J. A. Sellwood

Radial mixing in galactic discs

We show that spiral waves in galaxy discs churn the stars and gas in a manner that largely preserves the overall angular momentum distribution and leads to little increase in random motion. Changes in the angular momenta of individual stars are typically as large as ∼50 per cent over the lifetime of the disc. The changes are concentrated around the corotation radius for an individual spiral wave, but since transient waves with a wide range of pattern speeds develop in rapid succession, the entire disc is affected. This behaviour has profound consequences for the metallicity gradients with radius in both stars and gas, since the interstellar medium is also stirred by the same mechanism. We find observational support for stirring, propose a simple model for the distribution of stars over metallicity and age, and discuss other possible consequences.

Constraints from Dynamical Friction on the Dark Matter Content of Barred Galaxies

We show that bars in galaxy models having halos of moderate density and a variety of velocity distributions all experience a strong drag from dynamical friction unless the halo has large angular momentum in the same sense as the disk. The frictional drag decreases the bar pattern speed, driving the corotation point out to distances well in excess of those estimated in barred galaxies. The halo angular momentum required to avoid strong braking is unrealistically large, even when rotation is confined to the inner halo only. We conclude, therefore, that bars are able to maintain their observed high pattern speeds only if the halo has a central density low enough for the disk to provide most of the central attraction in the inner galaxy. We present evidence that this conclusion holds for all bright galaxies.

Dynamics of barred galaxies

Some 30% of disc galaxies have a pronounced central bar features in the disc plane and many more have weaker features of a similar kind. Kinematic data indicate that the bar constitutes a major non-axisymmetric component of the mass distribution and that the bar pattern tumbles rapidly about the axis normal to the disc plane. The observed motions are consistent with material within the bar streaming along highly elongated orbits aligned with the rotating major axis. A barred galaxy may also contain a spheroidal bulge at its centre, spirals in the outer disc and, less commonly, other features such as a ring or lens. Mild asymmetries in both the light and kinematics are quite common. The authors review the main problems presented by these complicated dynamical systems and summarize the effort so far made towards their solution, emphasizing results which appear secure.

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.

Differential Rotation and Turbulence in Extended H I Disks

When present, extended disks of neutral hydrogen around spiral galaxies show a remarkably uniform velocity dispersion of ~6 km s-1. Since stellar winds and supernovae are largely absent in such regions, neither the magnitude nor the constancy of this number can be accounted for in the classical picture in which interstellar turbulence is driven by stellar energy sources. Here we suggest that magnetic fields with strengths of a few microgauss in these extended disks allow energy to be extracted from galactic differential rotation through MHD-driven turbulence. The magnitude and constancy of the observed velocity dispersion may be understood if its value is Alfvenic. Moreover, by providing a simple explanation for a lower bound to the gaseous velocity fluctuations, MHD processes may account for the sharp outer edge to star formation in galaxy disks.

The Destruction of bars by central mass concentrations

More than two-thirds of disk galaxies are barred to some degree. Many today harbor massive concentrations of gas in their centers, and some are known to possess supermassive black holes (SMBHs) and their associated stellar cusps. Previous theoretical work has suggested that a bar in a galaxy could be dissolved by the formation of a mass concentration in the center, although the precise mass and degree of central concentration required is not well established. We report an extensive study of the effects of central masses on bars in high-quality N-body simulations of galaxies. We have varied the growth rate of the central mass, its final mass, and its degree of concentration to examine how these factors affect the evolution of the bar. Our main conclusions are the following: (1) Bars are more robust than previously thought. The central mass has to be as large as several percent of the disk mass to completely destroy the bar on a short timescale. (2) For a given mass, dense objects cause the greatest reduction in bar amplitude, while significantly more diffuse objects have a lesser effect. (3) The bar amplitude always decreases as the central mass is grown and continues to decay thereafter on a cosmological timescale. (4) The first phase of bar weakening is due to the destruction by the central mass concentration (CMC) of lower energy, bar-supporting orbits, while the second phase is a consequence of secular changes to the global potential that further diminish the number of bar-supporting orbits. We provide detailed phase-space and orbit analysis to support this suggestion. Thus, current masses of SMBHs are probably too small, even when dressed with a stellar cusp, to affect the bars in their host galaxies. The molecular gas concentrations found in some barred galaxies are also too diffuse to affect the amplitude of the bar significantly. These findings reconcile the apparent high percentage of barred galaxies with the presence of CMCs and have important implications for the formation and survival of bars in such galaxies.

Secular Evolution in Disk Galaxies

Disk galaxies evolve over time through processes that may rearrange both the radial mass profile and the metallicity distribution within the disk. This review of such slow changes is largely, though not entirely, restricted to internally-driven processes that can be distinguished from evolution driven by galaxy interactions. It both describes our current understanding of disk evolution, and identifies areas where more work is needed. Stellar disks are heated through spiral scattering, which increases random motion components in the plane, while molecular clouds redirect some fraction of the random energy into vertical motion. The recently discovered process of radial migration at the corotation resonance of a transient spiral mode does not alter the underlying structure of the disk, since it neither heats the disk nor causes it to spread, but it does have a profound effect on the expected distribution of metallicities among the disk stars. Bars in disks are believed to be major drivers of secular evolution through interactions with the outer disk and with the halo. Once the material that makes up galaxy disks is converted into stars, their overall angular momentum distribution cannot change by much, but that of the gas is generally far more liable to rearrangement, allowing rings and pseudo-bulges to form. While simulations are powerful tools from which we have learned a great deal, those of disks may suffer from collisional relaxation that requires some results to be interpreted with caution.

The lifetimes of spiral patterns in disc galaxies

The rate of internally driven evolution of galaxy discs is strongly affected by the lifetimes of the spiral patterns they support. Evolution is much faster if the spiral patterns are recurrent short-lived transients rather than long-lived, quasi-steady features. As rival theories are still advocated based on these two distinct hypotheses, I review the evidence that bears on the question of the lifetimes of spiral patterns in galaxies. Observational evidence from external galaxies is frustratingly inconclusive, but the velocity distribution in the solar neighbourhood is more consistent with the transient picture. I present simulations of galaxy models that have been proposed to support quasi-steady, two-arm spiral modes that in fact evolve quickly due to multi-arm instabilities. I also show that all simulations to date manifest short-lived patterns, despite claims to the contrary. Thus the transient hypothesis is favoured by both numerical results and the velocity distribution in the solar neighbourhood.

The Compression of Dark Matter Halos by Baryonic Infall

The initial radial density profiles of dark matter halos are laid down by gravitational collapse in hierarchical structure formation scenarios and are subject to further compression as baryons cool and settle to the halo centers. Here we describe an explicit implementation of the algorithm, originally developed by Young, to calculate changes to the density profile as the result of adiabatic infall in a spherical halo model. Halos with random motion are more resistant to compression than are those in which random motions are neglected, which is a key weakness of the simple method widely employed. Youngs algorithm results in density profiles in excellent agreement with those from N-body simulations. We show how the algorithm can be applied to determine the original uncompressed halos of real galaxies, a step that must be computed with care in order to enable a confrontation with theoretical predictions from theories such as ΛCDM.

Modeling noncircular motions in disk galaxies : Application to NGC 2976

We present a new procedure to fit nonaxisymmetric flow patterns to two-dimensional velocity maps of spiral galaxies. We concentrate on flows caused by barlike or oval distortions to the total potential, which 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 Hα data for the nearby, low-mass spiral NGC 2976, previously obtained by Simon et al., and find that a barlike 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 barlike distortions in other galaxies may either increase or decrease the estimated central attraction.