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 stability of some galaxy disks is still perplexing

The problem of how some disk galaxies avoid forming bars remains unsolved. Many galaxy models having reasonable properties continue to manifest vigorous instabilities that rapidly form strong bars and no widely-accepted idea has yet been advanced to account for how some disk galaxies manage to avoid this instability. It is encouraging that not all galaxies formed in recent cosmological simulations possess bars, but the dynamical explanation for this result is unclear. The unstable mode that creates a bar is understood as a standing wave in a cavity that reflects off the disk center and the corotation radius, with amplification at corotation. Here we use simulations to address one further idea that may perhaps inhibit the feedback loop and therefore contribute to stability, which is to make the disk center dynamically hot and/or to taper away mass from the inner disk, which could be masked by a bulge. Unfortunately, we find that neither strategy makes much difference to the global stability of the disk in the models we have tried. While deep density cutouts do indeed prevent feedback through the center, they still reflect incoming waves and thereby provoke a slightly different instability that again leads to a strong bar.Comment: Added reference to Smercina et al (2023). Appeared in ApJ 958:18

Spiral instabilities: Linear and nonlinear effects

We present a study of the spiral responses in a stable disc galaxy model to co-orbiting perturbing masses that are evenly spaced around rings. The amplitudes of the responses, or wakes, are proportional to the masses of the perturbations, and we find that the response to a low-mass ring disperses when it is removed -- behaviour that is predicted by linear theory. Higher mass rings cause nonlinear changes through scattering at the major resonances, provoking instabilities that were absent before the scattering took place. The separate wake patterns from two rings orbiting at differing frequencies, produce a net response that is an apparently shearing spiral. When the rings have low mass, the evolution of the simulation is both qualitatively and quantitatively reproduced by linear superposition of the two separate responses. We argue that apparently shearing transient spirals in simulations result from the superposition of two or more steadily rotating patterns, each of which is best accounted for as a normal mode of the non-smooth disc.Comment: 14 pages, 19 figures and one animation. To appear in MNRAS. The animation may be viewed at http://www.physics.rutgers.edu/~sellwood/blobs4.av