Accepted to appear in Reviews of Modern Physics, 2014
http://arxiv.org/abs/1310.0403

For a PDF version of the article, click here.

SECULAR EVOLUTION IN DISK GALAXIES

J. A. Sellwood


Department of Physics & Astronomy, Rutgers University, 136 Frelinghuysen Road, Piscataway, NJ 08854, USA


Abstract: 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 pseudobulges 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.


Table of Contents

INTRODUCTION

BACKGROUND
Galaxy formation
Relaxation time in the disks of galaxies
Standard theory for spherical systems
Applications to disk systems
Stellar orbits in disks
Action-angle variables
Orbital tori
Distribution function
Nonaxisymmetric disturbances
Resonances
Local stability
Angular momentum changes
Gas
Gravity softening in simulations
2D simulations
Simulations of thickened disks

TRANSIENT SPIRAL MODES
Origin and Recurrence
Scattering of stars
Solar neighborhood data
Scattering by spirals
Vertical heating
Scattering by dense mass clumps
Collective effects
Conclusions on scattering
Heating in simulations
Radial migration and mixing
Mechanism of radial migration
Other radial mixing processes
Radial migration in simulations
Adiabatic invariants
Tests for radial migration in the Milky Way
Smoothing rotation curves
Angular momentum redistribution

DISK THICKENING AND SURVIVAL
Formation of thickened disks
Survival of thin disks
Challenge to radial migration models

BARS
Stellar dynamics of bars
The origin of bars
Global bar-forming mode
Stabilizing mechanisms
Bar formation through nonlinear trapping
Slow trapping of orbits
Bar formation through tidal encounters
Recurring bars?
Continued growth of bars
Buckling instability
Gas response to bar forcing
Flows in two dimensions
Flows in 3D
Bar dissolution
Discussion of bar fraction
Bar pattern speeds
Bars within bars
Fueling of AGN by bars?
DYNAMICAL FRICTION
Mechanism
Improved treatment
Bar-halo friction
Constraint on halo density
Anomalously weak friction
Particle number
Change in halo density
Halo density reduction by bars
Halo density reduction by moving mass clumps

RINGS AND OUTER LIGHT PROFILES
Rings
Outer light profiles

PSEUDOBULGES AND LENSES

CONCLUSIONS

REFERENCES

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