbers of particles with increasing angular momentum, and friction can therefore be avoided if there is no gradient in the density of particles across the major resonances. Here we show that anomalously weak friction can occur for this reason if the pattern speed of the bar fluctuates upwards. After such an event, the density of resonant halo particles has a local inflexion created by the earlier exchanges, and bar slowdown can be delayed for a long period; we describe this as a metastable state. We show that this behavior in purely collisionless N-body simulations is far more likely to occur in methods with adaptive resolution. We also show that the phenomenon could arise in nature, since bar-driven gas inflow could easily raise the bar pattern speed enough to reach the metastable state. Finally, we demonstrate that mild external, or internal, perturbations quickly restore the usual frictional drag, and it is unlikely therefore that a strong bar in a galaxy having a dense halo could rotate for a long period without friction.Comment: 13 pages, 11 figures, to appear in Ap

Modeling Non-Circular Motions in Disk Galaxies: Application to NGC 2976

We present a new procedure to fit non-axisymmetric flow patterns to 2-D velocity maps of spiral galaxies. We concentrate on flows caused by bar-like or oval distortions to the total potential that may arise either from a non-axially symmetric halo or a bar in the luminous disk. We apply our method to high-quality CO and Halpha data for the nearby, low-mass spiral NGC 2976 previously obtained by Simon et al., and find that a bar-like model fits the data at least as well as their model with large radial flows. We find supporting evidence for the existence of a bar in the baryonic disk. Our model suggests that the azimuthally averaged central attraction in the inner part of this galaxy is larger than estimated by these authors. It is likely that the disk is also more massive, which will limit the increase to the allowed dark halo density. Allowance for bar-like distortions in other galaxies may either increase or decrease the estimated central attraction.Comment: 12 pages, 6 figures, accepted for publication in ApJ. v2: minor changes to match proofs. For version with high-resolution figures, see http://www.physics.rutgers.edu/~spekkens/papers/noncirc.pd

The counter-streaming instability in dwarf ellipticals with off-center nuclei

n many nucleated dwarf elliptical galaxies (dE,N's), the nucleus is offset by a significant fraction of the scale radius with respect to the center of the outer isophotes. Using a high-resolution N-body simulation, we demonstrate that the nucleus can be driven off-center by the m=1 counterstreaming instability, which is strong in flattened stellar systems with zero rotation. The model develops a nuclear offset on the order of 30% of the exponential scale length. We compare our numerical results with the photometry and kinematics of FCC 046, a Fornax Cluster dE,N with a nucleus offset by 1.2" we find good agreement between the model and FCC 046. We also discuss mechanisms that may cause counterrotation in dE,N's and conclude that the destruction of box orbits in an initially triaxial galaxy is the most promising.Comment: 5 pages, 4 figure

Physical Properties of Tidal Features in Interacting Disk Galaxies

We explore tidal interactions of a galactic disk with Toomre parameter Q ~ 2 embedded in rigid halo/bulge with a point mass companion moving in a prescribed parabolic orbit. Tidal interactions produce well-defined spiral arms and extended tidal features such as bridge and tail that are all transient, but distinct in nature. In the extended disks, strong tidal force is able to lock the perturbed epicycle phases of the near-side particles to the perturber, shaping them into a tidal bridge that corotates with the perturber. A tidal tail develops at the opposite side as strongly-perturbed, near-side particles overtake mildly-perturbed, far-side particles. The tail is essentially a narrow material arm with a roughly logarithmic shape, dissolving with time because of large velocity dispersions. Inside the disks where tidal force is relatively weak, on the other hand, a two-armed logarithmic spiral pattern emerges due to the kinematic alignment of perturbed particle orbits. While self-gravity makes the spiral arms a bit stronger, the arms never become fully self-gravitating, wind up progressively with time, and decay after the peak almost exponentially in a time scale of ~ 1 Gyr. The arm pattern speed varying with both radius and time converges to Omega-kappa/2 at late time, suggesting that the pattern speed of tidally-driven arms may depend on radius in real galaxies. We present the parametric dependences of various properties of tidal features on the tidal strength, and discuss our findings in application to tidal spiral arms in grand-design spiral galaxies. (Abridged)Comment: 49 pages, 17 figures, 1 table. Accepted for publication in Astrophysical Journal. PDF version with higher resolution figures is available at http://astro.snu.ac.kr/~shoh/research/publications/astroph/Tidally_Induced_Spiral_Structure.pd

Structure of the Draco Dwarf Spheroidal Galaxy

This article studies the structure of the Draco dwarf spheroidal galaxy with an emphasis on the question of whether the spatial distribution of its stars has been affected by the tidal interaction with the Milky Way, using R- and V-band CCD photometry for eleven fields. The article reports coordinates for the center, a position angle of the major axis, and the ellipticity. It also reports the results of searches for asymmetries in the structure of Draco. These results, and searches for a ``break'' in the radial profile and for the presence of principal sequences of Draco in a color-magnitude diagram for regions more than 50 arcmin from the center, yield no evidence that tidal forces from the Milky Way have affected the structure of Draco.Comment: 25 pages, 11 figures, 3 tables. Accepted for publication in A

High resolution simulations of unstable modes in a collisionless disc

We present N-body simulations of unstable spiral modes in a dynamically cool collisionless disc. We show that spiral modes grow in a thin collisionless disk in accordance with the analytical perturbation theory. We use the particle-mesh code SUPERBOX with nested grids to follow the evolution of unstable spirals that emerge from an unstable equilibrium state. We use a large number of particles (up to 40 million particles) and high-resolution spatial grids in our simulations (128^3 cells). These allow us to trace the dynamics of the unstable spiral modes until their wave amplitudes are saturated due to nonlinear effects. In general, the results of our simulations are in agreement with the analytical predictions. The growth rate and the pattern speed of the most unstable bar-mode measured in N-body simulations agree with the linear analysis. However the parameters of secondary unstable modes are in lesser agreement because of the still limited resolution of our simulations.Comment: 11 pages, 8 figures in 22 files, A&A in print: Oct. 1st 200