Proceedings of the XXIII Canary Islands Winter School of Astrophysics: 'Secular Evolution of Galaxies', edited by J. Falcon-Barroso and J.H. Knapen 2012, chapter 4.
astro-ph/1211.6752

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BARS AND SECULAR EVOLUTION IN DISK GALAXIES: THEORETICAL INPUT

E. Athanassoula


Aix Marseille Université, CNRS,
LAM (Laboratoire d'Astrophysique de Marseille)
UMR 7326, 13388, Marseille, France


Abstract: Bars play a major role in driving the evolution of disk galaxies and in shaping their present properties. They cause angular momentum to be redistributed within the galaxy, emitted mainly from (near-)resonant material at the inner Lindblad resonance of the bar, and absorbed mainly by (near-)resonant material in the spheroid (i.e., the halo and, whenever relevant, the bulge) and in the outer disk. Spheroids delay and slow down the initial growth of the bar they host, but, at the later stages of the evolution, they strengthen the bar by absorbing angular momentum. Increased velocity dispersion in the (near-)resonant regions delays bar formation and leads to less strong bars.

When bars form they are vertically thin, but soon their inner parts puff up and form what is commonly known as the boxy/peanut bulge. This gives a complex and interesting shape to the bar which explains a number of observations and also argues that the COBE/DIRBE bar and the Long bar in our Galaxy are, respectively, the thin and the thick part of a single bar.

The value of the bar pattern speed may be set by optimising the balance between emitters and absorbers, so that a maximum amount of angular momentum is redistributed. As they evolve, bars grow stronger and rotate slower. Bars also redistribute matter within the galaxy, create a disky bulge (pseudo-bulge), increase the disk scale-length and extent and drive substructures such as spirals and rings. They also affect the shape of the inner part of the spheroid, which can evolve from spherical to triaxial.


Table of Contents

INTRODUCTORY REMARKS

INTRODUCTION

ORBITS AND RESONANCES

N-BODY SIMULATIONS

ON ANGULAR MOMENTUM EXCHANGE AND THE ROLE OF RESONANCES: THE ANALYTIC APPROACH

ON ANGULAR MOMENTUM EXCHANGE AND THE ROLE OF RESONANCES: INPUT FROM SIMULATIONS
General comments
Calculating the orbital frequencies
Material at resonance
Angular momentum exchange
Types of models
Live versus rigid halo
Distribution of frequencies for MD- and MH-type models
Bar strength

BAR SLOWDOWN
Results from N-body simulations
A schematic view
What sets the pattern speed value?

BOXY/PEANUT BULGES
Peanuts: input from simulations, orbits and observations
Peanut-related orbital structure
Peanuts as parts of bars: shape and extent
The boxy/peanut system in the Milky Way

SECULAR EVOLUTION OF THE DISK AND OF ITS SUBSTRUCTURES
Redistribution of the disk mass: formation of the disky bulge
Redistribution of the disk mass: the disk scale-length and extent
Redistribution of the disk mass: maximum versus sub-maximum disks
Secular evolution of the halo component

COMPARISON WITH OBSERVATIONS

SUMMARY AND DISCUSSION

REFERENCES

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